Vertebrata

“ The highest class of Vertebrata affords us examples of these three modes of adaptation, the lowest of these being entirely aquatic, and even absolutely marine; next, we have fluviatile types of the large terrestrial mammalia, in the family of Manatees, again, a swimming family among Carnivora, another flying, most of them however walking upon their four extremities on solid ground, but at the head of all, man, standing upright, to look freely upwards, and to contemplate the whole universe. ”

“ Equally as throughout the Mollusca, there holds throughout the Vertebrata the correlative fact, that not even in its lowest any more than in its highest types, is the body divisible into homologous segments. The vertebrate animal, under its simplest as under its most complex form, is like the molluscous animal in this, that you cannot cut it into transverse slices, each of which contains a digestive organ, a respiratory organ, a reproductive organ, &c. The organs of the least-developed fish as well as those of the most developed mammal, form but a single physiological whole ”

“ When we see that the limbs of the most different Vertebrata, in spite of their exceedingly different external forms, nevertheless possess essentially the same internal structure; when we see that in the arm of a man and ape, in the wing of a man or a bird, in the breast fins of whales and sea-dragons, in the fore-legs of hoofed animals and frogs, the same bones always lie in the same characteristic position, articulation and connection—we can only explain this wonderful agreement and homology by the supposition of a common transmission by inheritance from a single primary form. ”

“ But, in addition to these, fishes among Vertebrata are introduced, but fishes only, all of which are exclusively marine. At a somewhat later period insects come in. We find next reptiles in addition to fishes—the lower classes, or invertebrates, continuing to be represented through all subsequent epochs, but by species changing gradually at each period, as all classes do after they have been once introduced. ”

“ In any classification of the animal kingdom, it is clear that large groups of animals exist which have so many common characteristics as to necessitate their being placed in one larger group or kingdom; thus zoologists are able to speak definitely of the Vertebrata, Arthropoda, Annelida, Echinodermata, Porifera, Cœlenterata, Mollusca, etc. ”

“ But a man and a fish are members of the same sub-kingdom 'Vertebrata', because they are much more like one another than either of them is to a worm, or a snail, or any member of the other sub-kingdoms. ”

“ According to him there are four forms on which animals have been modelled, and of which ulterior divisions are only slight modifications founded on the development or addition of some parts that do not produce any essential change of structure.
The four great branches of the animal world are the vertebrata, mollusca, articulata, and radiata. The vertebrata are those animals which (as man and other sucklers, birds and fishes) have a backbone and a skull with lateral appendages, within which the viscera are excluded, and to which the muscles are attached. ”

“ Besides the senses which we have enumerated, which Fishes possess in common with other Vertebrata, there is another faculty with which some species are endowed, quite peculiar to this Class. It is the power of communicating electric shocks at will to other creatures. ”

“ The third class of the Vertebrata, comprises the "Birds," or those whose young are not born alive, but in the form of eggs; who have warm blood, and breathe by lungs, the air-cells of which communicate with the hollow shafts of the bones, (these are full of air instead of marrow) , this gives them lightness, and serves as a reservoir of air while flying; they have two anterior extremities so modified as to form wings, and a heart with four cavities ”

“ The contribution of the Vertebrata.—In the vertebrates the dynamic or working organism may be said to reach its highest expression, unless it be in the flying insects, and their inorganic residue becomes relatively unimportant in rock formation. Although the greatest of all animal types in most respects, it has never formed more than trivial beds of rocks. ”

“ It has already been shewn that in all the higher Vertebrata muscle-plates appear, which closely resemble those in Elasmobranchii; so that all the higher Vertebrata pass through, with reference to their muscular system, a fish-like stage. The middle portion of the inner layers of their muscle-plates becomes, as in Elasmobranchii, converted into muscles at a very early period, and the outer layer for a long time remains formed of indifferent cells. ”

“ It has also never been doubted that of all the Vertebrata the Mammals are most closely allied to Man, and that he possesses all the characteristic features distinguishing them from all other Vertebrata. If then we further carefully examine the three different main groups or sub-classes of Mammals—the inter-connections of which were discussed in our last chapter—there cannot be the slightest doubt that Man belongs to the Placentals, and shares with all other Placentals, the important characteristics 267 which distinguish them from Marsupials and from Cloacals. ”

“ This structure is distinctive of that class of the vertebrata called mammals, and which includes man, most of the larger and commoner land animals, and whales and manatee. We shall find later that it is essentially connected with the perfection of the air breathing to which this group has attained. Another characteristic shared by all mammals, and by no other creature, is the presence of hair. In birds we have an equally characteristic cover in the feathers, the frog is naked, and the fishes we find either naked skins or scales. ”

“ It is now pretty generally understood that fishes, which rank lowest among the Vertebrata, have existed alone during the oldest periods; that the reptiles which, in the gradation of structure, rank next [Pg 162] above them, have followed at a later period; that still later the birds, which, according to their anatomy, rank above reptiles, have next made their appearance; and that mammalia, which stand highest, have been introduced last, and even among these the lower families seem to have been more numerous, before the higher ones prevailed over them. ”

“ Nature's mistake was rectified and further evolution secured, not by degeneration in the brain-region, for that means degradation not upward progress, but by the formation of a new food-channel, in consequence of which the brain was free to develop to its fullest extent. Thus the great and mighty kingdom of the Vertebrata was evolved with its culminating organism—man—whose massive brain with all its possibilities could never have been evolved if he had still been {67}compelled to pass the whole of his food through the narrow œsophageal tube, still existent in him as the infundibular tube. ”

“ Fishes, constituting the class Pisces, may be defined as Craniate Vertebrata, or Chordata, in which the anterior portion of the central nervous system is expanded into a brain surrounded by an unsegmented portion of the axial skeleton; which are provided with a heart, breathing through gills; and in which the limbs, if present, are in the form of fins, as opposed to the pentadactyle, structure common to the other Vertebrata. ”

“ It is, in fact, true of several groups of Vertebrata which have lived through a considerable range of time, that the endoskeleton (more particularly the spinal column) of the older genera presents a less ossified, and, so far, less differentiated, condition than that of the younger genera. ”

“ We are now in a position to appreciate the fact that the old and more popularly know division of animals into vertebrata and invertebrata scarcely represents the facts of the case, that the primary division should be into protozoa and metazoa, and that the vertebrata are one of several groups of metazoa with a fundamental bilateral symmetry and imperfect metameric segmentation. ”

“ When we look to the great fundamental characters particularly to the framework for the attachment of the muscles, what do we find?—why, that of these Placoids,—‘the highest types of their class,’—it is barely possible to establish their being vertebrata at all, the back-bone having generally been too slight for preservation, although the vertebral columns of later fossil fishes are as entire as those of any other animals. In many of them traces can be observed of the muscles having been attached to the external plates, strikingly indicating their low grade as vertebrate animals. ”

“ There can be no doubt that many of the Arthropods, a division of animals more ancient and even now more prevalent than the Vertebrata, have undergone more phylogenetic modification"—a beautiful phrase—"than even the most modified of vertebrated animals. ”

“ That there are a great number of points of similarity between ourselves and the lower animals, then, appears to be clearly admitted on all hands. It is, further, universally allowed that the Vertebrata resemble man more nearly than do any invertebrates; that among vertebrates the Mammalia, and of these the Quadrumana, approach him most closely. Lastly, I am aware of no dissentient voice to the proposition, that in the whole, the genera Troglodytes, Pithecus, and Hylobates, make the closest approximation to the human structure. ”

“ The legs of an insect, for instance, are small tubes with the muscles inside. The limbs of vertebrate animals, on the other hand, have the muscle outside the bone. Invertebrates commonly have the main nerve trunk in front, or underneath, instead of at the back, and likewise often have their brains in their abdomens. Some of them, such as the grasshopper, even hear with their abdomens. But all vertebrata have the great nerve trunk at the back, contained in the spine and with a bulb on the front or upper end constituting the brain. ”

“ From these lowest Vertebrata to the highest, and to the highest forms among these, the comparison between the embryonic conditions of the higher animals and the adult states of the lower can be more completely and thoroughly instituted than if the survey is extended to the Invertebrata, inasmuch as the latter are in many respects constructed upon an altogether too dissimilar type; indeed they often differ from one another far more than the lowest vertebrate does from the highest mammal ”

“ The axis of the backbone of all vertebrates is formed by an elastic rod known as the 'notochord,' which lasts throughout life in some of the lowest forms, but in the higher forms appears only in the embryo. The universal occurrence of this structure has been regarded as the most important characteristic of the Vertebrata and their allies, which are accordingly grouped together in the phylum Chordata. ”

“ Among the higher aquatic Invertebrata,—cuttlefishes and lobsters, for instance,—there is a very considerable power of locomotion; and the aquatic Vertebrata are, considered as a class, much more active in their movements than the other inhabitants of the water. But it is only when we come to air-breathing creatures that we find the vital characteristics of self-mobility manifested in the highest degree. Flying insects, mammals, birds, travel with velocities far exceeding those attained by any of the lower classes of animals. ”

“ They agree in being air-breathers and never having gills, except the rudiments present in the early embryo: this distinguishes them from Amphibia. They agree in having the skull set on to the back-bone by a single articulating surface or condyle; and thus differ alike from Amphibia and from Vertebrata. They agree in having the red corpuscles of the blood nucleated; and in this differ from the Mammalia, in which the red corpuscles are non-nucleated discs. From a popular point of view, we may say that the striking distinction between [155] birds and reptiles lies in beauty and ugliness. ”

“ These are perfectly identical with the eyes and ears of vertebrates. Notice, again, the organization of the mouth. Only vertebrates have such. In annulosa the jaws have lateral motion, and are modifications of the feet. One other consideration must be mentioned—the size of these animals. Only vertebrata and mollusca seem to have unlimited powers of increase. They only have been distinguished for the magnitude of individuals. Among the latter especially are to be noted the cephalopods for furnishing giants. Did space permit, a thousand homologues might be pointed out. ”

“ Now, as a terrestrial animal slowly became more and more aquatic in its habits, very slight changes, first in the curvature of the cornea or crystalline, and then in the density of the humours, or conversely, might successively occur, and would be advantageous to the animal whilst under water, without serious detriment to its power of vision in the air. It is of course impossible to conjecture by what steps the fundamental structure of the eye in the Vertebrata was originally acquired, for we know absolutely nothing about this organ in the first progenitors of the class. ”

“ Again, the change from mere indefinite and accidental processes to two regular pairs of symmetrical limbs, as the result of merely fortuitous, favouring variations, is a step the feasibility of which hardly commends itself to the reason, seeing the very different positions assumed by the ventral fins in different fishes. If the above suggestion made in opposition to the views here asserted be true, then the general constancy of position of the limbs of vertebrata may be considered as due to the position assumed by the primitive rugosities from which those limbs were generated. ”