Vertebrate Zoology G. R. De Beer (1928)

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PART V

Chapter XLIII Conclusions

Not the least of the interests aroused by the study of Vertebrates is due to the fact that they form a group which lends itself perhaps better than any to a consideration of general principles and matters of wide importance. This is largely because, although imperfect, present knowledge covers a considerable amount of the results of vertebrate evolution, and still more because between the most widely separated members of the group, between Amphioxus and man, there is sufficient similarity in plan of structure to enable comparisons to be made with advantage. Comparative Anatomy as an intellectual weapon is the more satisfactory when the number of correspondences of kind which can be established is great, regardless of course of matters of detail. So it is not astonishing that the Anatomy of, for example, Nematodes and Echinoderms when compared should be less fertile in conclusions of general interest than a comparison between Vertebrates as distant from one another as are fish and mammals. From the fact of the general homogeneity of the group as a whole, the variations to be observed in different vertebrates become all the more interesting.

It is very striking to find organs such as notochord, nerve-tube, dorsal and ventral nerve-roots, essentially the same in Amphioxus and man, but the most striking case of homologous organs is that of the thyroid. From the endostyle of Amphioxus, through Petromyzon with its tell-tale Ammocoete larva, to all the Craniates, the chain is complete, and not the least remarkable feature of it is the great change in function which has taken place from an organ connected with the ciliary method of feeding to a ductless gland regulating the metabolism of the body. This case is a good illustration of the fact that function is no criterion whatever in questions of homology, and that the sole condition which organs must fulfil to be homologous is to be descended from one and the same representative in a common ancestor.

A fact which the vertebrates illustrate well, is that the numerical correspondence of segments which give rise to particular structures is not a necessary criterion for homology. This is well shown by a consideration of the pectoral and pelvic limbs. The fore limb is formed from trunk-segments 2, 3, 4, and 5 in the newt (Salamandra), whereas in the lizard it arises from segments 6, 7, 8, and 9. Similarly the hind limb arises from segments 16, 17, and 18 in the newt, but segments 26 to 31 in the lizard. Countless similar examples are afforded by other vertebrates, and it is to be noticed that the limbs not only vary in their position, but also in the number of segments which have contributed to their formation. Yet wherever they may be and however many or few segments they may contain, fore limbs are homologous throughout the vertebrates, and so are hind limbs. During evolution transposition has occurred ; new adjacent segments have taken to contributing to the formation of the limb, and at the opposite end segments which hitherto contributed may cease to do so. In this way the limbs may become transposed over the trunk of the animal much as a tune can be transposed over the keys. But it is the same tune and the same limb.

Another case is that of the position of the occipital arch at the back of the skull. The neurocranium of Scyllium occupies 7 segments while that of a form as closely related to it as Squalus occupies 9. Although they are situated in different segments, there can be no doubt that the occipital arches of these two animals are descended from the occipital arch of a common ancestor, and are therefore homologous.

A very interesting example of the same kind is furnished by the number of gill-slits in various Selachii. Heptanchus has 7, Hexanchus and Pliotrema have 6, and the remaining Selachi have 5 gill-slits and branchial arches on each side. Now the remarkable thing is that the last branchial arch has a typical structure whether it be the 7th, 6th, or 5th. Its peculiarity consists in the fact that its pharyngobranchial element is attached to that of the preceding arch, and it receives a portion of the trapezius muscle. The function of the last branchial arch is to anchor the branchial basket on to the shoulder- girdle. This being so, in the course of the evolution of forms with 5 branchial arches from forms in which there were 6,* it is impossible to imagine that the transformation took place gradually by reduction from behind ; for if this had occurred, there would have been stages in which the " old last branchial " arch had partly disappeared and the " new last branchial " arch had partially been modified to replace the former, and it is difficult to see how such an arrangement could have fulfilled the function of providing attachment between the branchial basket and the shoulder-girdle. This case is therefore different from that of the limbs, for in the latter there is nothing to prevent gradual transposition of the limb by means of partial modification of adjacent segments. Still less can it be imagined that the number of branchial arches has been altered by reduction from in front because the first two visceral arches, the mandibular and hyoid, are constant throughout the Gnathostomes. The only explanation left is that there has been a sudden change in evolution, and that a formwith,say,6 branchial arches gave rise to offspring with 5, without any intermediate stage of functional inefficiency. This conclusion is, of course, interesting from the point of view of evolution, but it is also not without importance as regards the relation of metameric segmentation to differentiation during development. The only difference between this hypothetical offspring with 5, and its parent with 6 branchial arches, is that the raw material for the production of the branchial arches has in the one case been divided up between 5 segments and in the other between 6 segments, during development. It is this raw material which is homologous in the two forms, regardless of the numerical position of the segment in which it is situated. A matter like this is worth some attention, for it is an example of how principles of general and wide interest can be derived from comparative anatomical studies.

Or vice versa.

In sharp contrast to homologous structures are the resemblances between different and unrelated groups of animals as regards characters which can be proved to have been separately and independently evolved. These resemblances are analogies, and they give rise to the phenomenon of convergence in evolution which is well illustrated by vertebrates. The instances of convergence which might be given are so numerous that only very few need be mentioned here. A good example is the modification of the pentadactyl limb into a paddle, thereby losing its typical appearance and presenting a superficial resemblance to the fins of fish. But the interesting thing is that this process has occurred not once but several times, independently, in different groups of Tetrapods : Chelonia, Ichthyosaurs, Plesiosaurs, Mosasaurs, Thalattosaurs, Thalattosuchia, Penguins, Cetacea (whales), Carnivora pinnipedia (seals), and Sirenia. The Ichthyosaurs and some of the Cetacea are further interesting in that they have developed median dorsal fins which are superficially very similar to those of fish. The Urodela also have median fins ; but in all these cases, a little study suffices to show that these structures not only differ very much from the fins of fish, but also that they differ between themselves.

Convergence is also to be found in the case of the elongated and limbless condition of Gymnophiona, certain lizards such as Anguis, Amphisbaena, Scincus, and the snakes. Or again, the fore limb has been modified into a wing independently in Pterosaurs, birds, and bats. The marsupial " mole " Notoryctes is very similar to the true placental moles (Talpa).

Now it is noteworthy that these cases of convergence are each of them related to a particular mode of life. So the paddle- like modification of the limbs is an adaptation to life in the water, just as wings are adapted to life in the air ; the limbless condition is a form of adaptation to a burrowing habit, while another form of this habit characterises the " moles." It is because of their adaptations to their environment that these animals come to resemble one another, and these adaptations have of course no value in determining affinities or descent.

Another phenomenon may now be considered, which is in some ways intermediate between homology and convergence. It is often the case that in two groups of related animals which have recently diverged the same evolutionary changes take place. This may be called parallelism, and it is illustrated in certain groups of Ungulates such as the Titanotheres and the rhinoceroses. In several distinct stocks of Titanotheres peculiar bony knobs appear on the skull. These structures were not visibly present in the common ancestor of the forms which have evolved them ; the structures cannot therefore strictly be called homologous, yet they are so similar that it is impossible to avoid the impression that they have some common cause. The independent development of such similar structures in related groups of animals is often ascribed to a so-called process of " Orthogenesis,' ' or variation along " straight " and constant lines. The working of this process in two or more related groups is supposed to result in parallel evolution.

Now it is worthy of note that when tracing lines of descent through fossil forms, it is rarely possible to identify one form as the direct ancestor of another. Instead, it is more usual to find that one fossil form is related to the ancestor of another, because it possesses characters which that ancestor must have possessed, while at the same time showing other characters which proclaim that it had diverged from that ancestor. The characters of the ancestor in question are, of course, to a certain extent deducible from those of the form descended from it.

The incompleteness of knowledge of the fossil record makes it difficult to find " fathers," but it supplies a number of " uncles." The question now is this : why do the " fathers " and " uncles " resemble one another ? Cynognathus itself is not the ancestor of the mammals, for in several respects it is too specialised, but it must have evolved parallel with the ancestor of the mammals or it would not possess so many similarities. In the same way it can be shown that the later Stegocephalians, which were not on the line of descent of the reptiles, nevertheless show a number of changes in evolution which took place parallel to those which were going on in their " cousins " the reptiles.

The answer must be that the " fathers " and the " uncles " inherited something from the " grandfather " which determines the course of their evolution. This something need not, however, have been visible in the " grandfather," so that the " fathers " and the " uncles " in which the something does become visible appear to have evolved it independently. In these cases there appears what may be called a latent homology between the structures in question, and which accounts for the so-called " Orthogenesis." In any case, it is most important to avoid the impression that " Orthogenesis " implies a purposeful or directive force, or that evolution takes place in straight lines. Such impression is quickly dispelled by a consideration of the record of success and failure of the different groups of animals during evolution. If a directive force were responsible for evolution, it would seem to be peculiarly malicious, for most groups of animals have been " directed " to their doom by extinction.

An insight into what " Orthogenesis " really means is given by a study of the relative sizes of parts of animals to the whole animals, at different absolute sizes. It is found, for example, that the size of the antlers in Red deer is relatively larger in large animals than it is in small ones. That is to say, that the larger a Red deer grows, the relatively larger do its antlers become, on the average. These cases are susceptible of mathematical treatment, and it is found that the antlers not only grow faster than the body, but they grow faster at a constant rate, for the ratio of the growth-rates of antlers and body remains constant. Organs to which this principle applies are called heterogonic, and Heterogony is of wide occurrence in the horns and bony nobs of various groups of Ungulate mammals. Now just as the heterogonic organ is relatively smaller in small animals, it is found as a rule that in two species of one genus both of which possess this organ, the larger species will have the relatively larger heterogonic organ. So the antlers of the little Muntjack are relatively smaller than those of the larger Red deer. There are of course exceptions and complications, but from the present point of view, the main thing to notice is that for an organ which shows heterogonic growth to appear at all, the animal's body must have reached a certain absolute size. Now as the different races of Titanotheres evolved, their size increased, in common with nearly all the groups of mammals. Independently, each of these races of Titanotheres developed bony knobs on the skull, and as the size-increase of the animals continued, the bony knobs became relatively larger still. The bony knobs are heterogonic organs, and their independent appearance in different races is not due to any directive force, but automatically to the increase in size of the body of the animal. This increase of body-size was probably due to random variation selected by natural selection in the direction of greater size because it is (up to a point) advantageous, and has survival value. From the common ancestor of the different races it is only necessary to assume that the capacity was inherited to produce bony knobs if and when a certain body-size is reached. On this view, therefore, " Orthogenesis " does not mean directed evolution, but merely directional. It also enables an explanation to be given for the cases of extinction of animals in which the size of the heterogonic organ (consequent on the large size of the body) had become so great as to reduce the animal's chances of survival. This applies to the Irish elk, which was a very large deer with relatively immense antlers.

Attention must now be paid to the terms " primitive " and " specialised," which were defined early in this book, and which have been consistently used throughout. In the first place, it is necessary to notice that their meaning is relative, so that it is possible to find an animal which is primitive when compared with one and specialised when compared with another animal. A specialised animal is one which is committed to a particular line and so has a restricted potency of evolution. As a rule, specialised animals are adapted to a particular mode of life, and this adaptation has entailed either the development or loss of certain structures which render the animals unfit to live in any other environment but their own. Once committed, they are committed for always, for in its broad lines evolution is irreversible.

Primitive animals, on the other hand, are not committed to any particularly restricted mode of life ; they do not have any delicate adaptations with the structural modifications which they involve, and they are, in a word, generalised.

It is from generalised ancestors that the main groups of animals have evolved, and as these groups radiated out they became specialised in their various ways. Specialisation and evolutionary capacity are roughly inversely proportional.

The significance of primitiveness and specialisation is thus related to evolution. Amphioxus is primitive because it possesses many characters which the early ancestral Chordates must have had. But its specialised characters show that it was not itself that ancestor. Amphioxus is with regard to the higher Chordates not a " father " but an " uncle." It is worth noticing that the primitive arrangement of several structures was segmental, and that as evolution proceeded this simple scheme was departed from. So the gonads of Amphioxus, myotomes of Amphioxus, kidney tubules of Myxine, ribs of Cotylosaurs and respiratory centres of Raia show that " a pair of each in each segment " was the primitive outfit, on which evolution has worked.

When man is considered in relation to his ancestors, a significant fact emerges. Man is not adapted to any restricted mode of life at all ; instead he is fitted for almost all sorts of habits and circumstances ; he is generalised not specialised, and that is one of the secrets of his evolutionary success. His ancestors must have been among the most primitive and generalised of the mammals ; they did not live on the capital of their evolutionary capacities and spend it in exchange for delicate adaptations, which, while perhaps allowing of " easier living," would have resulted in side-tracking the race into a rut or backwater of life.

Lastly, mention may be made of the material which the vertebrates supply for a consideration of what is often called the Law of Recapitulation. It is not astonishing that a group as broad and as well known as the vertebrates should provide several examples of embryos which seem to reflect something in the ancestral stages of the forms to which the embryos in question belong. As an example, the gill-slits (or rather gill- pouches) of the mammals may be taken. It is rightly held that these structures in the embryo mammal represent the gill- pouches and slits of the fish-stage ancestor of the mammals. But the most important thing to notice is that it is the gill- pouches of embryo fish and not those of adult fish which the gill-pouches of mammalian embryos resemble ; indeed, not much observation is needed to see that between the gill-pouches of the mammalian embryo and the gill-slits of an adult fish there is but little resemblance, whereas the gill-pouches of embryonic stages are very similar in all groups of vertebrates. This explanation covers all cases of so-called recapitulation. It follows that it is inaccurate and misleading to say that Ontogeny (the development of the individual) recapitulates Phylogeny (the evolution of the race). What may be true is that Ontogeny recapitulates the Ontogeny of the ancestor, and even then, it is not necessarily true of all embryonic forms. While the gill-pouches do recapitulate in this sense, other organs such as the primitive streak or the extra-embryonic ccelom do not. It is also to be noted that the order of appearance of structures in Ontogeny is not necessarily the same as in Phylogeny. Denticles appeared early in evolution, bat they arise late in the development of the dogfish. The embryo is phylogenetically older than the amnion, but in the development of the mouse, the amnion arises first and the embryo afterwards.

Fig. 185. — Views of embryos of A, dogfish ; B, lizard ; C, chick ; D, rabbit ; and E, man ; showing the similarity at early stages between embryonic forms of related animals.

The real value of embryology from the point of view of evolution lies in the fact that embryonic forms are like the embryonic forms of related animals. As a rule, the younger the embryos are, and the closer akin the species to which they belong, the more closely do the embryos resemble one another. The more closely allied the species are, the longer does the resemblance between the embryos persist. Embryology furnishes valuable evidence therefore as to affinities, but it cannot profess to give definite information concerning the adult forms of ancestors.

Literature Bateson, W. Problems of Genetics. Yale University Press, 1913. Garstang, W. The Theory of Recapitulation : a Critical Restatement of the Biogenetic Law. Journal Linnean Society, London, Zoology, vol.

35> 1922. Goodrich, E. S. Metameric Segmentation and Homology. Quarterly Journal of Microscopical Science, vol. 59, 191 3. Huxley, J. S. Constant Differential Growth-ratios and their Significance.

Nature, vol. 114, December 20th, 1924.

Versluys, J. Uber die Riickbildung der Kiemenbogen bei den Selachii. Bijdragen tot de Dierkunde, vol. 22, 1922.

CLASSIFICATION OF THE ANIMALS AND GROUPS OF ANIMALS MENTIONED IN THIS BOOK (An asterisk denotes a totally extinct group) Chordata.

Hemichordata. Pterobranchia, e.g. Cephalodiscus. Enteropneusta, e.g. Balanoglossus. Protochordata (Acrania). Urochordata.

Ascidiacea, e.g. Ascidia. Thaliacea, e.g. Salpa. Larvacea, e.g. Fritillaria. Cephalochordata, e.g. Amphioxus. Craniata. Anamnia. Cyclostomata. Cyclostomata.

Petromyzontia, e.g. Petromyzon (? Palaeospondylus). Myxinoidea, e.g. Myxine, Bdellostoma. Qstracoderma,* e.g. Cephalaspis. Gnathostomata. Pisces. Chondrichthyes.

Selaehii, e.g. Scyllium, Squalus, Heptanchus, Hexanchus, Heterodontus, Pristis, Rhina, Pliotrema, Raia, Torpedo. Holocephali, e.g. Chimaera. Acanthodii,* e.g. Acanthodes. Pleuracanthodii* e.g. Pleuracanthus Cladoselachii,* e.g. Cladoselache. Osteiehthyes. Teleostomi.

Osteolepidoti * e.g. Osteolepis, Sauripterus. Coelacanthini,* e.g. Undina. Polypterini, e.g. Polypterus. Palseoniscoidei,* e.g. Cheirolepis. 487

Phylum.

Subphylum.

Class.

Subphylum.

Class.

Order.

Class.

Subphylum.

Grade.

Branch.

Class.

Subclass.

Class.

Branch.

Class.

Grade.

Order.

Order. Order. Order.

Order. Grade. Subclass. Order.

Order. Order. Order.

4 88

CLASSIFICATION OF ANIMALS

Order. Acipenseroidei, e.g. Acipenser.

Order. Amioidea, e.g. Amia.

Order. Lepidosteoidei, e.g. Lepido steus. Order. Teleostei, e.g. Gadus, Amiurus, Ipnops, Periophthalmus, Fierasfer, Gobiesox, Amblyopsis, Lucifuga, Solea, Exoccetus, Edriolychnus. Dipnoi, e.g. Ceratodus, Lepidosiren, Protopterus, Dipterus. Amphibia. Labyrinthodontia * (Stegocephalia), Embolomeri, e.g. Eogyrinus, Loxomma. Urodela, e.g. Triton, Salamandra, Proteus, Siren. Anura, e.g. Rana, Pipa, Rhinoderma, Alytes, Hylambates. Gymnophiona, e.g. Ichthyophys, Hypogeophys. Amniota. Reptilia. tCotylosauria * e.g. Seymouria. Sauropsida. tChelonia, e.g. Testudo, Chelone, Sphargis, Eunotosaurus, Triassochelys. Parapsida. Squamata.

Lacertilia, e.g. Lacerta, Varanus, Uromastix, Gecko, Anguis, Chalcides, Scincus, Amphisbaena, Chamaeleo, Mosasaurus. Ophidia, e.g. Vipera. Ichthyosauria, e.g. Mixosaurus, Ichthyosaurus, Ophthalmosaurus. Diapsida.

Rhynchocephalia, e.g. Sphenodon (? Thalattosaurus). Crocodilia. Pseudosuchia,* e.g. Euparkeria. Thallatosuchia,* e.g. Geosaurus. Eusuchia, e.g. Crocodilus.

t The Cotylosauria and Chelonia are often grouped together as Anapsida.

Subclass.

Class.

Order.

Suborder.

Order.

Grade.

Class.

Subdivision Subdivision Order.

Group.

Superorder Order.

Order.

Superorder and Order.

Group. Order.

Order. Suborder.

Suborder.

CLASSIFICATION OF ANIMALS

489

Order. Suborder.

Suborder.

Order. Subdivision. Group. Order.

Group. Order.

Class. Grade. Grade. Subclass.

Subclass.

Class.

Grade.

Grade and Subclass.

Grade.

Subclass.

Subclass. Order.

Order. Order.

Suborder.

Order.

Dinosauria.* Saurischia, e.g. Diplodocus, Tyrannosaurus. Predentata (or Ornithischia), e.g. Iguanodon, Stegosaurus, Triceratops. Pterosauria,* e.g. Pteranodon. Theropsida. Synapsida.

Theromorpha,* e.g. Cynognathus. Synaptosauria. Sauropterygia,* e.g. Nothosaurus, Plesiosaurus. Aves.

Archaeornithes,* e.g. Archseopteryx. Neornithes. Palseognathse, e.g. Struthio, Emu, Rhea, Cassowary, Apteryx (Kiwi), Moa, Tinamu. Neognathse, e.g. Columba, Gallus, Megapode, Grebe, Petrel, Diver, Gull, Flamingo, Duck, Phalarope, Dodo, Solitaire, Humming-bird, Penguin. Mammalia. Multituberculata.* Monotremata, e.g. Ornithorhynchus, Echidna. Ditremata. Trituberculata.* Marsupialia, e.g. Didelphys, Ccenolestes, Dasyurus, Thylacinus, Perameles, Phascolarctos, Phascolomys, Phalanger, Notoryctes, Thylacoleo, Macropus. Placentalia. DeltatheridiidaB,* e.g. Deltatheridium. - Creodonta.* Carnivora. Fissipedia, e.g.Canis, Felis, Ursus, Civet, Badger. Pinnipedia, e.g. Seal. Condylarthra.* Amblypoda,* e.g. Uintathe-

Order.

Suborder.

Family.

Ungulata. Perissodactyla. Titanotheridse,* e.g. Tita- notherium.

490

CLASSIFICATION OF ANIMALS

Family. Family.

Family.

Suborder Tribe. Family. Family.

Tribe.

Family.

Tribe.

Family.

Order. Order.

Order. Order. Order.

Order.

Order. Order. Order. Suborder.

Suborder.

Suborder. Series. Series. Family

Tapiridae, e.g. Tapir. Rhinocerotidae, e.g. Rhino- cerus. Equidae, e.g. Eohippus, Me- sohippus, Miohippus, Pliohippus, Equus. Artiodactyla. Suina. Suidae, e.g. Sus. Hippopotamidae, e.g. Hippo- potamus. Tylopoda. Camelidee, e.g. Protylopus, Poebrotherium, Proca- melus, Camelus. Pecora. Giraffid33, e.g. Giraffe. Cervidas, e.g. Cervus, Rein- deer, Muntjack, Irish Elk. Bovidae, e.g. Ox, Zebu, Sheep, Goat, Antelope, Antilocapridae, e.g. Anti- locapra. Hyracoidea,e.g. Hyrax (coney). Pro boscidea, e.g. Mceritherium , Palaeomastodon, Tetrabelo- don, Elephas. Sirenia, e.g. Manatus. Cetacea, e.g. Whale, Dolphin, Porpoise. Litopterna,* e.g. Thoatherium. Edentata, e.g. Bradypus, Cho- loepus, Armadilloe, Pango- lin. Rodentia, e.g. Lepus, Mus, Squirrel, Porcupine. Inseetivora, e.g. Mole, Hedge- hog, Shrew, Plesiadapis,Ma- croscelides, Tupaia. Cheiroptera, e.g. Bat. Dermoptera. Primates. Lemuroidea, e.g. Notharc- tus, Lemur. Tarsioidea, e.g. Tetonius, Tarsius. Anthropoidea. Platyrrhini,e.g. Marmoset. Catarrhini. Parapithecidae,* e.g. Parapithecus.

Family. Family.

Family.

Cercopithecidae, e.g. Cercopithecus, Man- drill, Baboon. Simiidse, e.g. Proplio- pithecus, Pliopithe- cus, Hylobates, Simia (Orang), Chimpan- zee, Gorilla, Austra- lopithecus. Hominidse, e.g. Pithe- canthropus, Eoan- thropus. Homo rhodesiensis, Homo heidelbergen- sis, Homo neandertha- lensis, Homo sapiens.

Historic Disclaimer - information about historic embryology pages
Pages where the terms "Historic" (textbooks, papers, people, recommendations) appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms, interpretations and recommendations may not reflect our current scientific understanding. (More? Embryology History \| Historic Embryology Papers)

Cite this page: Hill, M.A. (2024, April 23) Embryology Book - Vertebrate Zoology (1928) 43. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_Vertebrate_Zoology_(1928)_43

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