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Scientific Memoirs IV
[A course of six lectures to working men, delivered in the theatre of the Royal School of Mines.]
I
[Feb 28, 1876]
[163] TWENTY years ago the arguments as to the causes of the phenomena of organic nature, brought forward in support of the then recently advanced views of Mr. Darwin, were largely speculative; all pne could hope to show was that no valid objections could be urged against the theory of evolution. But since that time "many have run to and fro and knowledge has been increased"; the question has come out of the region of speculation into that of proof; every day increases our familiarity with the phenomena of life on the globe in antecedent ages, and so gives us the only valid evidence obtainable as to the evolution of living things.
When we consider any animal at the present day there are three theses which may be put forward with regard to its origin: that it arose out of nothing, that it had its origin from dead inorganic matter, or that it arose as a modification of some pre-existing living being. It is hardly worth while to consider the two first of these theses—for the first it would be utterly impossible to obtain any evidence, and the second is devoid of all ground of analogy, and opposed to all our knowledge of what actually takes place. The last, on the other hand, should, if true, be capable of some sort of proof—at any rate it can be brought to the test of facts.
It is quite conceivable that all evidence as to the origin of an animal may have disappeared, and that the problem becomes, in [164] consequence, insoluble by direct evidence, analogy and probability being the only guides left. As a matter of fact, however, we possess in the 70,000 feet of stone, gravel, sand, &c., which form the earth's crust, fossil remains imbedded in chrono-logical order, and in many cases so perfectly preserved, that all important details can be made out almost as well as in the recent state.
The plan adopted in these lectures will be not to give all obtainable evidence with regard to the origin of each group of vertebrate animals, but to select from each class one or two definite cases of living animals, and to see what evidences can be obtained, by going back in time, as to the way in which they have come about, or at any rate as to the extent of the duration of their existence.
To begin with fishes: we will take as our first example the very beautiful genus Beryx, a fish not unlike our sea-bream, found widely distributed through the deep seas, and extending to about 40° on each side of the equator. Like the perch or the sea-bream, it is a greatly specialised fish; the head is immensely large, the bony rays supporting the fins hard and unjointed, the ventral fins or representatives of the hind limbs situated just behind the head, under the throat, the operculum curiously ornamented, and the air-bladder completely shut off from the gullet; thus differing very markedly from a more generalised fish such as the herring or carp, in which the head is proportionally much smaller, the fin-rays soft and jointed, the ventral fins far back, the operculum not ornamented, and the air bladder communicating by a duct with the oesophagus.
We now know that at depths greater than five or six hundred fathoms, the sea-bottom is to a great extent composed of a very fine greyish-white mud, sticky when first removed from the water, but afterwards hardening into a delicate friable stone, not at all unlike chalk. This mud, which is largely made up of shells of the minute marine organism Globigerina forms the bed of thousands of square miles of sea in which Beryx lives, and there can be no doubt that when the fish dies it sinks to the bottom, and, its soft parts being destroyed, becomes gradually imbedded in the soft mud, there to remain until the present sea-bottom is upheaved and becomes dry land.
*****? those by which the recent species of Beryx are distinguished from one another,.
Now, as a matter of archæological evidence, what is known of the history of Beryx and of the source whence it proceeded? Naturally it is useless to seek for such evidence except in deposits formed under like conditions to those in which the fish lives at the present day. Through the whole of the Pliocene and Miocene epochs no deep-sea formations are known, but in the middle of the Eocene [165] period—a time so remote that tropical plants flourished on the banks of the Thames, and crocodiles abounded in this country—we acquainted with an extensive deep-see deposit, the Nummulitic limestone which, besides the fossil giving it its name, contains large quantities of Globigerina, in essential respects like that of the present day. In this formation we found two forms closely allied to Beryx, but with such slight differences as to receive different generic names; these are known as Acanus and Pristigaster. On passing from the Eocene or lowest tertiary to the chalk or uppermost secondary formation which bears the closely possible resemblance to belonging to one continuous series of deposits—we find an actual Beryx, a fish differing no more from the modern Berices than the various modern species of Beryx do from one another. This fossil, owing as to the fineness of the chalk-forming mud in which it was buried, is so perfectly preserved, that all the details of structure of its hard parts, even to the ornamentation of the scales, can be compared with those in the recent fish; and in this way the most conclusive evidence is obtained that the differences which separate it from its modern relatives are of no greater importance than those by which the recent species of Beryx are distinguished from one another.
Thus we have positive evidence that a fish altogether like the Beryx of the present day, existed millions of years ago, before the Alps, the Himalayas, or the Rocky Mountains were upheaved, and has continued to live ever since. In face of these facts we cannot but conclude that the modern Beryx is derived from that of the chalk, and that the hypotheses of its creation out of nothing, and of its origin from inorganic matter, are, for scientific purposes, simply non-existent.
As to the form from which the Beryx of the chalk was derived we have absolutely no evidence, for there is no trace of any such fish in any lower formation.
We now pass on to a fish of a far older and less specialised type than Beryx—the genus Ceratodus, recently discovered in Australia. This animal, which attains a length of six feet, is distinguished by the possession of very curious fins, consisting of a central lobe with a surrounding fringe of fin-rays, and by the character of its teeth which are produced into curious horn-like processes, so arranged that those of opposite jaws interlock. Ceratodus is probably a vegetable feeder, lives in fresh or brackish water, and is said sometime to leave it s native element and crawl about among the weeds on the back. It is enabled to do this by [166] the fact that it can breathe air directly as well as air dissolved in water it has, in fact, besides gills, an organ which is altogether a half-way house between the air-bladder of a fish and the lung of an amphibian.
In the Wealden, the most recent estuarine deposit of which we have any knowledge, there is no trace of Ceratodus to be found, but this is hardly surprising, as only two or three small patches of the earth's surface formed at this epoch have been examined, and animals have a geographical distribution at all times. But on passing back to the Trias, a formation as far from the chalk in point of time as the chalk from the present day, we find teeth belonging to an undoubted Ceratodus, in shape and in microscopic structure, exactly like those of the modern Australian fish. No other remains of the Triassic Ceratodus have as yet been found, but teeth are known to be so important a diagnostic character that no naturalist would have any hesitation in naming the genus to which the fish bearing such teeth should be assigned.
Thus we have a far more astonishing example of a persistent type than was afforded by our Beryx, and as in the case of the latter fish, all trace of the actual genus Ceratodus is lost at this point, and we are obliged to content ourselves with a few singular hints as to the way in which the type has come about. The most valuable of these hints are obtained by a study of a singular group of fish found in great abundance in the Old Red Sandstone. These are distinguished by the possession of curious fringed fins, unlike those of any other fish, except Lepidosiren and Ceratodus—in fact, one genus, Dipterus, has fins quite like those of Ceratodus, and its teeth and skeleton were formed on just the same type. It is doubtful whether there is any relation between Ceratodus and Dipterus in the way of ancestry, but the resemblance between them is remarkable.
It must seem rather strange for a known evolutionist to select as examples two fish like Beryx and Ceratodus, which, of all others seem most likely to support the notion that species are immutable. The adverse side only of the question has been stated to-night, the other side will be treated of on future occasions.
II
[March 6, 1876]
It was seen in the last lecture that no ultimate answer was obtainable as to the origin of the examples selected from the fish class, any more than is afforded as to the origin of the Anglo-Saxons [167] by showing that they came from Friesland in the sixth or seventh century. The same remark applies to the origin of nearly all fishes, in fact, only one clear case of progressive modification is known in the whole class; this is afforded by the group of the Pycnodonta.
These are fish not unlike our John Dory in shape, which appear for the first time in the carboniferous rocks, and become extinct in the older tertiaries; they are distinguished by the possession of rows of large crushing teeth, and in place of a vertebral column had a gelatinous chorda dorsalis or notochord. The spinal cord above this was embraced by arches of bone, placed at regular intervals along the chorda; and, immediately below these neural arches, were attached the ribs also bony. In the Carboniferous forms, both arches and ribs are quite distinct from one another, and are simply united by ligamentous fibres to the notochord; but, in the older Secondary species, they become expanded at their ends, and thus tend to embrace the notochord; and, lastly, in the Tertiary pycnodonts this process is carried to such an extent as almost to produce a ring of bone, like the body of a rudimentary vertebra.
Now let us turn to the next group of Vertebrate animals, that of Amphibia (frogs toads, newts, and salamanders), which are distinguished from fishes by certain very striking peculiarities. Fishes are all capable of breathing the air dissolved in water by means of gills, and—a far more important distinctive character—their limbs always have the character of fins, which organs are seen in their simplest form in Ceratodus. In this fish, there is a long jointed cartilaginous axis, running down the middle of the fin, with rows of rays of the same substance on each side of it; the whole is invested by a fold of the integument, the margins of which are beset with horny filaments called fin-rays. In all fishes these elements are to be found, generally in a curiously modified condition, in the bony fishes, for instance, the central axis with its side appendages are broadened out and shortened, the fin-rays becoming at the same time so much larger as to form the main part of the fin.
Some modification of this type of limb is possessed by all fishes which have limbs at all; but the first character, that afforded by the respiratory organs, is not absolute, for there are some fishes which, besides gills, possess an apparatus for breathing air directly. This apparatus, represented by the air-bladder of ordinary fishes, first takes on its new character, and becomes a lung in that remarkable genus, Ceratodus, in which it exists as a large cellular structure situated in the upper part of the abdominal cavity, just under the vertebral column, and connected with the gullet by a slit [168]—the glottis—by means of which the fish can pass air from the mouth into the lung. It is not, however, this pecutiarity of opening into the oesophagus which constitutes a lung, for the air-bladder of many fishes possesses an open duct of a similar nature; the great distinguishing feature is, that the blood taken to, this bladder does not pass into the ordinary venous channels, but is returned immediately to the heart, in a purified condition, by a special vein. In Ceratodus there is no special vessel to carry blood to the lung, in other words, although there is a pulmonary vein, the pulmonary artery has not appeared; but in the Mudfish (Lepidosiren) of Africa and eastern South America, the development of the lung goes a step further, a special pulmonary artery being present, as in all the higher animals. Thus Ceratodus and Lepidosiren are truly amphibious, for they can be suffocated neither by removal from water like most fish, nor by immersion in water like the higher animals.
What constitutes the difference between these amphibious fish, and the lowest of the true Amphibia? Not the nature of the respiratory process, for many of the latter group, such as the blind Proteus of the Austrian caves and the North American Menobranchus possess gills throughout life, but the structure of the limbs, which are now, no longer fins, but legs. A fish requires a broad surface for balancing itself in the water, locomotion being chiefly performed by the tail, but in land animals an apparatus is required capable of raising the body above the ground, and the limbs take on the form of a set of jointed levers. In its simplest form the higher vertebrate limb, consists, first, of a single piece of cartilage articulated with the body, then two pieces side by side, then a number of small nodules, and lastly, five series of short jointed pieces; all of these become in the adult state more or less converted into bone. The first or proximal division of the limb is called the humerus in the fore limb, the femur in the hind limb; the next segment consists of radius and ulna in the arm, tibia and fibula in the leg; the nodular pieces are respectively carpals and tarsals, and the series of jointed bones or cartilages, the five digits. From the lowest Amphibia upwards, the limbs, when present, are always constructed upon this type.
Nevertheless, the Amphibia still retain certain fish-like characters, which are lost in the groups above them. They all, at some period of life, breathe by means of gills, although all have, in the adult state, lungs in addition. Some forms, such as the Proteus and Menobranchus mentioned above, retain their gills throughout life and are hence called Perennibranchiates; others, such as Menopoma, [169] Amphiuma, &c., loose them in adult life, and are called Caducibranchiates. These two last genera, however, still retain traces of gill-clefts; but in all the Amphibia with which we are acquainted in this country, the frog, toad, and newt, even the clefts disappear, and the perfect air-breathing character is assumed.
These animals, in the course of their development, go through a very singular series of metamorphoses, comparable to those by which a grub is converted into a butterfly. At this season of the year, every pond is almost certain to contain frog-spawn, masses of transparent albuminous matter, with numberless imbedded eggs, consisting of yolk, black on one side and white on the other. A few hours after these eggs are laid, the process of development begins by the formation of a shallow groove, which appears quickly on the black, more slowly on the white hemisphere, and is just such a groove as would be produced by drawing a blunt instrument along the equator of a soft globe. The egg is thus divided into two masses. A second form appears at right angles to the first, dividing the whole egg into four others appear, in definite order, cutting it up into smaller and smaller masses, until the whole yolk becomes granular, or formed of microscopic cells.
Two ridges then appear, on the surface of the egg, and, uniting in the middle line, enclose a cavity, the lining membrane of which is converted into the brain and spinal cord. The head gradually becomes differentiated, and the mouth appears on its under side; the tail grows out, and the little creature, getting too long for the egg, becomes coiled upon itself, and before long, ruptures the egg membrane, and makes its exit from its mass of jelly.
It is now, to all intents and purposes, a fish; it has no limbs, its mouth is provided with horny jaws, and it breathes by means of a pair of plumose gills. It further differs from the adult frog in being herbivorous, feeding on water plants, to which it attaches itself by means of two suckers near the mouth. The tadpole grows rapidly, and, before long, a fold of skin appears on each side, which gradually closes over the gills, leaving, however, for a considerable time, a small opening on the left side. In the meantime the limbs appear, and the lungs are developed, the tadpole breathing for a time both by lungs and gills; the latter eventually disappear, the tail shortens, the limbs lengthen, the horny jaws are replaced by teeth, and an insect-eating, tail-less frog is formed, the adult air-breathing form having thus been attained by a wonderful series of changes, in which the fish, Lepidosiren, perennibranchiate, and triton, are all represented.
[170] One would be inclined to infer from these metamorphoses, that tracing the Amphibia back in time, the story of their origin should be told, but, as a matter of fact, paleontological history tells a different story altogether. Abundant remains of frogs and toads are found in the Miocene deposits, some of which are of so fine a character that even the tadpoles are preserved; but these tertiary frogs and toads do not differ, in any important particulars, from those of the present day, and the same is true of the tritons and salamanders. Some of the latter attained a very great size, and one of them—a near ally of the great Japanese salamander of the present day—has had a very singular fate, having been described, about the middle of the last century, as a fossil man, by the German naturalist Scheuchzer, who named it "Homo diluvii testis," the man who saw the flood!
In the Wealden and Purbeck formations no Amphibia have as yet been discovered, but, from the Lower Lias to the Carboniferous they turn up again in remarkable numbers, and of great size, but differing from existing forms in some important peculiarities, and affording no help whatever to our inquiries as to the origin of the existing or of the tertiary frogs, toads, and salamanders, Under the throat, these gigantic Amphibia had a remarkable shield of three bony plates, as well as a series of plates along the belly. Their teeth were large and powerful, and presented an extremely complicated structure, whence the group has received its name of Labyrinthodonta.
Thus, in tracing back the existing Amphibia, we find a great break in the secondary period, and then come upon a distinct group, the Labyrinthodonta, from which the existing forms cannot possibly be deduced. These, again, have been traced no farther back than the carboniferous epoch.
III
[March 13, 1876]
It will be necessary to preface our remarks as to the origin of the next highest group of Vertebrates—that of Reptiles—by some account of the distinction between them and the Amphibia, and by some observations on what zoologists mean by the terms "higher" and "lower" as applied to animals or groups of animals.
In external form there is little difference between such a reptile as a lizard, and such an amphibian as a newt, and there seems, at first sight, to be no reason why they should be placed in different primary groups. In former times, as a matter of fact, the essential [171] difference between reptiles and amphibians was not seen, and the two were united into a single class; but modern researches have shown that, beneath this external similarity, lie great and important differences, the chief of which we must now consider.
In the first place, no reptile, at any period of its life, possesses gills, and, in consequence, the breathing of air dissolved in water becomes impossible. Nevertheless, reptiles, in common with all the higher animals, have, at one period in their existence, slits leading from the throat to the exterior, in precisely the same position as the branchial clefts of an amphibian, but functionless.
Secondly, certain organs, known as "foetal appendages," are developed in connection with the young animal before it leaves the egg, and serve a temporary purpose in its economy. In the possession of these appendages, as well as in the absence of gills, reptiles agree with birds and mammals, and differ from fishes and amphibians.
The young reptile is produced from an egg of relatively large size, and consisting of a considerable mass of yolk, surrounded by a quantity of transparent "white" or albumen; the whole being invested by a hard or soft shell. The yolk does not divide as a whole, but the process of division is confined to a small patch on its surface; in fact, the reptilian egg answers to the amphibian egg, plus a quantity of additional matter, called accessory, or food-yolk, which is unaffected by the process of yolk-division. It is the small superficial patch, answering to the whole amphibian egg, which is converted into the body of the young reptile, the accessory yolk becoming gradually smaller and smaller, as its substance is used up in the nourishment of the embryo; in the meantime it forms a bag attached to the umbilicus of the embryo, and hence called the umbilical vesicle or yolk-sac; it is the first of the foetal appendages, and the only one which occurs in any vertebrate below a reptile, being possessed by certain fishes.
After the embryo has attained a certain size, and has come to lie, like an inverted boat, on the yolk-sac, a fold grows up, all round it, from the surface of the yolk, and, the edges of the fold coming together above, a bag is formed enclosing the embryo into the interior of which a watery fluid is secreted, in which the little creature lies. This natural water-bed is called the amnion; it is the second of the foetal appendages, and no trace of it is to be found in any fish or amphibian.
The third and last of these curious embryonic appendages, the allantois, grows out from the tail-end of the embryo as a pear-shaped body, solid at first, but soon converted into a sac, which extends round the embryo and yolk-sac, immediately beneath the membrane [172] of the shell. The cavity of the allantois acts as a receptacle for the nitrogenous waste of the embryonic body, but its chief function is as a respiratory organ; for this purpose it is supplied by blood-vessels which form a close network over its outer layer, and the blood contained in these coming into close relation with the external air, through the porous shell, readily exchanges its carbonic acid for the atmospheric oxygen.
As the embryo grows, the yolk-sac becomes smaller and smaller and is eventually completely drawn into the interior of the body of the young reptile, which by this time completely fills the shell. In many cases a horny knob is developed on the nose, and with this, the now ripe embryo breaks the shell from the interior; the amnion and other membranes are burst, the allantoic circulation is stopped, the first inspiration is taken, and the little creature is born.
There are several minor points in which reptiles are distinguished from amphibia, amongst which we will only mention the articulation of the skull to the first vertebra by one condyle instead of two, the presence of a bone called the basi-occipital in the hinder part of the skull floor, and the fact that the branchial apparatus is reduced in the adult to the small hyoid bone or cartilage, which supports the tongue.
In what respects is a reptile a higher organism than an amphibian? When one animal is said to be higher than another, one of two things may be meant: its structure may be more complicated, as a carved platter is higher than a simple trencher; or its parts may be so arranged as to form a more complicated mechanism. The mere repetition of parts does not raise an animal in the scale; a worm with a hundred segments is no higher than one with ten, any more than a mill with ten pairs of stones is a higher kind of machine than one with a single pair. But if, instead of multiplying the number of millstones, two pairs only were used, one of which was adapted for coarse, the other for fine grinding, a machine of a far higher order would be produced, and it is a similar differentiation of parts for special uses and co-adaptation of structures to given purposes which raises an animal above its fellows.
Judged by this standard, a reptile is a decidedly higher animal than an amphibian; its skeleton, for instance, is a better piece of work, the joints being more neatly finished, and the whole mechanism much more perfect.
A third test is based on the facts of development. We saw that a frog, in the course of its development, went through a stage [173] in which it was, to all intents and purposes, a fish, and that it was only after passing through this stage, as well as that of a branchiate amphibian, that it attained its higher adult character. Now the reptile stands in just the same relation to the amphibian, with regard to its development, as the amphibian to the fish. During the earlier stage of its growth it presents certain amphibian characters, such as the presence of gill-clefts; but these lower stages are passed over; the reptile goes beyond the highest amphibian in its development, and is therefore, in this respect also, to be considered as a higher animal,
At the present day there are four types of reptiles: the lizards (Laartilia), snakes (Ophidia), turtles and tortoises (Chelonia), and crocodiles (Crocodilia). We will now direct our attention to the first of these groups.
Most existing lizards have four well developed limbs, a long tail, a scaly armour, sometimes supplemented with plates of bone, and teeth, not set in distinct sockets, but firmly fixed to the jaw. The skull is so constructed that the hinder nostrils open far forwards into the mouth. The vertebræ have a peculiar and characteristic form, their articular surfaces being concave in front and convex behind, except in the Geckos or wall-lizards, and that remarkable New Zealand genus Hatteria or Sphenodon. The heart is composed of three chambers, two auricles and a single ventricle, the latter being again partly divided into two, and thus showing a slight advance on the amphibian heart, in which the ventricle is quite single.
Lizards are very abundant, especially in hot climates; most of them are land animals, a few only being inhabitants of fresh water, and one—the genus Amblyrhynchus of the Galapagos Archipelago—lives on the seashore, and, if hard pressed, takes to the sea.
Through the whole of the Tertiary epoch the lizards are essentially the same as those now existing. Some of the Secondary species, also, have the same characters, but in the chalk are found, in addition, strange marine lizards, such as the genus Mosasaurus, which attained a length of thirty feet. As far back as the Purbecks, the lizards have vertebræ like the existing kinds, but on descending to the Solenhofen slates we find abundant remains, which present the lower character of bi-concave vertebræ, and the same is true of all the still older forms, such as the Telerpeton of the Triassic sandstones of Elgin and the Permian Prolorosaurus.
Thus the older lizards have a slightly simpler structure than those of the present day, but resemble them, on the whole, so closely, that we must conclude our existing forms to have been derived from the [174] ancient ones, and have no need whatever to assume their special creation. Lizards, then, offer another example of what is meant by a persistent type.
A remarkable instance of this persistence is afforded by a case of quite the same order as that of Ceratodus, described in the first lecture. The Hatteria, mentioned above, differs from all other lizards in many particulars. Its jaws are armed with a horny beak, and its upper jaw has two rows of teeth, one on the maxillary, the other on the palatine bones; the teeth of the lower jaw bite between these, like a pair of scissors with a double upper blade. The vertebræ are bi-concave, and, along the belly, are placed a number of bony plates.
No other existing form whatever is known presenting these characters, but, about the year 1858, a number of fossils were discovered in the sandstone of Elgin, and amongst them the remains of a large lizard with bi-concave vertebræ, abdominal plates, a horny beak, a double row of upper jaw teeth, and, in fact, altogether like the existing Hatteria.
The crocodiles are the only other reptiles the history of which it will be possible to notice in this course. Two of the most important characters by which they are distinguished from lizards are, the lodgment of the teeth in distinct sockets and the position of the hinder nostrils or posterior nares. The maxillary, palatine, and pterygoid bones are so disposed as to form a remarkable shelf or partition in the roof of the mouth, thus bringing the posterior nares to the hindermost part of the throat. The soft palate forms a veil in front of these apertures, and hangs down so as to rest on the back part of the rudimentary tongue, and thus, except when the animal is swallowing, entirely shuts off the cavity of the mouth from the air passages. This arrangement has been prettily explained by the crocodile's habit of killing its prey by drowning; it is said that it can hold a captured animal under water, while its own nostrils—placed at the end of the long snout—are just above the surface, and thus is enabled to breathe freely, the air passing through the posterior nares, behind the veil of the palate, and so to the lungs, while its prey is being suffocated. This is an admirable explanation as far as the crocodile is concerned, but, unfortunately, it is probably untrue, for precisely the same arrangement is found in the Gavial and other crocodilians which live upon fish.
IV
[March 20,1876]
The crocodiles form the highest group of existing reptiles; they are higher than lizards as a steam-vessel is higher than a sailing-ship; for, while built essentially on the same lines, and exhibiting altogether the same fundamental structure, they are in some respects peculiarly modified, and that always in the direction of greater complexity.
Besides the characters of the skull mentioned in the last lecture, they are distinguished from lizards by having a four-chambered heart, one in which the separation of the ventricle into two distinct cavities is completed, so that, in the heart itself, the blood from the lungs is kept separate from that returned from the body generally. A mixture, however, takes place subsequently, through an aperture between the two aortæ, one of which springs from each ventricle.
Crocodiles are found in Central America, India, Africa, and Australia. Of the many species, the greater number are short-snouted; the fish-eating Gavial of the Ganges, on the other hand, has an extremely long and narrow snout.
All the existing crocodiles are fresh-water or estuarine animals, but, fortunately, this was not the case with the ancient forms, many of which were exclusively marine, seeming, so to say, to take the place in the sea of their own epoch, of our porpoises and dolphins.
Besides Tertiary species, crocodiles are found in the Chalk, Oolite, Lias, and Trias often in the best possible state of preservation; they therefore extend back to the very commencement of the Mesozoic epoch.
If we had specimens of all known forms of crocodiles, recent and extinct, and set to work to classify them according to their degrees of likeness and unlikeness, we should find that they naturally fell into three series.
In the first of these it would be found that the skull had all the characters mentioned at the end of the last lecture, the posterior nares being small apertures opening into the cavity of the mouth behind the pterygoid bones; the vertebræ would be concave in front and convex behind; the two bones composing the shoulder-girdle, the shoulder-blade or scapula and the coracoid, would be similar in shape, both being long and narrow; in the hip-girdle, the haunch-bone or ilium would be much cut away in front and excavated below, the ischium and pubis being both long, blade-like bones; and there would be seven or eight longitudinal rows of bony plates on the back.
[176] In the second set we should find the posterior nares to be much larger and placed farther forwards, immediately behind the palatine bones, the pterygoids not uniting as in the first group. The vertebræ would be slightly hollowed out at each end. In the shoulder and hip girdles there would be no important difference from the first group, with which also the more minute structure of the limbs would correspond closely. A difference would, however, be found in the fact of there being not more than two rows of plates on the back.
In the third series, we should notice certain very striking changes. The posterior nares would be actually as far forward as in a lizard; neither the palatine nor the pterygoids uniting in the floor of the mouth; the vertebræ would be completely amphicoelous or biconcave; the coracoid no longer long and narrow, but expanded and rounded like that of a lizard; the ilium more elongated and without the notch on its lower edge; and the ischium considerably broadened. As in the preceding group, the rows of bony plates on the back would not exceed two.
Thus we should find that the second group held an exactly intermediate place between the first and third, and that the third set, in every respect in which it differed from the normal crocodilian structure approached to that of lizards.
It is a very interesting point to see how these three groups appear in time. We should find that in the first are included all the Recent and Tertiary forms, and that there are no indications of the type below the later Cretaceous.
The second group would be found to extend from the older Cretaceous down to the Lias; moreover, a careful examination would show that there were lesser modifications among the individual species of a very instructive nature; those from the Wealden, for instance, would be seen to have the posterior nares farther back (i.e., nearer the typical crocodilian position) than those of the middle Mesozoic, and these again than those of the Lias.
The third group would contain exclusively Triassic forms, such as the dragon-like Belodon and the Stagonolepis of the Elgin sandstones. In this latter formation the fossils are in a very curious condition; after the sand accumulated round the bodies of the Triassic animals had hardened, water, percolating through the porous rock, completely dissolved out the bones, leaving nothing but cavities. Thus we have only the remains of remains to deal with, but casts taken from the cavities enable us to make out, with perfect certainty, even important characters, although there may be hardly a bone left.
We see then, that our third set of forms is the oldest, our first [177] the youngest, and the study of crocodilian remains seem to show that that has happened in the history of crocodiles, which should have happened, if the theory of evolution be true. Anatomical characters show that crocodiles are a modification of the lacertian type, and to this type the Triassic species, from which we are certainly justified in supposing that existing forms are descended, exhibit a marked approximation.
Still we are very far from knowing the whole story: it is certainly allowable to assume that our third group of crocodilian forms was evolved from a common stock with lizards, but this is as far as the facts of the case will take us at present.
There seems, at first sight, to be something unnatural in speaking of birds and reptiles together, for no two animals can be, to all appearance, more unlike. The wonderfully constructed feathers of the one group, compared with the scutes and scales of the other, the cold blood of the reptile contrasted with the hot fluid which circulates through the vessels of a bird and raises its body several degrees above our own in temperature; the dumbness and general sluggishness of the reptile as compared with the vocal powers and the rapid flight of birds; all these compel us to say, and justly so, that nothing can be more different than the character of the two classes.
Even when we go more into details, similar differences are apparent. The bird has a small head, set on a long flexible neck, and provided with a horny beak in lieu of teeth; its bones arc hollow and full of air; its breastbone, instead of being a small plate of cartilage, is a huge bony plate, usually provided with a large keel for the attachment of the powerful muscles of flight; the fore-limb is of no use in progression on the ground, and, the body having to be supported entirely by the hind limbs, the femora are placed parallel to the long axis of the body, instead of almost at right angles to it as in a reptile, so that the body is well raised from the ground, and a gait the very opposite of a reptile's sprawling waddle is the result.
The scapula and coracoid are not so very different from the corresponding bones in the lower class; the humerus, ulna, and radius, can also be perfectly well identified, but the modification of the distal division of the limb—the part answering to the reptile's fore-paw or to our own hand—is very great. First come two small bones answering to carpals, then three longer ones all united together, which represent the metacarpus, and are followed by the rudiments of the phalanges of the three corresponding digits. In the ostrich two of these three "fingers" are terminated by claws, the use of which it is rather hard to divine, unless the bird uses [178] them for scratching itself, an operation in which a very large portion of the activity of the lower animals is taken up.
The haunch-bone, or ilium, is of enormous size, and extends a long way in front of, as well as behind, the acetabulum; in correspondence with this, a great number of vertebræ are fused together to form a sacrum of sufficient size for the attachment of the ilia and the support of the weight of the body. The ischium and pubis are long slender bones, and the latter, as well as the former, is bent back, so that they both come to lie nearly parallel with the vertebral column.
To allow of the femur taking up its position parallel with the axis of the body, its well-finished globular head is set on at right-angles to the shaft; moreover, its further end has a characteristic notch for the reception of the upper extremity of the fibula. The shin-bone is provided with a large and very characteristic crest for the attachment of the strong muscles of the anterior part of the thigh, its lower extremity is pulley-shaped, and, in a young bird, the pulley-like end continued into a tongue of bone running up the back of the tibia, can be separated as a perfectly distinct ossification; its shaft also is so twisted that its two ends come to lie in different planes.
Following upon the tibia comes a bone with an easily separable piece at its upper end, and showing signs of a longitudinal division into three separate bones; this is the tarso-metatarsus, and represents the metatarsals and all the tarsals except one—the astragalus—which is represented by the pulley of the tibia. As a rule there are four toes, three of which are turned forwards and articulate with the tarsometatarsus, while the fourth, the representative of our hallux or great toe, is turned backwards and articulates with a small distinct bone.
The heart has four perfectly distinct chambers, so that the pure blood from the lungs, and the impure blood from the rest of the body, are kept quite separate. There is a single aorta which turns to the right side after leaving the heart.
V
[March 27, 1876]
We saw in the last lecture that the differences between birds and reptiles were very great; nevertheless, many of them tend to disappear on a closer examination. For instance, the extremely avian character of the absence of teeth, and the presence of a horny beak, is found in turtles and tortoises; that of the penetration of the bones by air cavities exists in the skull of crocodiles; and, although [179] no existing reptile possesses the power of flight, or a forelimb in any way approaching in structure to a bird's wing, yet, in the crocodiles, the fourth and fifth digits—those we found to be wholly absent in the bird—are much smaller than the others, and have no claws.
On passing to the internal organs, and the mode of development, we find far greater points of resemblance; as to the latter, in fact, the correspondence is wonderful, the account given of the development of a reptile. [Nature, vol. xiii, p. 429], applying in every respect to that of a bird.
On the whole it is certain, from anatomical characters alone, that birds are modifications of the same type as that on which reptiles are formed, and if this similarity of structure is the result of community of descent, we should expect to find, in the older formations, birds more like reptiles than any existing bird, and reptiles more like birds than any existing reptile. If the Geological record were sufficiently extensive, and the conditions of preservation favourable, we ought to find an exact series of links, but this, of course, is hardly to be expected, and it will be a great step if we can show that certain forms tend to bridge over the gulf between the two groups.
Let us see, then, what the facts of Palæontology tell us in this matter: and first, as to birds.
It is a curious fact that, just as in the case of Crocodiles, all the birds found in the Tertiary deposits differ in no essential respects from those of the present day. Great numbers of remains have been found in beds of Miocene age—beds found at the bottoms of great lakes—and the very perfectly preserved specimens show, beyond any doubt, that the Miocene birds are referable to precisely the same groups as those of our own time. Our knowledge of the Eocene forms is less perfect, but enough is known to show that the same fact held good at the commencement of the Tertiary epoch.
Throughout the secondary period remains of birds are very rare; until lately, in fact, there were none at all. But within the last ten or fifteen years some remarkable discoveries have been made—one or two in Europe, and a whole series in America, which give us some very precise information as to the nature of the Mesozoic birds.
Two of the most interesting of these—the genera Hesperornis and Ichthyornis—occur in certain beds in the United States, corresponding in age to our later Cretaceous. Hesperornis is stated, by its describer, to have had nearly the organisation of our Northern Diver (Columbus); it was five or six feet in length, of swimming habits, had small wings, like those of the Penguin or Auk, and a long beak like [180] the Diver. But—and this is the interesting feature in its organisation—both jaws were beset with teeth: not mere serrations of the jaw, such as many existing birds have, but true teeth like those of a reptile. Here then we have the appearance of a true reptilian character.
Ichthyornis was, in some respects, even more curious. It was about as large as a good-sized pigeon, had large wings adapted for powerful flight, and teeth in both jaws, like Hesperornis. In another character it showed a still greater approximation to the lower reptilian type: the bodies of its vertebræ, instead of having the cylindroidal or saddle-shaped form so characteristic of nearly all birds, were bi-concave. [In this peculiarly avian form of vertebra, the front face of the centrum is convex from above downwards, and concave from side to side, the hinder face being concave from above downwards, and convex from side to side. The Penguins have the dorsal vertebrae opisthocoelous, i.e., with a ball in front and a cup behind.] Thus, in tracing birds back in time, we find a parallel series of modifications to those described in the Crocodilia.
Beyond this point, the history of birds is almost a blank, the only other remains being—curiously enough—one or two feathers, and the Archæopteryx of the Solenhofen slates, a formation which has been of great service in the preservation of organic remains, the same qualities which make it so useful for purposes of lithography having fitted it for the preservation of even such perishable structures as jelly-fish.
Archæopteryx, known only by a single specimen now in the British Museum, was a bird about the size of a crow. Its head is unfortunately wanting; its tail is quite unlike that of any existing bird, being long, composed of a great number of vertebræ, and having two rows of feathers attached, one to each side of it. The leg is quite like that of any ordinary perching bird. Unluckily, the bones of the wing are detached, so that the exact structure of the manus is not known, but it is quite certain that the metacarpal bones were not united together, but were separate and terminated by distinct claws; there was thus an approximation in structure to a true forepaw. Long quills were attached to the wings, and both they and the tail-feathers are in an exquisite state of preservation.
With Archaeopteryx we come to the end of all precise information as to the history of birds, and the only possible trace of the group in earlier formations are certain footprints found in the Trias of Connecticut, and referred to the genus Brontozoum. These were prints of some gigantic three-toed animal, which certainly walked on its hind legs, and was always supposed to be an ostrich-like bird until some recent discoveries, presently to be mentioned, have shown that Brontozoum may have been a reptile.
[181] It would at first seem easy to show an equally striking approximation of reptiles to birds, for we have, throughout the greater part of the secondary rocks, and notably in the Solenhofen slates, remains of a group of reptiles known as Pterodactyles. These remarkable creatures had teeth set in distinct sockets, sometimes extending to the end of the long snout, sometimes stopping short, and having their place taken by a horny beak. The neck was long; the sacrum consisted of from three to six vertebræ; the tail was short in some, long in other genera. The breast-bone had a great keel, like that of a bird, the shoulder-girdle was also quite bird-like, as also were the humerus and the bones of the fore-arm. The manus, on the other hand, was quite different to anything found in birds; the first, second, and third digits were of the usual reptilian character and bore claws, but the fourth was immensely prolonged, produced downwards, and clawless. The pelvis was, in some respects, birdlike, in others quite peculiar: the hind-limb was reptilian.
It is certain that the Pterodactyles were animals of flight, and that there was a membrane, like a bat's wing, stretched between the fourth finger and the sides of the body; it is also certain that it was unable to walk, though it may have used its hind-limbs, as bats do, for hanging itself head downwards from branches.
Although these creatures are, in many respects, very bird-like, yet it can hardly be said that they give us any direct help, or that they connect reptiles and birds any more than bats connect birds and mammals. Their avian characters seem to have been purely adaptive, or produced in relation to their peculiar mode of life, and we must therefore try some other line of reptiles for the origin of birds.
In the rocks from the Trias to the later Cretaceous there are, in many places, abundant remains of a group of wholly extinct terrestrial reptiles known as Dinosauria. Most of these are of great size, the genus Iguanodon, for instance, must have attained a length of fully thirty feet. Our knowledge of most of them is imperfect, but many points of the greatest possible interest are perfectly well known.
Some genera have the snout turned downwards like a turtle's beak, and both it and the large lower jaw were unsheathed in horn. In some the vertebræ are slightly excavated on both faces, and are penetrated with air cavities. The shoulder-girdle consists of a long blade-bone and a short coracoid like that of many lizards. Of the fore-limb nothing is known for certain in the larger species. The sacrum is composed of as many as six vertebræ, which often take on [182] a remarkably birdlike character. More curious still, the ilium has a great forward process, and the ischium and pubis arc both turned backwards, parallel with one another, so as to have almost exactly the same position as in birds. There can be no doubt about this most remarkable point now, as the parts have been found in place in the genus Hypsilophodon. The femur was evidently brought parallel to the long axis of the body, and it has the characteristic ridge between the places of articulation of the tibia and fibula. The tibia has a great crest on its front surface the fibula is quite small, and the flattened end of the tibia fits on to a pulley-shaped bone exactly like the ankylosed astragalus of a bird. The middle or third toe is the largest, and the outer and inner toes small; the metatarsals, although separate from one another, have their faces so modelled that they must have been quite incapable of movement. Substitute ankylosis for ligamentous union, and a bird's metatarsus is produced; in fact the whole structure of the Dinosaurian hind-limb is exactly that of an embryonic bird.
In the very remarkable genus Compsognathus of the Solenhofen slates, which is nearly allied to the Dinosauria, and included, with them, in the order, Ornithoscelida, the head is small, the neck extremely long, and the peculiarities of the hind-limb are entirely birdlike; it also seems that the tibia and astragalus were actually united. The fore-limb, moreover, was very small, and it is certain that Compsognathus must have walked on its hind-legs.
The question, then, naturally arises, did the gigantic Dinosauria, such as Iguanodon and Megalosaurus, have the same mode of progression? This seems, at first sight, hard to believe, but there is considerable reason for thinking that it may have been the case, for, in the case mentioned above of the great three-toed footprints of the Connecticut valley and others found in the Wealden formation, no impression of a fore-foot has ever been found; so that, even if we suppose that the impressions of the fore-feet were entirely obliterated, as the animal walked, by those of the hind-feet, the former must, at any rate, have been very small.
When we consider what a very strong piece of evidence this is, we are forced to the conclusion that the evolution of birds from reptiles, by some such process as these facts indicate, is by no means such a wild speculation as it mighl from a priori considerations, have been supposed to be.
[184] VI
[April 3,1876]
In the highest group of Vertebrates, the Mammalia, the perfection of animal structure is attained. It will hardly be necessary, indeed it will be impossible, in the time at our disposal, to give the general characters of the group, but our purpose will be answered as well by devoting a short time to considering the peculiarities of a single well-known animal, the evidence as to the origin of which approaches precision.
The horse is one of the most specialised and peculiar of animals, its whole structure being so modified as to make it the most perfect living locomotive engine which it is possible to imagine. The chief points in which its structure is modified to bring about this specialisation, and in which, therefore. it differs most markedly from other mammals, we must now consider.
In the skull the orbit is completely closed behind by bone, a character found only in the most modified mammals. The teeth have a very peculiar character. There are, first of all, in the front part of each jaw, six long curved incisors or cutting teeth, which present a singular dark mark on their biting surfaces, caused by the filling, in of a deep groove on the crown of each tooth, by the substances on which the animal feeds. After the incisors, comes on both sides of each jaw a considerable toothless interval, or diastema, and then six large grinding teeth, or molars and premolars. In the young horse a small extra premolar is found to exist at the hinder end of the diastema, so that there are, in reality, seven grinders on each side above and below; furthermore, the male horse has a tusklike tooth, or canine, in the front part of the diastema immediately following the last incisor. Thus, the horse has, on each side of each jaw, three incisors, one canine, and seven grinders, making a total of forty-four teeth.
The grinding surfaces of the molars and premolars are very curious, In the upper jaw, each tooth is marked by four crescentic elevations, concave externally, the inner pair having each a curious folded mass connected with it. These projecting marks are formed of dentine and enamel, and, consequently, wear away more slowly than the intervening portions of the tooth, which are composed of cement. The lower grinders are marked with two crescents and two accessory masses, but the crescents are convex externally, and, consequently, when the opposite teeth bite together, the elevations do not correspond at any point. In this way a very perfect grinding surface [184] is obtained. The teeth are of great length, and go on growing for a long time, only forming roots in old animals. All these points contribute to the perfection of the horse as a machine, by rendering the mastication of the food, and its consequent preparation for digestion in the stomach, as rapid and complete a process as possible.
It is, however, in the limbs that the most striking deviation from the typical mammalian structure is seen, the most singular modifications having taken place to produce a set of long, jointed levers, combining great strength with the utmost possible spring and lightness.
The humerus is a comparatively short bone inclined backwards the radius is stout and strong, but the ulna seems to be reduced to its upper end—the olecranon or elbow; as a matter of fact, however, its distal end is left, fused to the radius, but the middle part has entirely disappeared: the carpus or wrist—the so-called "knee" of the horse—-is followed by a long "cannon-bone"; attached to the sides of which are two small "splint-bones": the three together evidently represent the metacarpus, and it can be readily shown that the great cannon-bone is the metacarpal of the third finger, the splint-bones those of the second and fourth. The splint-bones taper away at their lower ends and have no phalanges attached to them, but the cannon-bone is followed by the usual three phalanges, the last of which, the "coffin-bone," is ensheathed by the great nail or hoof.
The femur, like the humerus, is a short bone, but is directed forwards; the tibia turns backwards, and has the upper end of the rudimentary fibula attached to its outer angle. The latter bone, like the ulna, has disappeared altogether as to its middle portion, and its distal end is firmly united to the tibia. The foot has the same structure as the corresponding part in the fore-limb-a great cannon-bone, the third metatarsal; two splints, the second and fourth; and the three phalanges of the third digit, the last of which bears a hoof.
Thus, in both fore and hind limb one toe is selected, becomes greatly modified and enlarged at the expense of the others, and forms a great lever, which, in combination with the levers constituted by the upper and middle divisions of the limb, forms a sort of double C-spring arrangement, and thus gives to the horse its wonderful galloping power.
In the river-beds of the Quaternary age—a time when England formed part of the Continent of Europe—abundant remains of horses are found, which horses resembled altogether our own species, or perhaps are still more nearly allied to the wild ass. The same is the case in America, where the species was very abundant in the [185] Quaternary epoch-a curious fact, as, when first discovered by Europeans, there was not a horse from one end of the vast continent to the other.
In the Pliocene and older Miocene, both of Europe and America, are found a number of horse-like animals, resembling the existing horse in the pattern and number of the teeth, but differing in other particulars, especially the structure of the limbs. They belong to the genera Protohippus, Hipparion, &c., and are the immediate predecessors of the Quaternary horses.
In these animals the bones of the fore-arm are essentially like those of the horse, but the ulna is stouter and larger, can be traced from one end to the other, and, although firmly united to the radius, was not ankylosed with it. The same is true, though to a less marked extent, of the fibula.
But the most curious change is to be found in the toes. The third toe, though still by far the largest, is proportionally smaller than in the horse, and each of the splint bones bears its own proper number of phalanges; a pair of "dew-claws," like those of the reindeer, being thus formed, one on either side of the great central toe. These accessory toes, however, by no means reached the ground, and could have been of no possible use, except in progression through marshes.
The teeth are quite like those of the existing horse, as to pattern, number, presence of cement, &c.; the orbit also is complete, but there is a curious depression on the face-bones, just beneath the orbit, a rudiment of which is, however, found in some of the older horses.
On passing to the older Miocene, we find an animal, known as Anchitherium, which bears, in many respects, a close resemblance to Hipparion, but is shorter-legged, stouter-bodied, and altogether more awkward in appearance. Its skull exhibits the depression mentioned as existing in Hipparion, but the orbit is incomplete behind, thus deviating from the specialised structure found in the horse, and approaching nearer to an ordinary typical mammal. The same is the case with the teeth, which are short and formed roots at an early period; their pattern also is simplified, although all the essential features are still retained. The valleys between the various ridges are not filled up with cement, and the little anterior premolar of the horse has become as large as the other grinders, so that the whole forty-four teeth of the typical mammalian dentition are well developed. The diastema is still present between the canines and the anterior grinding teeth—a curious fact in relation to the theory [186] that the corresponding space in the horse was specially constructed for the insertion of the bit; for, if the Miocene men were in the habit of riding the Anchitherium, they were probably able to hold on so well with their hind legs as to be in no need of a bit.
The fibula is a complete bone, though still ankylosed below to the tibia; the ulna also is far stouter and more distinct than in Hipparion. In both fore and hind foot the middle toe is smaller, in relation to the size of the animal, than in either the horse or the Hipparion, and the second and fourth toes, though still smaller than the third, are so large that they must have reached the ground in walking. Thus, it is only necessary for the second and fourth toes, and the ulna and fibula to get smaller and smaller for the limb of Anchitherium to be converted into that of Hipparion, and this again into that of the horse.
Up to the year 1870 this was all the evidence we had about the matter except for the fact that a species of Palæotherium from the older Eocene was, in many respects, so horse-like, having, however, well-developed ulna and fibula, and the second and fourth toes larger even than in Anchitherium, that it had every appearance of being the original stock of the horse. But within the last six years some remarkable discoveries in central and western North America have brought to light forms which are, probably, nearer the direct line of descent than any we have hitherto known.
In the Eocene rocks of these localities, a horse-like animal has been found, with three toes, like those of Anchitherium, but having, in addition, a little style of bone on the outer side of the fore foot, evidently representing the fifth digit. This is the little Orohippus, the lowest member of the Equine series.
This evidence is conclusive as far as the fact of evolution is concerned, for it is preposterous to assume that each member of this perfect series of forms has been specially created; and if it can be proved—as the facts adduced above certainly do prove—that a complicated animal like the horse may have arisen by gradual modification of a lower and less specialised form, there is surely no reason to think that other animals have arisen in a different way.
This case, moreover, is not isolated. Every new investigation into the Tertiary mammalian fauna brings fresh evidence, tending to show how the rhinoceros, the pigs, the ruminants, have come about. Similar light is being thrown on the origin of the carnivora, and also, in a less degree, on that of all the other groups of mammals.
It may well be asked why such clear evidence should be obtainable as to the origin of mammals, while in the case of many other groups—fish, for instance—all the evidence seems to point the other way. This [187] question cannot be satisfactorily answered at present, but the fact is probably connected with the great uniformity of conditions to which the lower animals are exposed, for it is invariably the case that the higher the position of any given animal in the scale of being, the more complex are the conditions acting on it.
It is not, however, to be expected that there should be, as yet, an answer to every difficulty, for we are only just beginning the study of biological facts from the evolutionary point of view. Still, when we look back twenty years to the publication of the 'Origin of Species,' we are filled with astonishment at the progress of our knowledge, and especially at the immense strides it has made in the region of palæontological research. The accurate information obtained in this department of science has put the fact of evolution beyond a doubt; formerly, the great reproach to the theory was, that no support was lent to it by the geological history of living things; now, whatever happens, the fact remains that the hypothesis is founded on the firm basis of paIæontological evidence.
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