In the never-ceasing struggle for existence all forms of life upon the earth, whether consciously or unconsciously, are continuously striving for improvement; striving to flee from adverse environments, or to adapt themselves better to those which must be endured; to escape their enemies, or to find means whereby they may withstand them; to find more or better food, or to prevent others from despoiling them of what they have. There is always more or less of unrest, more or less of discontent, if such terms may be used of the lower organisms. It sometimes happens with groups of organisms that by reason of unusual or extraordinary traits they become so perfectly adapted to their environments, to their surroundings, or so easily adaptable to changes in their environments, that they remain for long ages securely protected and little changed. But, as with man himself, improvement is usually the result of adversity—adversity which stimulates but does not destroy. And the word improvement, translated into biological language, means simply specialization, that specialization which adapts the organism better to its mode of life, which fits it the better to excel its less ambitious or less capable competitors. No animals or plants are perfect; if they were, there would be no advancement, no struggle. If all physical conditions stood still, or remained uniform, perhaps life would stand still, but conditions never have and never will stand still, and life must change to meet changed conditions.

Thus it is that which makes life easier, which lessens the dangers of destruction, which insures the continued prosperity of the race, is seized upon and utilized by all plants and animals, so far as possible. As said long ago by Tennyson,[2] the first law of life is not the preservation of self, but the prosperity of the race. Whatever the causes may be whereby the offspring are better adapted to conquer in the struggle for existence, whatever may be the laws governing changes and specialization, whether heredity, Mendelism, mutation, natural selection, or Lamarckism, we call the process evolution.

To escape from the severe competition of the overcrowding animals of the sea, some of those creatures we call fishes long ago became air-breathers and took possession of the unoccupied land. From among the myriads which were driven into unbreathable water, by accident or by their enemies, or led there in the search for more easily acquired or better food, some survived and found that the oxygen of the air was quite as breathable as that of the water. Steadily their progeny became better and better adapted to the unusual life until they ceased to be fishes and became amphibians, from which have arisen in like manner all the reptiles and birds and mammals that live or have lived upon the earth.

With more and better powers, developed under better opportunities, not a few of these descendants have repeatedly sought safety from their newly acquired enemies of the overcrowded land, or a better supply of food in the sea; gradually, perhaps incidentally at first, as we shall see is the case with some lizards today, but later with increased adaptation to their new surroundings, they become truly sea or water animals, no longer able to live upon the land. In these changed conditions and with concomitantly changed habits they never reverted to the primitive condition of fishes, never became water-breathing animals again, for that would be actual retrogression, a seeming impossibility in evolution. Nor indeed does it seem possible that a land creature after its reversion to water life ever can return to the land again.

A fish through long ages of evolution has become well adapted to its environments; its shape is the best for speed or varied evolutions in the water; its teeth and mouth-organs are best suited for the food it requires. Now it is evident that if animals of very different habits and form should go back to the water and seek to compete with creatures already well adapted to their surroundings, they must, so far as possible, acquire like forms and like habits. And any improvement on such forms and habits that their higher development permits them to attain will of course be of advantage in their competitive struggles. A fish makes most use of its tail fin for propulsion. It follows that a land animal seeking to compete with it under like conditions must acquire a tail fin or some other organ which subserves its purpose as fully. The body fins are of little use to a fish, save for equilibration, for preserving its position, for stopping quickly, or for changing the direction of its movements quickly—very different functions from those of the corresponding organs, the limbs, of higher vertebrates. There are few better examples of predaceous, fish-eating fishes than the common gar-pike of our rivers, fishes with a slender body covered with very smooth scales, a strong tail, a short neck, and long jaws armed with numerous slender and sharp teeth. Such a fish, darting into a school of smaller fishes, by quick, sudden changes of movement, actively opening and closing its jaws, is sure to seize some of its sought-for prey. In a direct trial of speed with its victims it would most likely be worsted.

There have been many animals of high and low rank which in the past and present have gone back from a terrestrial existence to a life in the water, finding at last a congenial home away from the shores. Or, perhaps, like the monitor lizards of today, they have found temporary safety in the water when hard pressed by their land enemies, and finally found, not only protection, but an abundant supply of easily obtainable food therein. As in every vocation of life there have been many failures in such attempts, many partial successes only. But not a few have found abounding and enduring success and final prosperity—success that has led possibly to undue adaptation to surroundings, and to the acquirement of great size, for that has been the invariable end of water air-breathers of long duration—specializations which finally prevented them from meeting new exigencies. It seems to be a law of evolution that no large creatures can give rise to races of smaller creatures; and as we shall see, the largest sea-animals have been the final evolution of their respective races.

There are no better examples of such success today, nor has there been in all the geological ages, so far as we know, more perfect examples of the adaptation of air-breathing animals to an aquatic life than the great whalebone whales. In Eocene times their ancestors were walking and running land animals; of that there can be not the slightest doubt, since we cannot conceive, as did the older naturalists, of their direct descent from the fishes while having all the essential structure of mammals, i.e., lungs, circulatory system, manner of breeding and rearing the young, etc. Of the living whales, or Cetacea, there are now in existence two very distinct types, so different from each other that some have supposed them to have been evolved from different types of land mammals. One of these is best exemplified by the great baleen whale, having a broad, short head and no teeth. It feeds upon crustaceans chiefly, which are strained from the water by the great fringe or net of “whalebone.” The other type is seen in the porpoise or dolphin. These cetaceans have numerous, pointed and recurved teeth, which they use as did many of the reptiles, hereinafter described, for the seizure and retention of fishes and other swimming animals. So great have been the changes in all these cetaceans, in the adaptation to an aquatic life, that we are almost at a loss to conjecture from what kinds of land animals they have descended. The great zeuglodont whales of early Tertiary times have long been thought to be a sort of connecting link between them and their land ancestors, and it is still probable that they were. The forms of zeuglodont whales that have been discovered in Africa within recent years bear so much resemblance in their skull and teeth to the contemporary carnivores, that many paleontologists think, with good reason, that they were descended from them, that is, from the ancestors of all our dogs, cats, weasels, bears, etc., of modern times. And we have much reason to believe that future discoveries will bring further and more decisive proof of their origin before many years have elapsed. The modern Sirenia, the dugongs and manatees, exclusively aquatic mammals, which feed upon seaweeds at the bottoms of shallow bays and harbors, or in the mouths of rivers, are now known, practically with certainty, to be the descendants in these same African regions of the earliest ancestors of our sheep, oxen, and horses, known so certainly that they are often classed with them, or at least with the elephants, which approach them in their ancestral line even more closely.

A third type of living aquatic air-breathers is seen in the seals, sea-lions, etc. They are much less highly specialized, however, than the whales or sirenians, since they are still capable of considerable freedom upon land, which they recurrently seek for the breeding of their young. They still retain the primitive covering of hair, lost almost entirely by the cetaceans and sirenians and functionally replaced for the conservation of heat by a thick layer of blubber. Instead of losing the hind legs and developing the tail as a propelling organ like the whales, the seals encountered precisely the reverse experience. The hind legs have been developed into most efficient paddles or sculls, and the tail has been for the most part lost. They are fish-eaters, it is true, but they do not have the long jaws possessed by the porpoises and toothed whales.

In the sea-otters, beavers, and even the muskrats, we have examples of less complete adaptation of land mammals to water life, the most of them showing the beginnings at least of structural adaptations similar to those of the seals. From an attentive examination of all these animals, living as well as extinct, which have attained partial or complete success as air-breathing water animals, we find certain laws existing, if we may call them such, which we may discuss a little in detail. As we have seen in the comparison of the whale with the seal, the methods of adaptation have not always been the same, and some recent writers have endeavored to classify aquatic animals under many groups, to which they have given learned technical names, most of which will not concern us here in dealing with the reptiles only.

Beginning with the head, we find that all those reptiles and most of the mammals which have become aquatic fish-eaters have an elongated skull, or rather an elongated face. The jaws are long and slender, and the teeth are not only numerous but also sharp and slender, much like those of the gar-pike, indeed. It is remarkable, too, that in most such animals the external nostrils are situated, not at the extremity of the snout, as in all terrestrial mammals and reptiles, but far back near the eyes. In the whales this position of the nostril enables the animals to breathe without continuous muscular exertion while floating on the surface; that is, the nostrils are at the top of the head. In the sirenians, on the other hand, which live habitually at the bottom of shallow waters, coming to the surface to breathe only, the nostrils are situated so that they are the first to emerge, that is, they are near the front end. The crocodiles, with a more or less elongated face, as also the Choristodera, described farther on, are exceptions, since their nostrils are at the extremity of the snout. Both of these types, however, notwithstanding the elongation of the face, are only partly aquatic in habit, and in the crocodiles the breathing organs have undergone a strange modification in accordance with habits peculiarly their own, as will be explained later on. Whether this recession of the nostril toward the eyes can be explained in all cases by the peculiar breathing habits is, however, doubtful. Possibly in some cases, such as the phytosaurs, described later, the creatures used their long beaks to probe in the mud while breathing. Possibly the posterior position has been in some cases rather the result of the elongation of the face, leaving the nostrils behind in some forms, or carrying them forward in others. Nevertheless posterior nostrils always indicate more or less aquatic habits.

In all the earliest reptiles, as we have seen, the neck was short, like that of their immediate progenitors, the ancient amphibians. The shoulders were close to the skull, with not more than two vertebrae that could be called cervical. It happens that most of the earliest reptiles, as we know them, were more or less amphibious in habit, and all of them were probably good swimmers; nevertheless in all likelihood reptiles began their career as a class with a very short neck. The earliest known distinctly terrestrial reptiles had a moderately long neck composed of six or seven cervical vertebrae. It may therefore be assumed with much probability that all later reptiles with a greater or less number of cervical vertebrae are specialized animals, so far as the neck is concerned. Most living reptiles have eight cervical vertebrae; a few have nine, and still fewer have but five. Birds may have as many as twenty-four, while all mammals, with two or three exceptions, have the primitive number seven. Among extinct reptiles, however, there were not a few with more numerous neck vertebrae, some having the enormous number of seventy-six.

An ordinary fish has apparently no neck whatever, the trunk being seemingly attached to the head, nearly as in the primitive amphibians and primitive reptiles. It is evident that a movable neck of considerable length would not only be of no use to the swiftly swimming fish, but a positive disadvantage to it. The body is quickly and easily turned by the powerful tail fin, and a long neck could be of no use that the tail would not better subserve. It is therefore of interest to learn that, as a rule, aquatic animals of all kinds having a powerful propelling tail have also a short neck, acquired either by the loss of neck vertebrae, or, as in the mammals, by the shortening and coalescence of the normal number of seven. There are very few exceptions to this rule of a short neck and a long tail. Those strange little reptiles of Paleozoic times, the first that we know that returned to the water, the Proganosauria, have not only a long, flattened tail, but also an unduly elongated neck of from nine to twelve vertebrae.

On the other hand, certain unrelated reptiles of the past, the dolichosaurs, nothosaurs, and plesiosaurs, with a short non-propelling tail, developed a long neck—sometimes an excessively long one in the plesiosaurs. The turtles, some of which have attained a high adaptation to water life, have invariably a short tail and a freely movable, relatively long neck, a neck which Dr. Hay tells us has increased in length from the beginning of their race by the simple elongation of the vertebrae, as in the giraffe, and never by the addition of vertebrae. We may then account it a rule that swimming animals with a long neck have a short tail, and those with a short tail have a long flexible neck. Even in the plesiosaurs there is some variation of the length of the tail in correlation with the neck. Short-tailed animals must necessarily propel themselves through the water by the aid of their legs, especially the hind legs. If one watches an actively swimming alligator he will observe that the front legs are folded or collapsed by the side of the body, while the hind legs, much bent, are used only slightly in propulsion. The animal swims by a marked sinuous or serpentine movement, like that of a snake upon land, extending throughout the tail and part of the body, at least. An animal propelling itself by its limbs could not move sinuously, and use its legs actively at the same time, and it is probable that the long neck has been evolved compensatorily.

With this shortening of the neck and sinuosity of movement there is developed in every case a long trunk as well as a long tail. The trunk becomes more slender and cylindrical, more like that of a snake, with an actual increase of the bones composing it, reaching the great number of forty-three vertebrae in that most sinuous of all water reptiles with legs, Pleurosaurus of the Protorosauria. And the tail, primitively having perhaps sixty or seventy vertebrae, may have as many as one hundred and fifty in the more typical aquatic forms. This elongation of trunk and tail must be of great advantage to the swimming reptile, just as the racing scull is a more perfect type of speedy craft than a flat-bottomed scow. Dr. Woodward has said that the fate of all fishes, if they continue their evolution long enough, is to become eel-like.

Not only was the tail greatly elongated in swimming reptiles, but it was also more or less flattened. In the beginning of water adaptation the flattening was throughout the tail, as in the living alligators and crocodiles. As the adaptation to water life became more perfect, the flattening became more and more restricted to the extremity; that is, the flattening begins like that of a salamander and in the end becomes like that of a fish, a terminal fin. And some of the actual stages in the evolution of the fish-like fin have been observed by Dr. Merriam in the earlier and more primitive ichthyosaurs of California. In those animals swimming chiefly in a horizontal direction the tail fin has become like that of fishes, that is, vertical; but in those animals which use the tail chiefly for ascending and descending rapidly in the water the fin is developed in a horizontal position, examples of which are seen in the flukes of whales and sirenians.

All animals living upon the land require firm articulations between the different bones of the skeleton, and especially between the vertebrae, for the support and control of the body. Among aquatic animals there is a strong tendency toward looseness of joints, with increasing flexibility. Fishes have the articular processes between the arches of the vertebrae feebly or not at all developed, and the centra or bodies of the vertebrae have thick pads of cartilage between them. Firm union between the vertebrae would restrict freedom of movement, and firmness is not required when the body is surrounded on all sides by water of nearly the same specific gravity as the body itself. And it is doubtless for the same reasons that the articulations of all strictly aquatic reptiles have for the most part become looser and less firm, especially those between the different vertebrae.

The same looseness of articulation is also found in the ribs of aquatic animals. In most animals, and in all those which walk erect, like the mammals, each rib is firmly attached to the backbone by two distinct joints, the head and tubercle, with an interval between them. This double attachment prevents much in-and-out movement of the ribs and gives a firm support for the attachment of the muscles of respiration, as well as for those supporting the viscera. This firmness is unnecessary in animals living always in the water, and the ribs therefore in all aquatic animals tend to become single-headed and loose. The lower or capitular articulation has been lost in part, or almost wholly, in many cetaceans. It has been said that a whale cast up on land will die of suffocation, not for the lack of air, for it is an air-breathing animal like ourselves, but because it can no longer use its respiratory muscles attached to the loosely articulated ribs; it suffocates because the ribs collapse.

As would be expected, the greatest modifications of structure in the adaptation of air-breathers to water life are found in the limbs. No other parts of the body have such different functions in water and on land as the limbs and fins. The limbs of a dog, or a cat, or a man are feeble organs for swimming in comparison with the fins of a fish, and if the land animal must compete with fishes to prey upon them for food it must acquire like swimming powers. As a matter of fact, the limbs of all typically aquatic air-breathing animals have lost nearly all external resemblance to the legs of walking and running animals, and have become more or less fin-like in function—fin-like in shape and function, but never fin-like in actual structure. No creature can go back and begin over again, any more than a man can again become a child with all its possibilities for improvement and development. If an animal cannot modify the organs it already possesses so as to adapt them to new and changed uses by the aid of evolutionary forces it must fail in the struggle. It can never acquire new material, never get new fingers and toes, new organs or parts of organs; all its possibilities lie in the improved and new uses it can make of the material which it received from its ancestors.

The beginning of aquatic adaptation of the limbs lies in the membranous webs between the toes of frogs, salamanders, ducks, seal, otters, etc., where the feet are used largely or entirely for propulsion through the water, in the absence of a propelling tail. And this membrane, in the majority of cases, is the extent of aquatic adaptation in air-breathing animals. In those animals, however, such as most of the reptiles described in the following pages, where the tail has developed as the propelling organ, the limbs lose to a greater or less extent their propelling function and become merely organs of equilibration and control. Of the two pairs of fins of fishes it is evident that the anterior ones have the more important equilibrational function; the hind ones have a much less important use as guiding organs; as a matter of fact, in not a few fishes the hind or pelvic fins have actually migrated forward to supplement the function of the pectoral fins. It is for these reasons that those animals best adapted of all for life in the water—the whales and sirenians—have lost the hind legs completely. In other tail-propelled air-breathers the hind legs have become progressively smaller and less powerful than the front ones. In all short-tailed water animals, however, where the legs, and especially the hind legs, have the important function of propulsion to subserve, they still retain the large size and firm connections with the body, examples of which will be seen in the seals, sea-otters, marine turtles, and plesiosaurs.

Because the legs are no longer needed for the support or propulsion of the body in long-tailed air-breathers, their connection with the body becomes less and less firm, long before their entire disappearance. In animals using the legs for crawling or walking the bones of an arm and thigh are elongated, and the joints are always well formed, permitting varied, extensive, and firm movements. Just the reverse is the tendency in all those animals that propel themselves by the aid of the tail in the water, since here what is needed is broad, short limbs, not long and slender ones.

Most reptiles have five digits on each hand or foot; the bones of the wrist and ankle are well formed, as in mammals, and the digits are elongate, with a very definite arrangement of the bones composing them, as already described, never exceeding five in any one finger or toe.

In the paddles of water reptiles, as the limbs are usually called, the bones of the first segment, that is, the humerus and femur, are always greatly shortened in those having a propelling tail, and even in some with a short tail, such as the seals, and in a lesser degree in the sea-otters. On the other hand, in those animals which use the legs chiefly for direct propulsion these bones are elongated, as exemplified by the plesiosaurs and marine turtles. In all save the seals and their kind, and the otters, whose legs are used rather as sculls than as oars, the bones of the next segment, the radius and ulna of the front pair, the tibia and fibula of the hind pair, are always shortened, and one can tell the stage of aquatic adaptation, as exemplified, for instance, in the plesiosaurs and ichthyosaurs by the degree of shortening of these bones. Indeed, the first suggestion in any crawling animal of water habits is shown in the relative lengths of the epipodial bones, as these bones are called. Furthermore, cursorial or terrestrial habits are suggested by the relative size of the smaller bone of the leg, that on the little-toe side, the fibula. In birds, pterodactyls, and most running animals, it disappears in part or wholly. In swimming animals it tends to grow larger than the tibia, as will be conspicuously seen in the paddle of the mosasaurs.

The bones of the wrist change in two ways: by becoming cartilaginous, as in whales and salamanders, or by becoming more firmly ossified and more closely united, as in the plesiosaurs. The digits always are elongated, often extraordinarily so, either by the elongation of individual bones or phalanges, or by the development of new bones. These new bones, when they occur, are new growths, not the reproduction of the old elements of fishes, and there may be as many as twenty such new elements or phalanges in a single digit. There is one marked exception among reptiles to this hyperphalangy, as the increased number of phalanges is called, and that is the turtles. As we have seen, in the elongation of the neck among turtles there never has been an actual increase in the number of vertebrae; so also in the elongation of the digits the normal number of three in each digit has never been exceeded, except among the river turtles, where there are four in the fourth digit—possibly a relic of original conditions rather than the beginning of hyperphalangy; but the individual bones have become greatly elongated. In living reptiles, birds and mammals of the land, the fifth toe is always shorter than the fourth. In the seals, the sea-otter, and to a less degree in the muskrat, the fifth toe has become elongated. And the elongation of this toe is the first and most decisive indication of a webbed foot of strong propelling power among the aquatic reptiles of the past, as exemplified especially by the proganosaurs. Finally, in one order of extinct reptiles, the ichthyosaurs, there has been an actual increase in the number of digits, in some to as many as nine in each paddle.

In addition to all these modifications of the skeleton, the bones themselves tend to become softer and more spongy in aquatic animals. The bones of the whale, as is well known, are very spongy in texture, and those of the seals and sea-lions contain an unusually large amount of oily matter. So, too, the bones of the extinct water reptiles—of many of them at least—were more spongy than those of their land relatives; and this is due in part perhaps to their lessened use as muscular supports, in part perhaps to the necessity of a lessened specific gravity. As a rule sea-animals need to be of the same specific gravity as the water in which they live, or a little less. The bones of the living sirenians, the manatees and dugongs, so far from being light and porous, are unusually dense and solid. The sirenians live habitually at the bottom of shallow waters, feeding upon vegetable growths; and doubtless their bottom-feeding habits account for the solidity of the bones. A whale would float to the top, while a dugong would sink to the bottom, on the relaxation of all muscular movement. And we shall see that certain reptiles in the past had in all probability like bottom-feeding habits, because of the solidity of the bones of their skeletons.

Many birds and fishes have a peculiar ossification of the usually tendinous outer covering of the eyeball, called the sclerotic membrane. These ossifications form a flattened or somewhat projecting conical bony ring about the pupil of the eye. The individual bones are flat and more or less imbricated plates, with some motion between them. Accommodation for vision in reptiles, birds, and fishes is not the simple process that it is in mammals, where it is controlled by simple ciliary muscles which compress the lens, causing it to assume a more spherical or a more flattened form, thus changing the focus. In reptiles accommodation is effected by the compression of the eyeball by means of external muscles, elongating it and causing its front part to expand or project. The imbricated sclerotic plates permit this expansion and contraction of the eyeball. Under great internal or external air pressure the cornea, the only unprotected part, must necessarily change its contour unless some compensatory force is brought to bear to counterbalance it; and this doubtless was the function of the sclerotic plates so commonly present in aquatic reptiles.

Among terrestrial reptiles there are not a few examples of the ossification of such sclerotic plates, notably among the skink lizards. Every known form of extinct reptiles of aquatic habit had them, and even some of the subaquatic dinosaurs, like Diplodocus and Trachodon. One may say with assurance that it is impossible for any reptile to become thoroughly adapted to aquatic life without acquiring large and strong sclerotic plates.

Most land reptiles are or were covered by horny scales or bony plates; the pterodactyls are the only order of terrestrial reptiles with no such covering of which we have any evidence. Such coverings are wholly unneeded for animals living in the water. Not only are they unnecessary, but the increased resistance to the water would be more or less detrimental to rapid swimming. It is for these reasons doubtless that bony plates or horny scales disappeared for the most part from the skin of all truly aquatic reptiles and mammals.

The foregoing are the chief acquired characteristics of aquatic air-breathing animals and especially aquatic reptiles in adaptation to their new mode of life. The resemblances, sometimes striking, thus brought about in animals of very different origin and remote relationships have often been mistaken for evidences of kinship, that is, direct inheritance from common ancestors. Such acquired resemblances in unrelated animals are known as parallel or convergent evolution. It has often been difficult to distinguish between convergent evolution and direct evolution, and difficulties still perplex and trouble the student of natural history in every branch of life. Not till all such problems are solved can we hope to attain the true classification of animals and plants. The whales a century ago were considered merely breathing fishes; the ichthyosaurs until a quarter of a century ago were supposed to be the direct descendants of fishes; lizards and crocodiles were grouped together in a single order; and salamanders were called reptiles not very long ago.

Perhaps the reader will be able from the foregoing to understand and appreciate better some of the difficulties that confront the paleontologist in his attempts to solve the problems of past life; to understand why he sometimes makes mistakes, for he has by no means yet learned all the permutations of the skeleton in any class of vertebrates, and is not sure that the laws he accepts are not subject to modifications and exceptions. If he is truly scientific he hesitates long in prophesying or conjecturing.