The Genealogy Of Animals

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Title:     The Genealogy Of Animals
Author: Thomas Henry Huxley [More Titles by Huxley]

THE GENEALOGY OF ANIMALS.[1]

Considering that Germany now takes the lead of the world in scientific investigation, and particularly in biology, Mr. Darwin must be well pleased at the rapid spread of his views among some of the ablest and most laborious of German naturalists.

[Footnote 1: "The Natural History of Creation." By Dr. Ernst Haeckel (_Natuerliche Schoepfungs-Geschichte._--Von Dr. Ernst Haeckel, Professor an der Universitaet Jena.) Berlin, 1868.]

Among those, Professor Haeckel, of Jena, is the Coryphaeus. I know of no more solid and important contributions to biology in the past seven years than Haeckel's work on the _Radiolaria_, and the researches of his distinguished colleague Gegenbaur, in vertebrate anatomy; while in Haeckel's _Generelle Morphologie_ there is all the force, suggestiveness, and, what I may term the systematizing power, of Oken, without his extravagance. The _Generelle Morphologie_ is, in fact, an attempt to put the doctrine of Evolution, so far as it applies to the living world, into a logical form; and to work out its practical applications to their final results. The work before us, again, may be said to be an exposition of the _Generelle Morphologie_ for an educated public, consisting, as it does, of the substance of a series of lectures delivered before a mixed audience at Jena, in the session 1867-8.

"The Natural History of Creation,"--or, as Professor Haeckel admits it would have been better to call his work, "The History of the Development or Evolution of Nature,"--deals, in the first six lectures, with the general and historical aspects of the question, and contains a very interesting and lucid account of the views of Linnaeus, Cuvier, Agassiz, Goethe, Oken, Kant, Lamarck, Lyell, and Darwin, and of the historical filiation of these philosophers.

The next six lectures are occupied by a well-digested statement of Mr. Darwin's views. The thirteenth lecture discusses two topics which are not touched by Mr. Darwin, namely, the origin of the present form of the solar system, and that of living matter. Full justice is done to Kant, as the originator of that "cosmic gas theory," as the Germans somewhat quaintly call it, which is commonly ascribed to Laplace. With respect to spontaneous generation, while admitting that there is no experimental evidence in its favour, Professor Haeckel denies the possibility of disproving it, and points out that the assumption that it has occurred is a necessary part of the doctrine of Evolution. The fourteenth lecture, on "Schoepfungs-Perioden und Schoepfungs-Urkunden," answers pretty much to the famous disquisition on the "Imperfection of the Geological Record" in the _Origin of Species_.

The following five lectures contain the most original matter of any, being devoted to "Phylogeny," or the working out of the details of the process of Evolution in the animal and vegetable kingdoms, so as to prove the line of descent of each group of living beings, and to furnish it with its proper genealogical tree, or "phylum."

The last lecture considers objections and sums up the evidence in favour of biological Evolution.

I shall best testify to my sense of the value of the work thus briefly analysed if I now proceed to note down some of the more important criticisms which have been suggested to me by its perusal.

I. In more than one place, Professor Haeckel enlarges upon the service which the _Origin of Species_ has done, in favouring what he terms the "causal or mechanical" view of living nature as opposed to the "teleological or vitalistic" view. And no doubt it is quite true that the doctrine of Evolution is the most formidable opponent of all the commoner and coarser forms of Teleology. But perhaps the most remarkable service to the philosophy of Biology rendered by Mr. Darwin is the reconciliation of Teleology and Morphology, and the explanation of the facts of both which his views offer.

The Teleology which supposes that the eye, such as we see it in man or one of the higher _Vertebrata_, was made with the precise structure which it exhibits, for the purpose of enabling the animal which possesses it to see, has undoubtedly received its death-blow. Nevertheless it is necessary to remember that there is a wider Teleology, which is not touched by the doctrine of Evolution, but is actually based upon the fundamental proposition of Evolution. That proposition is, that the whole world, living and not living, is the result of the mutual interaction, according to definite laws, of the forces possessed by the molecules of which the primitive nebulosity of the universe was composed. If this be true, it is no less certain that the existing world lay, potentially, in the cosmic vapour; and that a sufficient intelligence could, from a knowledge of the properties of the molecules of that vapour, have predicted, say the state of the Fauna of Britain in 1869, with as much certainty as one can say what will happen to the vapour of the breath in a cold winter's day.

Consider a kitchen clock, which ticks loudly, shows the hours, minutes, and seconds, strikes, cries "cuckoo!" and perhaps shows the phases of the moon. When the clock is wound up, all the phenomena which it exhibits are potentially contained in its mechanism, and a clever clockmaker could predict all it will do after an examination of its structure.

If the evolution theory is correct, the molecular structure of the cosmic gas stands in the same relation to the phenomena of the world as the structure of the clock to its phenomena.

Now let us suppose a death-watch, living in the clock-case, to be a learned and intelligent student of its works. He might say, "I find here nothing but matter and force and pure mechanism from beginning to end," and he would be quite right. But if he drew the conclusion that the clock was not contrived for a purpose, he would be quite wrong. On the other hand, imagine another death-watch of a different turn of mind. He, listening to the monotonous "tick! tick!" so exactly like his own, might arrive at the conclusion that the clock was itself a monstrous sort of death-watch, and that its final cause and purpose was to tick. How easy to point to the clear relation of the whole mechanism to the pendulum, to the fact that the one thing the clock did always and without intermission was to tick, and that all the rest of its phenomena were intermittent and subordinate to ticking! For all this, it is certain that kitchen clocks are not contrived for the purpose of making a ticking noise.

Thus the teleological theorist would be as wrong as the mechanical theorist, among our death-watches; and, probably, the only death-watch who would be right would be the one who should maintain that the sole thing death-watches could be sure about was the nature of the clock-works and the way they move; and that the purpose of the clock lay wholly beyond the purview of beetle faculties.

Substitute "cosmic vapour" for "clock," and "molecules" for "works," and the application of the argument is obvious. The teleological and the mechanical views of nature are not, necessarily, mutually exclusive. On the contrary, the more purely a mechanist the speculator is, the more firmly does he assume a primordial molecular arrangement, of which all the phenomena of the universe are the consequences; and the more completely is he thereby at the mercy of the teleologist, who can always defy him to disprove that this primordial molecular arrangement was not intended to evolve the phenomena of the universe. On the other hand, if the teleologist assert that this, that, or the other result of the working of any part of the mechanism of the universe is its purpose and final cause, the mechanist can always inquire how he knows that it is more than an unessential incident--the mere ticking of the clock, which he mistakes for its function. And there seems to be no reply to this inquiry, any more than to the further, not irrational, question, why trouble oneself about matters which are out of reach, when the working of the mechanism itself, which is of infinite practical importance, affords scope for all our energies?

Professor Haeckel has invented a new and convenient name, "Dysteleology," for the study of the "purposelessnesses" which are observable in living organisms--such as the multitudinous cases of rudimentary and apparently useless structures. I confess, however, that it has often appeared to me that the facts of Dysteleology cut two ways. If we are to assume, as evolutionists in general do, that useless organs atrophy, such cases as the existence of lateral rudiments of toes, in the foot of a horse, place us in a dilemma. For, either these rudiments are of no use to the animal, in which case, considering that the horse has existed in its present form since the Pliocene epoch, they surely ought to have disappeared; or they are of some use to the animal, in which case they are of no use as arguments against Teleology. A similar, but still stronger, argument may be based upon the existence of teats, and even functional mammary glands, in male mammals. Numerous cases of "Gynaecomasty," or functionally active breasts in men, are on record, though there is no mammalian species whatever in which the male normally suckles the young. Thus, there can be little doubt that the mammary gland was as apparently useless in the remotest male mammalian ancestor of man as in living men, and yet it has not disappeared. Is it then still profitable to the male organism to retain it? Possibly; but in that case its dysteleological value is gone.

II. Professor Haeckel looks upon the causes which have led to the present diversity of living nature as twofold. Living matter, he tells us, is urged by two impulses: a centripetal, which tends to preserve and transmit the specific form, and which he identifies with heredity; and a centrifugal, which results from the tendency of external conditions to modify the organism and effect its adaptation to themselves. The internal impulse is conservative, and tends to the preservation of specific, or individual, form; the external impulse is metamorphic, and tends to the modification of specific, or individual, form.

In developing his views upon this subject, Professor Haeckel introduces qualifications which disarm some of the criticisms I should have been disposed to offer; but I think that his method of stating the case has the inconvenience of tending to leave out of sight the important fact--which is a cardinal point in the Darwinian hypothesis--that the tendency to vary, in a given organism, may have nothing to do with the external conditions to which that individual organism is exposed, but may depend wholly upon internal conditions. No one, I imagine, would dream of seeking in the direct influence of the external conditions of his life for the cause of the development of the sixth finger and toe in the famous Maltese.

I conceive that both hereditary transmission and adaptation need to be analysed into their constituent conditions by the further application of the doctrine of the Struggle for Existence. It is a probable hypothesis, that what the world is to organisms in general, each organism is to the molecules of which it is composed. Multitudes of these, having diverse tendencies, are competing with one another for opportunity to exist and multiply; and the organism, as a whole, is as much the product of the molecules which are victorious as the Fauna, or Flora, of a country is the product of the victorious organic beings in it.

On this hypothesis, hereditary transmission is the result of the victory of particular molecules contained in the impregnated germ. Adaptation to conditions is the result of the favouring of the multiplication of those molecules whose organizing tendencies are most in harmony with such conditions. In this view of the matter, conditions are not actively productive, but are passively permissive; they do not cause variation in any given direction, but they permit and favour a tendency in that direction which already exists.

It is true that, in the long run, the origin of the organic molecules themselves, and of their tendencies, is to be sought in the external world; but if we carry our inquiries as far back as this, the distinction between internal and external impulses vanishes. On the other hand, if we confine ourselves to the consideration of a single organism, I think it must be admitted that the existence of an internal metamorphic tendency must be as distinctly recognized as that of an internal conservative tendency; and that the influence of conditions is mainly, if not wholly, the result of the extent to which they favour the one, or the other, of these tendencies.

III. There is only one point upon which I fundamentally and entirely disagree with Professor Haeckel, but that is the very important one of his conception of geological time, and of the meaning of the stratified rocks as records and indications of that time. Conceiving that the stratified rocks of an epoch indicate a period of depression, and that the intervals between the epochs correspond with periods of elevation of which we have no record, he intercalates between the different epochs, or periods, intervals which he terms "Ante-periods." Thus, instead of considering the Triassic, Jurassic, Cretaceous, and Eocene periods, as continuously successive, he interposes a period before each, as an "Antetrias-zeit," "Antejura-zeit," "Antecreta-zeit," "Antecocen-zeit," &c. And he conceives that the abrupt changes between the Faunae of the different formations are due to the lapse of time, of which we have no organic record, during their "Ante-periods."

The frequent occurrence of strata containing assemblages of organic forms which are intermediate between those of adjacent formations, is, to my mind, fatal to this view. In the well-known St. Cassian beds, for example, Palaeozoic and Mesozoic forms are commingled, and, between the Cretaceous and the Eocene formations, there are similar transitional beds. On the other hand, in the middle of the Silurian series, extensive unconformity of the strata indicates the lapse of vast intervals of time between the deposit of successive beds, without any corresponding change in the Fauna.

Professor Haeckel will, I fear, think me unreasonable, if I say that he seems to be still overshadowed by geological superstitions; and that he will have to believe in the completeness of the geological record far less than he does at present. He assumes, for example, that there was no dry land, nor any terrestrial life, before the end of the Silurian epoch, simply because, up to the present time, no indications of fresh water, or terrestrial organisms, have been found in rocks of older date. And, in speculating upon the origin of a given group, he rarely goes further back than the "Ante-period," which precedes that in which the remains of animals belonging to that group are found. Thus, as fossil remains of the majority of the groups of _Reptilia_ are first found in the Trias, they are assumed to have originated in the "Antetriassic" period, or between the Permian and Triassic epochs.

I confess this is wholly incredible to me. The Permian and the Triassic deposits pass completely into one another; there is no sort of discontinuity answering to an unrecorded "Antetrias;" and, what is more, we have evidence of immensely extensive dry land during the formation of these deposits. We know that the dry land of the Trias absolutely teemed with reptiles of all groups except Pterodactyles, Snakes, and perhaps Tortoises; there is every probability that true Birds existed, and _Mammalia_ certainly did. Of the inhabitants of the Permian dry land, on the contrary, all that have left a record are a few lizards. Is it conceivable that these last should really represent the whole terrestrial population of that time, and that the development of Mammals, of Birds, and of the highest forms of Reptiles, should have been crowded into the time during which the Permian conditions quietly passed away, and the Triassic conditions began? Does not any such supposition become in the highest degree improbable, when, in the terrestrial or fresh-water Labyrinthodonts, which lived on the land of the Carboniferous epoch, as well as on that of the Trias, we have evidence that one form, of terrestrial life persisted, throughout all these ages, with no important modification? For my part, having regard to the small amount of modification (except in the way of extinction) which the Crocodilian, Lacertilian, and Chelonian _Reptilia_ have undergone, from the older Mesozoic times to the present day, I cannot but put the existence of the common stock from which they sprang far back in the Palaeozoic epoch; and I should apply a similar argumentation to all other groups of animals.

IV. Professor Haeckel proposes a number of modifications in Taxonomy, all of which are well worthy of consideration. Thus he establishes a third primary division of the living world, distinct from both animals and plants, under the name of the _Protista_, to include the _Myxomycetes_, the _Diatomaceae_, and the _Labyrinthulae_, which are commonly regarded as plants, with the _Noctilucae_, the _Flagellata_, the _Rhizopoda_, the _Protoplasta_, and the _Monera_, which are most generally included within the animal world. A like attempt has been made, by other writers, to escape the inconvenience of calling these dubious organisms by the name of plant or animal; but I confess, it appears to me, that the inconvenience which is eluded in one direction, by this step, is met in two others. Professor Haeckel himself doubts whether the _Fungi_ ought not to be removed into his _Protista_. If they are not, indeed, the _Myxomycetes_ render the drawing of every line of demarcation between _Protista_ and Plants impossible. But if they are, who is to define the _Fungi_ from the _Algae_? Yet the sea-weeds are surely, in every respect, plants. On the other hand, Professor Haeckel puts the sponges among the _Coelenterata_ (or polypes and corals), with the double inconvenience, as it appears to me, of separating the sponges from their immediate kindred, the _Protoplasta_, and destroying the definition of the _Coelenterata_. So again, the _Infusoria_ possess all the characters of animality, but it can hardly be said that they are as clearly allied to the worms as they are to the _Noctilucae_.

On the whole, it appears to me to be most convenient to adhere to the old plan of calling such of these low forms as are more animal in habit, _Protozoa_, and such as are more vegetal, _Protophyta_.

Another considerable innovation is the proposition to divide the class Pisces into the four groups of _Leptocardia, Cyclostomata, Pisces_, and _Dipneusta_. As regards the establishment of a separate class for the Lancelet _(Amphioxus)_, I think there can be little doubt of the propriety of so doing, inasmuch as it is far more different from all other fishes than they are from one another. And there is much to be said in favour of the same promotion of the _Cyclostomata_, or Lampreys and Hags. But considering the close relation of the Mudfish with the _Ganoidei_, and the wide differences between the _Elasmobranchii_ and the _Teleostei_, I greatly doubt the propriety of separating the _Dipneusta_, as a class, from the other _Pisces_.

Professor Haeckel proposes to break up the vertebrate sub-kingdom, first, into the two provinces of _Leptocardia_ and _Pachycardia; Amphioxus_ being in the former, and all other vertebrates in the latter division. The _Pachycardia_ are then divided into _Monorhina_, which contains the Cyclostome fishes, distinguished by their single nasal aperture; and _Amphirhina_, comprising the other _Vertebrata_, which have two nasal apertures. These are further subdivided into _Anamnia (Pisces, Dipneusta, Amphibia)_ and _Amniota (Reptilia, Aves, Mammalia)_. This classification undoubtedly expresses many of the most important facts in vertebrate structure in a clear and compendious way; whether it is the best that can he adopted remains to be seen.

With much reason the Lemurs are removed altogether from the _Primates_, under the name of _Prosimiae_. But I am surprised to find the _Sirenia_ left in one group with the _Cetacea_, and the _Plesiosauria_ with the _Ichthyosauria_; the ordinal distinctness of these having, to my mind, been long since fully established.

V. In Professor Haeckel's speculations on Phylogeny, or the genealogy of animal forms, there is much that is profoundly interesting, and his suggestions are always supported by sound knowledge and great ingenuity. Whether one agrees or disagrees with him, one feels that he has forced the mind into lines of thought in which it is more profitable to go wrong than to stand still.

To put his views into a few words, he conceives that all forms of life originally commenced as _Monera_, or simple particles of protoplasm; and that these _Monera_ originated from not-living matter. Some of the _Monera_ acquired tendencies towards the Protistic, others towards the Vegetal, and others towards the Animal modes of life. The last became animal _Monera_. Some of the animal _Monera_ acquired a nucleus, and became amoeba-like creatures; and, out of certain of these, ciliated infusorium-like animals were developed. These became modified into two stirpes: A, that of the worms; and B, that of the sponges. The latter by progressive modification gave rise to all the _Coelenterata_; the former to all other animals. But A soon broke up into two principal stirpes, of which one, _a_, became the root of the _Annelida, Echinodermata_, and _Arthropoda_, while the other, _b_, gave rise to the _Polyzoa_ and _Ascidioida_, and produced the two remaining stirpes of the _Vertebrata_ and the _Mollusca_.

Perhaps the most startling proposition of all those which Professor Haeckel puts before us is that which he bases upon Kowalewsky's researches into the development of _Amphioxus_ and of the _Ascidioida_, that the origin of the _Vertebrata_ is to be sought in an Ascidioid form. Goodsir long ago insisted upon the resemblance between _Amphioxus_ and the Ascidians; but the notion of a genetic connection between the two, and especially the identification of the notochord of the _Vertebrate_ with the axis of the caudal appendage of the larva of the Ascidian, is a novelty which, at first, takes one's breath away. I must confess, however, that the more I have pondered over it, the more grounds appear in its favour, though I am not convinced that there is any real parallelism between the mode of development of the ganglion of the _Ascidian_ and that of the _Vertebrate_ cerebro-spinal axis.

The hardly less startling hypothesis that the _Echinoderms_ are coalesced worms, on the other hand, appears to be open to serious objection. As a matter of anatomy, it does not seem to me to correspond with fact; for there is no worm with a calcareous skeleton, nor any which has a band-like ventral nerve, superficial to which lies an ambulacral vessel. And, as a question of development, the formation of the radiate _Echinoderm_ within its vermiform larva seems to me to be analogous to the formation of a radiate Medusa upon a Hydrozoic stock. But a Medusa is surely not the result of the coalescence of as many organisms as it presents morphological segments.

Professor Haeckel adduces the fossil _Crossopodia_ and _Phyllodocites_ as examples of the Annelidan forms, by the coalescence of which the Echinoderms may have been produced; but, even supposing the resemblance of these worms to detached starfish arms to be perfect, it is possible that they may be the extreme term, and not the commencement, of Echinoderm development. A pentacrinoid Echinoderm, with a complete jointed stalk, is developed within the larva of _Antedon_. Is it not possible that the larva of _Crossopodia_ may have developed a vermiform Echinoderm?

With respect to the Phylogeny of the _Arthropoda_, I find myself disposed to take a somewhat different view from that of Professor Haeckel. He assumes that the primary stock of the whole group was a crustacean, having that _Nauplius_ form in which Fritz Mueller has shown that so many _Crustacea_ commence their lives. All the _Entomostraca_ arose by the modification of some one or other of these Naupliform "_Archicarida_." Other _Archicarida_ underwent a further metamorphosis into a _Zoaea_-form. From some of these "_Zoeopoda_" arose all the remaining Malacostracous _Crustacea_; while, from others, was developed some form analogous to the existing _Galeodes_, out of which proceeded, by gradual differentiation, all the _Myriapoda, Arachnida,_ and _Insecta_.

I should, be disposed to interpret the facts of the embryological history and of the anatomy of the _Arthropoda_ in a different manner. The _Copepoda_, the _Ostracoda_, and the _Branchiopoda_ are the _Crustacea_ which have departed least from the embryonic or _Nauplius_-forms; and, of these, I imagine that the _Copepoda_ represent the hypothetical _Archicarida_ most closely. _Apus_ and _Sapphirina_ indicate the relations of these Archaeocarids with the _Trilobita_, and the _Eurypterida_ connect the _Trilobita_ and the _Copepoda_ with the _Xiphosura_. But the _Xiphosura_ have such close morphological relations with the _Arachnida_, and especially with the oldest known Arachnidan, _Scorpio_, that I cannot doubt the existence of a genetic connection between the two groups. On the other hand, the _Branchiopoda_ do, even at the present day, almost pass into the true _Podophthalmia_, by _Nebalia_. By the _Trilobita_, again, the _Archicarida_ are connected with such _Edriophthalmia_ as _Serolis_. The _Stomapoda_ are extremely modified _Edriophthalmia_ of the amphipod type. On the other side, the _Isopoda_ lead to the _Myriapoda_, and the latter to the _Insecta_. Thus the Arthropod phylum, which suggests itself to me, is that the branches of the _Podophthalmia_, of the _Insecta_ (with the _Myriapoda_), and of the _Arachnida_, spring separately and distinctly from the Archaeocarid root--and that the _Zoaea_-forms occur only at the origin of the Podophthalmous branch.

The phylum of the _Vertebrata_ is the most interesting of all, and is admirably discussed by Professor Haeckel. I can note only a few points which seem to me to be open to discussion. The _Monorhina_, having been developed out of the _Leptocardia_, gave rise, according to Professor Haeckel, to a shark-like form, which was the common stock of all the _Amphirhina_. From this "Protamphirhine" were developed, in divergent lines, the true Sharks, Rays, and _Chimaerae_; the Ganoids, and the _Dipneusta_. The _Teleostei_ are modified _Ganoidei_. The _Dipneusta_ gave rise to the _Amphibia_, which are the root of all other _Vertebrata_, inasmuch as out of them were developed the first _Vertebrata_ provided with an amnion, or the _Protamniota_. The _Protamniota_ split up into two stems, one that of the _Mammalia_, the other common to _Reptilia_ and _Aves_.

The only modification which it occurs to me to suggest in this general view of the Phylogeny of the _Vertebrata_ is, that the "Protamphirhine" was possibly more ganoid than shark-like. So far as our present information goes the Ganoids are as old as the Sharks; and it is very interesting to observe that the remains of the oldest Ganoids, _Cephalaspis_ and _Pteraspis_, have as yet displayed no trace of jaws. It is just possible that they may connect the _Monorhina_, with the Sturgeons among the _Amphirhina_. On the other hand, the Crossopterygian Ganoids exhibit the closest connection with _Lepidosiren_, and thereby with the _Amphibia_. It should not be forgotten that the development of the Lampreys exhibits curious points of resemblance with that of the _Amphibia_, which are absent in the Sharks and Rays. Of the development of the _Ganoidei_ we have unfortunately no knowledge, but their brains and their reproductive organs are more amphibian than are those of the Sharks.

On the whole, I am disposed to think that the direct stem of ascent from the _Monorhina_ to the _Amphibia_ is formed by the Ganoids and the Mudfishes; while the Osseous fishes and the Sharks are branches in different directions from this stem.

What the _Protamniota_ were like, I do not suppose any one is in a position to say, but I cannot think that the thoroughly Lacertian _Protorosaurus_ had anything to do with them. The reptiles which are most amphibian in their characters, and therefore, probably, most nearly approach the _Protamniota_, are the _Ichthyosauria_ and the _Chelonia_.

That the _Didelphia_ were developed out of some ornithodelphous form, as Professor Haeckel supposes, seems to be unquestionable; but the existing Opossums and Kangaroos are certainly extremely modified and remote from their ancestors the "_Prodidelphia_," of which we have not, at present the slightest knowledge. The mode of origin of the _Monodelphia_ from these is a very difficult problem, for the most part left open by Professor Haeckel. He considers the _Prosimiae_, or Lemurs, to be the common stock of the _Deciduata_, and the _Cetacea_ (with which he includes the _Sirenia_) to be modified _Ungulata_. As regards the latter question, I have little doubt that the _Sirenia_ connect the _Ungulata_ with the _Proboscidea_; and none, that the _Cetacea_ are extremely modified _Carnivora_. The passage between the Seals and the _Cetacea_ by _Zeuglodon_ is complete. I also think that there is much to be said for the opinion, that the _Insectivora_ represent the common stock of the _Primates_ (which passed into them by the _Prosimiae_), the _Cheiroptera_, the _Rodentia_, and the _Carnivora_. And I am greatly disposed to look for the common root of all the _Ungulata_, as well, in some ancient non-deciduate Mammals which were more like _Insectivora_ than anything else. On the other hand, the _Edentata_ appear to form a series by themselves.

The latter part of this notice of the _Natuerliche Schoepfungs-Geschichte_, brings so strongly into prominence the points of difference between its able author and myself, that I do not like to conclude without reminding the reader of my entire concurrence with the general tenor and spirit of the work, and of my high estimate of its value.

[The end]
Thomas Henry Huxley's essay: The Genealogy Of Animals

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