THE CONQUEST OF THE LAND

The Amphibia are the oldest and the lowest group of vertebrates that are able to lead an active existence on land, and the characters which distinguish them most definitely from the fishes are all to be interpreted as adaptions to the new mode of life. One of the most obvious external differences between the two groups is in the structure of the extremities, the fish having fins, while the amphibian has limbs constructed on the same general lines as our own arms and legs. The fish's fin is to be regarded as an extremity with a very great number of fingers or toes. It has the function of a paddle, and is obviously useless whether for supporting or propelling the body on land. The first obvious necessity for a land existence is some mechanism by which the limb can be alternately pushed forward and, being fixed to some solid object, drawn upon, so as to pull the body after it. A different arrangement of bones and muscles, so as to give a much more complex lever system than that of a fin, and some kind of clawing arrangement at the end, were thus necessary. The similarity in the limbs of all the land vertebrates is very striking, as is indicated by the comparison of human and a frog's limbs on Fig. 84. In each case there is a single bone in the upper arm or thigh, which is attached to a bony girdle in the trunk. There are two elements in the forearm and in the lower leg respectively, below which, in either case, is a group of small bones constituting a complex joint at the wrist or ankle. Then follows the set of five bones in the foot or hand, to each of which is attached a jointed finger or toe. We have no reason to believe that this particular arrangement, and the particular number of [105]
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[107]digits, was arrived at except by accident. Once arrived at, however, the arrangement was adhered to with considerable strictness. For although the number of digits is in some groups—in the birds especially—reduced, the primary design is almost always readily recognisable. The second function of the limb, that of supporting the body, was developed very slowly. In the amphibians and reptiles, and even in the lower mammals, the legs are comparatively weak and sprawling, and the creature crawls on the belly.

The second great change which required to be made was of course in the method of breathing. An ordinary fish, when taken out of the water, dies of suffocation, because its gills become inefficient for respiration as soon as they become dry. An entirely new type of organ had therefore to be evolved, and this occurred on the same lines as in the Dipnoi, by the development of a pair of sacs from the upper part of the digestive canal, in which the blood is made to circulate, and which are kept filled with air taken direct from the atmosphere. It is of course very well known that an ordinary amphibian is not a lung breather throughout its whole life. The metamorphosis of a gill-breathing tadpole into a lung-breathing frog, illustrated in Fig. 85, is a phenomenon with which everyone is familiar. And this condition, in which a change in the mode of life is made by each individual in the course of its development, is the typical one. But the modern amphibians include types ranging from completely water to perfect land forms. Some, like the Austrian Olm (Fig. 86), are gill breathers throughout their whole life. One which is normally of this type, the Axolotyl, illustrated in Fig. 87, can be made to acquire lungs and assume a land mode of life. Others, which normally make the metamorphosis, can be prevented from doing so by being confined to the water, and complete their life-histories in the condition of gill breathers. In still other forms (e.g. the Coecilians, Fig. 89) the change is made before the young creature leaves the egg, and the independent life is commenced in the condition of a land animal.

Correlated with the development of the lungs is a change in the structure of the nostrils, from the condition of blind sacs, as they occur in the fishes, to that of air passages, communicating with the upper part of the alimentary canal, and thence with the lungs.

The living forms of the Amphibia differ considerably from the types which constituted the group in those long-distant ages when it was in the heyday of its prosperity. The latter forms were characterised especially by a system of armour-plating over the head, which frequently extended under the breast and even covered the greater part of the lower surface, and which appears to have formed a protection against the multitude of sharks which populated the waters in which the amphibians partly lived. The [109]
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[111]armour-plated type has long ago become extinct, but it is in it, rather than in the modern forms of Amphibia, that we must look for the direct ancestor of man. A fossil of this earlier group is shown on Fig. 90.

Our next group in the order of Evolution is that of the reptiles, the main differences between which and the Amphibia are of the nature of more complete adaptions to a life on land. The reptiles have in fact completed the conquest of the land which was undertaken by the previous group, and many of them, living as they do in dry and hot deserts, are as independent of the water as any form of animal life. Thus whereas the amphibian has a thin skin, which is kept moist by the secretions of numerous skin glands, the reptile has a body-covering of scales, which form an effective protection against a too rapid loss of moisture. Evidently with the same object, the reptile egg is enclosed in a hard and resistant shell. Correlated with the change in the skin is a much more perfect development of the lungs, for while the amphibian breathes to a considerable extent through its thin moist skin, this method of assisting respiration is not available to the reptile.

Again connected with the improvement of the respiratory process, there is a partial development of a septum dividing the ventricle or pumping chamber of the heart. The value of this division of course lies in the fact that the purified and oxygenated blood from the lungs is prevented from mixing with the venous blood from the body. The course of the blood is from the body to the right auricle, thence to the right ventricle, and thence to the lungs. The pure blood from the lungs returns to the left auricle, passes thence to the ventricle on the same side to be pumped to the general circulation. The disadvantage of a single ventricle, such as occurs in the Amphibia, and the advantage of the regular double circulation, such as that in man, are sufficiently obvious. The division of the ventricle into two chambers is less complete in the lizards and snakes, very nearly perfect in the crocodiles.

The reptiles, like the Amphibia, are 'cold blooded,' by which is meant, not that their blood is necessarily cold, but that its temperature varies with that of the surroundings, while that of the blood of the mammals and birds is practically constant.

A very important feature of the reptiles, which they possess in common with the mammals and the birds, is that the embryo produces two membranous outgrowths called respectively the amnion and the allantois, which completely envelop it, and which have important functions in connection with nutrition, respiration, and excretion during the period when the young creature is enclosed in the egg. It is, of course, not until we reach the higher mammals that these membranes assume their greatest importance.

For a considerable time in the world's history the reptiles were the dominant vertebrate class, and in the chalk period especially they were represented by a great variety of forms, and by a number of species of colossal stature, one at least of which was over a hundred feet long. In those times the reptiles were by no means all condemned to crawl on their bellies, for they [113]
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[115]included a large number of marine forms, comparable to the porpoises and whales among the mammals, and flying forms whose aspect must have resembled, and been equally terrifying with, that of our mythical dragons. A few reconstructions of these are shown in Fig. 91.

All living reptiles, with a single exception, belong to the four comparatively modern types of the lizards, snakes, tortoises, and crocodiles, of which examples are illustrated in Figs. 92 to 95. None of these are closely related to the mammals or birds. For the common ancestor of all these types we must go back to some primitive reptile form. Fortunately such a type is represented at the present day in a single peculiar species found in New Zealand, which bears the Maori name of the Tuatara. It was formerly found commonly on the mainland, but is now confined [117]
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[119]to a few small islands in the Bay of Plenty, North Island, where it enjoys Government protection. It is, as the illustration in Fig. 96 shows, a lizard-like creature, and reaches a length of about two feet. It lives in burrows near the shore, and feeds on small animals that are left behind by the tide. The Sphenodon, as zoologists have named it, has apparently been preserved owing to the absence of competition by the mammals, and by adopting the rather curious mode of life just described. In all its features, but especially in the primitive condition of its vertebrÆ, it is very much lower than any other living reptile, and it connects the higher groups with the Amphibia. Many closely related fossil species are known, one of which is shown in Fig. 97.

To the lay mind the distinctions between the Amphibia and the reptiles are not very obvious, and indeed in the older classifications the former group was not separated from the latter. The differences between a reptile and a bird, on the other hand, are very striking. It might therefore be regarded as a matter for surprise that zoologists now make the greater distinction between the Amphibia and the reptiles, grouping the former in one great class with the fishes, the latter in a second great section with the birds. But in fact there are many fundamental points of agreement between reptiles and birds, and it is impossible to doubt that the latter have sprung from a reptilian stock. Indeed, a most interesting connecting link is known, in the fossil Archiopteryx shown in Fig. 98, of which only two specimens have been found, and which is the only creature of its type of which we have any record. In all its skeletal features, the Archiopteryx is reptilian, and it would undoubtedly have been classed as a new type of reptile but for the obvious and unmistakable traces of feathers. From what particular class of reptiles the birds have sprung is not known.

The birds have assumed the position of almost unquestioned masters of the air, but like other great groups they show possibilities of evolution in other directions wherever opportunity offers, and types like the kiwi and the penguin shown in Figs. 100 and 101 have forsaken their native element—the one for the land, the other for the water.

The birds agree with the mammals in the development of a four-chambered heart, in their warm blood, in their external covering for the skin, and in the development of arrangements and instincts for the parental care of the young. Their line of Evolution has thus been to some extent parallel to that of the mammals. On the other hand, they differ obviously in the structure and function of their fore limbs, in the absence of a diaphragm, and in their special methods for the care of the young, and there can be no doubt that the two groups have had quite different origins in the reptile class.

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