[457] THERE IS evidence, the value of which has not been disputed, and which, in my judgment, amounts to proof, that, between the commencement of the Tertiary epoch and the present time, the group of the Equidæ has been represented by a series of forms, of which the oldest is that which departs least from the general type of structure of the higher Mammalia, while the latest is that which most widely differs from that type.

In fact, the earliest known equine animal possesses four complete subequal digits on the fore foot, three on the hind foot; the ulna is complete and distinct from the radius; the fibula is complete and distinct from the tibia ; there are 44 teeth, the full number of canines being present, and the cheek-teeth having short crowns with simple patterns and early-formed roots. The latest, on the other hand, has only one complete digit on each foot, the others being represented by larger or smaller rudiments; the ulna is reduced and ankylosed with the radius; the fibula is still more reduced and partially ankylosed with the tibia; the canine teeth are partially or completely suppressed in the females; the first cheek-teeth usually remain undeveloped, and when they appear are very small; the other cheek-teeth have long crowns with highly complicated patterns and late-formed roots. The Equidæ of intermediate age exhibit intermediate characters.

With respect to the interpretation of these facts, two hypotheses, and only two, appear to be imaginable. The one assumes that these [458] successive forms of equine animals have come into existence independently of one another. The other assumes that they are the result of the gradual modification undergone by the successive members of a continuous line of ancestry.

As I am not aware that any zoologist maintains the first hypothesis, I do not feel called upon to discuss it. The adoption of the second, however, is equivalent to the acceptance of the doctrine of evolution, so far as Horses are concerned; and, in the absence of evidence to the contrary, I shall suppose that it is accepted.

Thus, since the commencement of the Eocene epoch, the animals which constitute the family of the Equidæ have undergone processes of modification of three kinds

1. There has been an excess of development of some parts in relation to others.

2. Certain parts have undergone complete or partial suppression.

3. Certain parts, which were originally distinct, have coalesced.

Employing the term "law" simply in the sense of a general statement of facts ascertained by observation, I shall speak of these three processes by which the Eohippus form has passed into Equus as the expression of a threefold law of evolution.

It is of profound interest to remark that this law, or generalized statement of the nature of the ancestral evolution of the Horses, is precisely the same as that which formulates the process of individual development in animals generally, from the period at which the broad characters of the group to which an animal belongs are discernible, onwards. After a mammalian embryo, for example, has taken on its general mammalian characters, its further progress towards its specific form is effected by the excessive growth of one part in relation to another, by the arrest of growth or the suppression of parts already formed, and by the coalescence of parts primarily distinct.

This coincidence of the laws of ancestral and individual development creates a strong confidence in the general validity of the former and a belief that we may safely employ it in reasoning deductively from the known to the unknown. The astronomer who has determined three places of a new planet, calculates its place at any epoch however remote; and if the law of evolution is to be depended upon, the zoologist who knows a certain length of the course of that evolution in any given case, may with equal justice reason backwards to the earlier but unknown stages.

Applying this method to the case of the Horse, I do not see that [459] there is any reason to doubt that the Eocene Equidæ were preceded by Mesozoic forms which differed from Eohippus in the same way as Eohippus differs from Equus. And thus we are necessitated to conceive of a first term of the Equine series, which, if the law is of general validity, must needs have been provided with five subequal digits on each plantigrade foot, with complete, subequal antebrachial and crural bones, with clavicles, and with, at fewest, 44 teeth, the cheek-teeth having short crowns and simple-ridged or tuberculated patterns. Moreover, since Lartet's and Marsh's investigations have shown that the older forms of any given mammalian group have developed cerebral hemispheres than the later, there is a prima facie probability that this primordial Hippoid had a low form of brain. Further, since the existing Horse has a diffuse allantoic placentation, the primary form could not have presented a higher, and may have possessed a lower, condition of the various modes by which the foetus derives nourishment from the parent among vertebrated animals.

Such an animal as this, however, would find no place in any of our systems of classification of the Mammalia. It would come nearest to the Lemuroidea and the Insectivora, though the non-prehensile pes would separate it from the former, and the placentation from the latter group.

A natural classification is one which associates together all those forms which are closely allied and separates them from the rest. But, whether in the ordinary sense of the word "alliance," or in its purely morphological sense, it is impossible to imagine groups of animals more closely allied than the primordial Hippoids are with their descendants. Yet, according to existing arrangements, the ancestors would have to be placed in one order of the class Mammalia and their descendants in another.

It may be suggested that it might be as well to wait until the primordial Hippoid is discovered before discussing the difficulties which will be created by its appearance. But the truth is, that the problem is already pressing in another shape. Numerous "Lemurs," with marked ungulate characters, are being discovered in the older Tertiaries of the United States and elsewhere; and no one can study the more ancient mammals with which we are already acquainted, without being constantly struck with the "Insectivorous" characters which they present. In fact, there is nothing in the dentition of either Primates, Carnivores, or Ungulates which is not foreshadowed in the Insectivora; and I am not aware that there is any means of deciding whether a given fossil skeleton, with skull, teeth, and limbs [460] almost complete, ought to be ranged with the Lemurs, the Insectivores, the Carnivores, or the Ungulates.

In whatever order of Mammals a sufficiently long series of forms has come to light, they illustrate the threefold law of evolution as clearly, though perhaps not so strikingly, as the Equinæ series does. Carnivores, Artiodactyles, and Perissodactyles all tend, as we trace them back through the Tertiary epoch, towards less modified forms which will fit into none. of the recognized orders, but come closer to the Insectivora than to any other. It would, however, be most inconvenient and misleading to term these primordial forms "Insectivora," the mammals so called being themselves more or less specialized modifications of the same common type; and only, in a partial and limited sense, representatives of that type.

The root of the matter appears to me to be that the palæontological facts which have come to light in the course of the last ten or fifteen years have completely broken down existing taxonomical conceptions, and that attempts to construct fresh classifications upon the old model are necessarily futile.

The Cuvierian method, which most modern classifiers up to the time of the appearance of Haeckel's "Generelle Morphologie" have followed, has been of immense value in leading to the close investigation and the clear statement of the anatomical characters of animals. But its principle, the construction of sharp logical categories defined by such characters, was sapped when Von Baer showed that, in estimating the likenesses and unlikenesses of animals, development must be fully taken into account; and if the importance of individual development is admitted, that of ancestral development necessarily follows.

If the end of all zoological classification is the clear and concise expression of the morphological resemblances and differences of animals, then all such resemblances must have a taxonomic value. But they fall under three heads:–(1) those of adult individuals; (2) those of successive stages of embryological development or individual evolution; (3) those of successive stages of the evolution of the species, or ancestral evolution.

An arrangement is "natural" (that is, logically justifiable in view of the purpose of classification defined above) exactly in so far as it expresses the relations of likeness and unlikeness enumerated under these heads. Hence, in attempting to classify the Mammalia, we must take into account not only their adult and embryogenetic characters, but their morphological relations, in so far as the several groups represent different stages of evolution. And thus, just as [461] the persistent antagonism of Cuvier and his school to the essence of Lamarck's teachings (imperfect and objectionable as these often were in their accidents) turns out to have been a reactionary mistake, so Cuvier's no less definite repudiation of Bonnet's "échelle des étres" must be regarded as another unfortunate effort to oppose the development of just biological conceptions. For though no one will pretend to defend Bonnet's "échelle" at the present day, the existence of a "scala animantium" is a necessary consequence of the doctrine of evolution; and its establishment constitutes, I believe, the foundation of scientific taxonomy.

If all the Mammalia are the results of a process of evolution analogous to that which has taken place in the case of the Equidæ, and if they exhibit different degrees of that process, then a natural classification will arrange them, in the first instance, according to the place which they occupy in the scale of evolution of the mammalian type, or the particular rung of the "scala mammalium" on which they stand. The determination of the position thus occupied by any group may, I think, be effected by the deductive application of the laws of evolution. That is to say, those groups which approach the non-mammalian Vertebrata most closely, present least inequality of development, least suppression and least coalescence of the fundamental parts of the type, must belong to earlier stages of evolution; while those which exhibit the contrary characters must appertain to later stages.

Judged from this point of view, there can be no doubt that the Monotremes embody that type of structure which constitutes the earliest stage of mammalian organization :

1. The mammary glands are devoid of teats; and thus the essential feature of the mammal could hardly be presented under a simpler form.

2. There is a complete and deep cloaca, as in Vertebrata lower in the scale.

3. The openings of the ureters are hypocystic; that is to say, they open not into the bladder of these animals, but behind it, into the dorsal wall of the genito-urinary passage. As this answers to the neck of the allantois, the ureters of the Monotremes retain their primitive embryonic position.

4. There is no vagina apart from the genito-urinary passage; and the oviducts are not differentiated into distinct uterine and Fallopian regions.

5. The penis and the clitoris are attached to the ventral wall of the cloaca.

[462] 6. The epiphyses of the vertebræ are but slightly, or not at all, developed.¹

7. The malleus is relatively very large; and the "processus gracilis," which is singularly long and strong, passes between the tympanic and the periotic bones to the pterygoid, with which it is firmly united. Thus the palato-pterygoid apparatus is directly connected by a "suspensorium" with the periotic, as in the Amphibia and Sauropsida. As in these, the representative of the incus is extremely small, and that of the stapes columelliform.

8. The coracoid is complete, distinct, and articulates with the sternum.

9. The hip-girdle is provided with large epipubes; and the iliac axis is inclined at a large angle to the sacral axis.

10. The corpus callosum is very small.

11. There appears to be no allantoic placenta, though, from the obvious remains of the ductus arteriosus and of the hypogastric arteries, there can be little doubt that the foetus has a large respiratory allantois. It is quite possible that, with a large umbilical sac, there may be an imperfect "umbilical" placentation.

But, while the Ornithorhynchus and the Echidna are thus the representatives of the lowest stage of the evolution of the Mammalia, I conceive it to be equally unquestionable that, as Haeckel has already suggested, they are greatly modified forms of that stage Echidna, on the whole, representing a greater, and Ornithorhynchus a less, departure from the general type. The absence of true teeth in both genera is an obvious sign of extreme modification. The long tongue, extraordinary external auditory passages, and relatively large convoluted brain of Echidna, and the cheek-pouches and horny mouthplates of Ornithorhynchus, are other indications of the same kind.

Hence the primary mammals, which were less modified, and the existence of which is necessarily postulated in the conception of the evolution of the group, cannot, without risk of confusion, be called Monotremata or Ornithodelphia, since in all probability they were as widely different from Ornithorhynchus and Echidna as the Insectivora are from the Edentata, or the Ungulata from Rhytina. Hence it will be convenient to have a distinct name, Prototheria, [463] for the group which includes these, at present, hypothetical embodiments of that lowest stage of the mammalian type, of which the existing Monotremes are the only known representatives.

A similar reasoning applies to the Marsupialia. In their essential and fundamental characters they occupy an intermediate position between the Prototheria and the higher mammals.

1. The mammary glands are provided with teats.

2. The cloaca is so greatly reduced that it is often said to have disappeared.

3. The openings of the ureters are entocystic; that is to say, the ureters open into what is called the "base" of the bladder, in front .of the narrowed "neck" by which it passes into the tubular "urethra." This means, I conceive, that, morphologically, the bladder of the Marsupial represents the bladder of the Monotreme + the anterior part of the genito-urinary passage–the so-called "trigonum," if not more, of the bladder of the Marsupial, being the homologue of that anterior segment of the genito-urinary passage of the Monotreme.

4. There is a distinct and long vagina, quite separated from the cystic urethra, in the female; and the oviducts are differentiated into uterine and Fallopian portions.

5. The penis is larger, and the corpora cavernosa are connected by fibrous tissue and muscles with the pelvis. The spongy body has a large bifurcated bulb; and Cowper's glands are very largely developed.

6. The vertebræ have distinct epiphyses.

7. The malleus is small; and its connections are similar to those which it possesses in the higher mammals. The incus is relatively larger, and the stapes more or less stirrup-shaped.

8. The coracoid is short, does not articulate with the sternum, and becomes ankylosed with the scapula.

9. The hip-girdle is provided with epipubes, usually of large size and well ossified; and the iliac axis is inclined at a small angle to the sacral axis.

10. The corpus callosum is small.

11. In the few forms of which the foetus is known there is no allantoic placenta ; while the umbilical sac is so large that the possibility of the existence of a transitory umbilical placentation must be taken into account.

It will be observed that in the characters 1, 2, 3, 4, 5, 6, 7, 8, and the latter part of the 9th, the Marsupials agree with the higher mammals; while in the former part of the 9th, the 10th, and the [464] 11th they present Prototherian characters. So far, therefore, they constitute an intermediate type between that of the Prototheria and that of the higher mammals, which may be termed that of the Metatheria. And if there were any known animals which combined these characters, with a complete double dentition, unmodified pentadactyle manus and pes, and normal uterogestation, they would furnish us with the exact transition between the Prototheria and the higher mammals, which must have existed if the law of evolution is trustworthy.

No known Marsupial, however, possesses these additional characters. None has more than a single successional tooth on each side of each jaw., and, as Prof. Flower (to whom we owe the highly important demonstration of this fact) has pointed out, the question arises whether we have here a primary dentition with only one secondary tooth, or a secondary dentition with only one tooth of the primary set left. I have no doubt that the answer given to this question by Prof. Flower is correct, and that it is the milk-dentition of which only a vestige is left in the Marsupialia. Among existing Rodents, in fact, all conditions of the milk-dentition exist, from a number equal to that of the permanent incisors and premolars (as in the Rabbit²) to none at all.

The same thing is observed in the Insectivora, where the Hedgehog, and probably Centeles, have a full set of milk-teeth, while none have yet been found in the Shrews. In these cases it is obvious that the milk-dentition has gradually been suppressed in the more modified forms; and 1 think that there can be no reasonable doubt that the existing Marsupials have undergone a like suppression of the deciduous teeth, in the course of their derivation from ancestors which possessed a full set.

Again, no existing Marsupial possesses an unmodified pentadactyle pes. If the hallux is present, it presents an extensive movement in adduction and abduction; in fact, the pes is prehensile. This is the case in the Phascolomyidæ, Phalangistidæ, Phascolarctidæ, and Didelphidæ. The Dasyuridæ present the same type of pes, with the hallux reduced or suppressed. Hence, considering the relations of the Macropodidæ and the Peramelidæ with the Phalangers, it seems [465] likely that the hind foot in these groups is also a reduced prehensile pes; in which case this special modification of the foot would characterize the whole of the existing Marsupialia.

Thirdly, the most marked peculiarities of the reproductive organs and processes in the Marsupial are in no wise transitional, but are singularly specialized characters. The suspension of the scrotum in front of the root of the penis is unlike any arrangement in the higher mammals; and the development of the bulb and of Cowper's glands is in excess of anything observable in them. In the female, the cystic urethra is as completely separated from the vagina as it is in the higher mammals; while the doubling of the vagina must, in my opinion, also be considered a special peculiarity which leads from, rather than towards, the higher mammals. In a Monotreme, in fact, the anterior end of the genito-urinary passage exhibits two very short dilatations or cornua, one on each side. In the middle line, a little distance behind these, the ureters open on a prominent ridge-like papilla. The opening of the bladder lies in front of and below the genital cornua. Now, if we compare this arrangement with that which obtains in the lower forms of the higher Mammalia, we find that the ureteric papillæ have separated laterally and moved forwards, in such a manner as to occupy the base of the bladder, and the genital cornua come to lie behind and somewhat dorsad of them. At the same time a longitudinal separation has taken place between what may be called the "ureteric" region of the genito-urinary passage and the "genital" region. The first is taken into the bladder and becomes connected by a longer or shorter cystic "urethra" with the latter, which is converted into the longer or shorter vagina. In the Marsupial the same general modification has taken place ; but the "genital cornua" become immensely elongated, and give rise to the so-called "double" vagina.

Lastly, the marsupium, where it exists, is a no less special feature,of the Marsupialia, and, like the peculiarities of the female genital organs, appears to be related with the abnormally early birth of the foetus. Among the higher Mammalia, it is well known that the foetus is born in a relatively much earlier state in some cases than in others, even among closely allied species. Thus Rabbits are born hairless and blind, while Hares are born hairy and with their eyes open. I think it probable, from the character of the pes, that the primitive forms whence the existing Marsupialia have been derived, were arboreal animals; and it is not difficult, I conceive, to see that, with such habits, it may have been highly advantageous to an animal to get rid of its young from the interior of its body at as early a period of [466] development as possible, and to supply it with nourishment during the later periods through the lacteal glands, rather than through an imperfect form of placenta.

However this may be, the characters of the existing Marsupialia leave no doubt on my mind that they are greatly modified members of the Metatherial type; and I suspect that most, if not all, of the Australian forms are of comparatively late origin. I think it probable that the great majority of the Metatheria, of which I doubt not a great multitude will shortly be discovered in Mesozoic formations, differed widely from our existing Marsupials–not only lacking the pouch, as do some existing "Marsupialia," but possessing undivided vaginæ, and probably bringing forth their young not earlier than existing Carnivores and Rodents do, the nutrition of the foetus during prolonged gestation being provided for, in all probability, by an umbilical placental apparatus, and its respiration by a non-placental allantois.

In the remaining group of the Mammalia, hitherto spoken of as the "higher Mammalia,"

1 The mammary glands are provided with teats.³

2. The cloaca has usually disappeared. Sometimes, however (Beavers, Sloths), a shallow cloaca is present, especially in the female.

3. The openings of the ureters are always entocystic; but their position varies greatly, from close to the neck (e.g. Sorex) to the anterior end of the bladder (e.g. Hyrax).

4. There is a distinct vagina, which is almost always undivided. The oviducts are differentiated into uterine and Fallopian portions.

5. The penis is usually large, the bulb single or partially divided, and the corpora cavernosa almost always directly attached to the ischia.

6. The vertebræ have epiphyses.

7. The malleus is usually small, the incus relatively large, the stapes stirrup-shaped.

8. The coracoid is almost always much reduced, and it is ankylosed with the scapula.

9. The iliac axis makes a small angle with the sacral axis; and there is no epipubis, or only a fibrous vestige of it.

10. The corpus callosum and the anterior commissure vary widely. In such forms as Erinaceus and Dasypus they are almost Monotreme-like.

[467] 11. The foetus is connected with the uterus of the mother by an allantoic placenta. The umbilical sac varies in size; and in some lower forms (e.g. Lepus) it is, at first, highly vascular, and perhaps plays a quasi-placental part during the early stages of development.

It is obvious that, in all these respects, we have the mammalian type in a higher stage of evolution than that presented by the Prototheria and the Metatheria. Hence we may term forms which have reached this stage the Eutheria.

It is a fact, curiously in accordance with what might be expected on evolutionary principles, that while the existing members of the Prototheria-and the Metatheria are all extremely modified, there are certain forms of living Eutheria which depart but little from the general type. For example, if Gymnura possessed a diffuse placentation, it would be an excellent representative of an undifferentiated Eutherian. Many years ago, in my lectures at the Royal College of Surgeons, I particularly insisted on the central position of the Insectivora among the higher Mammalia; and further study of this order and of the Rodentia has only strengthened my conviction, that any one who is acquainted with the range of variation of structure in these groups possesses the key to every peculiarity which is met with in the Primates, the Carnivora, and the Ungulata. Given the common plan of the Insectivora and of the Rodentia, and granting that the modifications of the structure of the limbs, of the brain, and of the alimentary and reproductive viscera which occur among them may exist and accumulate elsewhere, and the derivation of all the Eutheria from animals which, except for their simpler placentation, would be Insectivores, is a simple deduction from the law of evolution.

There is no known Monotreme which is not vastly more different from the Prototherian type, and no Marsupial which has not far more widely departed from the Metatherian type, than Gymnura, or, indeed, Erinaceus, have from the Eutherian type.

The broadest physiological distinction between the Prototheria, the Metatheria, and the Eutheria, respectively, lies in the differences which the arrangements for prolonging the period of intra-uterine and extra-uterine nutrition by the parent present in each. The possibility of a higher differentiation of the species is apparently closely connected with the length of this period. Similarly, the broadest morphological distinction which can be drawn among the Eutheria lies in their placentation. All forms of deciduate placentation commence by being non-deciduate; and the intimate connection [468] of the foetal with the maternal structures is subsequent to their loose union. Hence Eutheria, with deciduate placentæ, are in a higher stage of evolution than those with non-deciduate placentæ.

In discussing the relations of the various existing groups of the higher Mammalia with one another, it would be a mistake to attempt to trace any direct genetic connection between them. Each, as the case of the Equidæ suggests, has probably had a peculiar line of ancestry; and, in these lines, Eutherian forms with deciduate placentation constitute the latest term, Eutherian forms with non-deciduate placentation the next latest, Metatherian forms the next, Prototherian forms the earliest among those animals which, according to existing definition, would be regarded as Mammals.

The accompanying Table (p. 469) presents, at a glance, the arrangement of the Mammalia in accordance with the views which I have endeavoured to express. The sign 0 marks the places on the scheme which are occupied by known Mammals; while x indicates the groups of which nothing is known, but the former existence of which is deducible from the law of evolution.

I venture to express a confident expectation that investigation into the Mammalian fauna of the Mesozoic epoch will sooner or later fill up these blanks. But if deduction from the law of evolution is to be justified thus far, it may be trusted much further. If we may confidently expect that Eohippus had a pentadactyle claviculate ancestor, then we may expect, with no less confidence, that the Prototheria proceeded from ancestors which were not mammals–in so far as they had no mammary glands, and in so far as the mandible was articulated with a quadrate bone, of which the malleus of the true mammal is the reduced representative. Probably also the corpus callosum had not appeared as a distinct structure.

Our existing classifications have no place for this "sub-mammalian" stage of evolution (already indicated by Haeckel under the name of Promammale). It would be separated from the Sauropsida by its two condyles, and by the retention of the left as the principal aortic arch; while it would probably be no less differentiated from the Amphibia by the presence of an amnion and the absence of branchiæ at any period of life. I propose to term the representatives of this stage Hypotheria ; and I do not doubt that when we have a fuller knowledge of the terrestrial Vertebrata of the later Palæozoic epochs, forms belonging to this stage will be found among them Now, if we take away from the Hypotheria the amnion and the corpus callosum, and add the functional branchiæ–the existence of which in the ancestors of the Mammalia is as clearly indicated by

[469]

[following chart modification of that in text]

[470] their visceral arches and clefts, as the existence of complete clavicles in the ancestral Canidæ is indicated by their vestiges in the dog–the Hypotheria, thus reduced, at once take their place among the Amphibia. For the presence of branchiæ implies that of an incompletely divided ventricle and of numerous aortic arches, such as exist in the mammalian embryo, but are more or less completely suppressed in the course of its development.

Thus I regard the Amphibian type as the representative of the next lower stage of vertebrate evolution; and it is extremely interesting to observe that even the existing Amphibia present us with almost every degree of modification of the type, from such forms as the oviparous, branchiate, small-lunged Siredon and Menobranchus, which stand in the same relation to it as Gymnura to the Eutheria, to the exclusively air-breathing Salamanders and Frogs, in which the period of intraovular development, either within the uterus itself or in special receptacles, may be as much prolonged as it is in the Mammalia.

A careful study, on full materials, of the development of the young of such forms as Hyloides will probably throw great light on the nature of the changes which ended in the suppression of the branchiæ, and the development of the amnion and of the extra-abdominal part of the allantois in the foetus of the higher Vertebrata.

The recent researches of Boas⁴ on the structure of the heart and the origin of the pulmonary arteries of Ceratodus fell into my hands when 1 happened to be working afresh at the subject, and had arrived, so far as the heart is concerned, at results which are entirely confirmatory of his. This wonderful creature seems contrived for the illustration of the doctrine of Evolution. Equally good arguments might be adduced for the assertion that it is an amphibian or a fish, or both, or neither–the reason of this being that, as it appears to me, Ceratodus is an extraordinarily little-modified representative of that particular stage of vertebrate evolution of which both the typical Fishes and the typical Amphibia are special modifications. I think it will be convenient to have a name for the representatives of this stage; and I propose that of Herpetichthyes.

If we were to take away from Ceratodus the membrane-bones of the head and the pneumatocoele, and slightly simplify the structure of the heart, the result would be an animal which would undoubtedly be classed among the Chimeroidei; and if, in such a Chimæroid, the [471] lamellar septa of the branchiæ were not reduced, as they are in the Chimæroids, while the opercular fold remained undeveloped, the product would be a little-modified representative of the Selachian group, to which, among actually known forms, Heptanchus and Cestracion present the nearest approximations. Vertebrated animals in this stage of evolution may be termed Chondrichthyes.

Suppose the limbs and the genital ducts of the Chondrichthyan stage to be undeveloped, and let the two nasal sacs be represented by a partially divided sac with a single external aperture, the result will be a still lower grade of vertebrate life, which may be termed Myzichthyes, represented only by the greatly modified Lampreys and Hags of the existing fauna.

Finally, let the head retain its primitive segmentation, and the heart its primitive character of a contractile tube, and we reach, in the Hypichthyes, a stage of simplification of the vertebrate type, from which it would be difficult to remove any essential feature without reaching a point at which it is questionable whether an animal should be called "vertebrate." This stage is at present represented only by a singularly modified form, the living Amphioxus.

Thus, in the order of Evolution–all the Vertebrata hitherto considered may be arranged in nine stages :-- 1, that of the Hypichthyres; 2, that of the Myzichthys; 3, that of the Chondrichthys; 4, that of the Herpetichthyes; 5, that of the Amphibia; 6, that of the Hypotheria; 7, that of the Prototheria; 8, that of the Metatheria; and 9, that of the Eutheria. All these stages, except that of the Hypotheria, are represented by existing groups of vertebrated animals, which in most cases are composed of greatly modified forms .of the type to which they belong, only the Amphibia and the Eutheria exhibiting near approximations to the unmodified type in some of their existing members.

It will be observed that I have omitted to mention the Ganoid and the Teleostean Fishes and the Sauropsida. I have done so because they appear to me to lie off the main line of evolution–to represent, as it were, side tracks starting from certain points of that line, The Ganoidei and the Teleostei I conceive to stand in this relation to the stage of the Herpetichthyes, and the Sauropsida to the stage of the Amphibia.

There is nothing, so far as 1 can see, in the organization of the Ganoid and Teleostean fishes which is not readily explicable by the application of the law of Evolution to the Herpetichthyes. They may [472] be interpreted as effects of the excessive development, reduction, or coalescence of the parts of a Herpetichthyan.⁵

Similarly, the suppression of the branchiæ, the development of an amnion, and of a respiratory extra-abdominal allantois, and that enlargement of the basioccipital relatively to the exoccipitals which gives rise to a single skull-condyle, is all the change required to convert a Urodele Amphibian into a Lizard. It is needless to recapitulate the evidence of the transition from the Reptilian to the Bird type which the study of extinct animal remains has brought to light.

The scheme of arrangement of the Vertebrata which naturally flows from the considerations now brought forward will stand thus:–

It appears to me that everything which is at present known respecting the Vertebrata of past epochs agrees with the assumption that the law which expresses the process of ancestral evolution of the higher Mammalia is of general application to all the Vertebrata. If this is admitted, I think it necessarily follows that the Vertebrata must have passed successively through the stages here indicated, and that the progress of discovery, while it will obliterate the lines of demarcation between these stages, and convert them into a continuous series of small differentiations, will yield no vertebrate form for which a place does not exist in the general scheme.

¹ Dr. Albrecht ("Die Epiphysen und die Amphiomphalie der Säugethier-Wirbelkörper," Zoologischer Anzeiger, 1879, No. A), while admitting that Echidna has no epiphyses, describes epiphyses of an incomplete character between the posterior twelve caudal vertebræ of Ornithorhynchus. So far as I am aware, the memoir, of which Dr. Albrecht has given a preliminary notice, has not yet been published; I content myself therefore with remarking that iny own recent observations are in harmony with Dr. Albrecht's statement.

PREVIEW TABLE of CONTENTS BIBLIOGRAPHIES 1.   THH Publications 2.   Victorian Commentary 3.   20th Century Commentary INDICES 1.   Letter Index 2.   Illustration Index TIMELINE FAMILY TREE Gratitude and Permissions C. Blinderman & D. Joyce Clark University 1998

THE HUXLEY FILE

GUIDES § 1. THH: His Mark § 2. Voyage of the Rattlesnake § 3. A Sort of Firm § 4. Darwin's Bulldog § 5. Hidden Bond: Evolution § 6. Frankensteinosaurus § 7. Bobbing Angels: Human Evolution § 8. Matter of Life: Protoplasm § 9. Medusa § 10. Liberal Education § 11. Scientific Education § 12. Unity in Diversity § 13. Agnosticism § 14. New Reformation § 15. Verbal Delusions: The Bible § 16. Miltonic Hypothesis: Genesis § 17. Extremely Wonderful Events: Resurrection and Demons § 18. Emancipation: Gender and Race § 19. Aryans et al.: Ethnology § 20. The Good of Mankind § 21.  Jungle Versus Garden