The Doctrine of Evolution: Its Basis and Its Scope

by Henry Edward Crampton

The members of another tribe of the familiar carnivora display in their own way the same kind of evidences of relationship. The varieties of domesticated dogs differ far more widely among themselves than do common cats, yet their community of ancestry is demonstrated not only by structural resemblances, but also by the striking fact that forms as diverse as the greyhound and the fox terrier can be crossed. Here again there are wild forms, like the wolf and fox and jackal, so like the domesticated members of the dog tribe that we cannot fail to recognize a common "dog-ness" and its significance as evidence of the relationship in ancestry of all these animals.

Extending our survey so as to include the other tribes of flesh-eaters, identical principles come to light. One is compelled to regard the polar and grizzly bears as obvious blood relatives of the brown bear, and even of the raccoon of our own territory. Instead of walking upon their toes like cats and dogs, these animals plant their feet flat upon the ground; and they agree in many other details of structure that place them together, but somewhat apart from the other tribes. The many kinds of seals and walruses and sea elephants form still another group displaying similar bodily characters, but differing more widely from the "cat theme" in these differences. They are all true carnivora, but in the course of their evolution they have progressively changed so as to be adapted to life in the water where they find their prey. The bones of the limbs are the same in number and arrangement as in the cat's limb, but the seal's anterior appendage or "arm" has altered in numerous ways so as to become an efficient flexible paddle, while the hind limbs have shifted posteriorly, very much as screw propellers have evolved in the history of steam vessels. How the members of the seal tribe have changed in their descent from purely terrestrial ancestors is partly explained by such intermediate animals as the otter. This form is adapted by its slender body and partly webbed feet to a semi-aquatic life; it seems to have halted at a point beyond which all of the seals have passed in their evolution.

Each one of these tribes by itself provides conclusive evidence of evolution, for it is most reasonable to regard the "theme" in every case as a product of common inheritance, while the variations of any theme are best understood as the results of adaptive changes in various directions. But the examples have disclosed a larger relation and a principle of wider scope, as indeed the assignment of all these tribes to the single natural group of the carnivora implies. These tribes are put together because comparative anatomy finds that the common characters of all cats are fundamentally like those of all dogs and bears and seals, and in these common qualities the carnivora differ from all other mammalia. Does this mean that the branches which bear respectively the various members of the several tribes are outgrowths of a single limb of the evolving animal tree? Science does not hesitate to give an affirmative answer, because, as in the case of the similar but varying domestic cats, no other explanation of tribal resemblance in structure seems so reasonable and natural.

So far the examples have been taken from one order of the highest class of backboned animals, called mammalia. When our survey is extended to other divisions of this class, additional laws of organic relationship are discovered. If in a series of evolving generations the line of modification proceeding from a terrestrial animal like a cat to semi-aquatic and marine types substantially like an otter and a seal should be carried further, it will inevitably lead to forms possessing characters such as those displayed by whales and the related porpoises, dolphins, and narwhals of the order cetacea. In their make-up all of these animals clearly possess the general characteristics of mammals, and they constitute collectively another limb which has sprung from the same stock as the carnivora, although at an earlier time. This we believe because of their plan of body and because their peculiar organization fits them even more perfectly than the seals for aquatic existence that is their only possible mode of life. In the case of the whales the bony framework of the fore limb is again like that of the cat's leg, although the whole structure is a flexible finlike paddle. The hind limb has disappeared as an efficient organ, but the significant fact is that small rudiments of hind limbs are present just where corresponding structures are placed in the seal. These vestiges cannot be reasonably accounted for, unless they are the degenerate hinder limbs of a remote four-footed ancestor. Furthermore the unborn whale possesses a complete coat of hair, which is afterwards replaced by blubber; but hair is a thatchlike coat to shed rain, as the way the hairs lie on a terrestrial mammal indicates. We are therefore forced to conclude that whales have originated from four-footed animals walking about on land, because no opposed explanation gives so reasonable an interpretation of the observed facts.

Another group of familiar animals materially reinforces the results already established. After what has been said, it will not be difficult to perceive the meaning of the resemblances among mice of the house and field, and of rats and rabbits and squirrels. All of them possess heavy curved gnawing teeth, or incisors, and lack the flesh-tearing or canine teeth. They agree in many other respects which distinguish them as a separate natural order of the mammals called the rodentia. Again we find a highly aberrant form in the flying squirrel, which leads toward an order with another plan of body. This animal is a true rodent, which lengthens its leap from branch to branch by means of a fold of skin stretching between its fore and its hind limbs. It is an animated aeroplane, and it shows in part how bats have originated. The wing of a bat is an elastic membrane stretching not only between the two legs of one side, but also between the greatly lengthened "fingers" of the fore limb. But the bones of arm, wrist, and fingers are almost precisely the same in number and relation as in walking forms. The fact that this peculiar wing adheres to a plan belonging to the anterior legs of walking or climbing types has no reasonable explanation save that of evolution.

The well-known group of hoofed animals, including horses and cattle, is also valuable for our present purposes, as well as in a later connection when the evidence of fossils is described. The elephant possesses five toes armed with well-developed nails or hoofs. A tapir has four or three toes, and it would seem that its ancestor had had five toes, of which one or two had been lost. A rhinoceros possesses three toes, and its foot is constructed internally like the elephant's with the outer elements absent. The horse comes last with one large toe and hoof, but on either side of the main bones of this digit are vestiges of what must have been toes in its ancestors. Among the even-toed forms the hippopotamus has four which reach the ground, with a vestige of a fifth, so this animal has apparently descended from a typical mammal with the full number along a different line from that taken by the odd-toed forms. A pig has a cloven hoof, made up of what we may call the third and fourth members of a series of five digits, but the second and fifth fingers and toes are present, though they are withdrawn from the ground so as to be no longer functional; this animal seems to have proceeded further along the same line taken by the hippopotamus. A deer, with still smaller rudiments at the sides of its double foot, leads in the comparative series to the camel with a cloven hoof devoid of any such relics.

We must pass with only brief mention the lower orders of mammalia, like the insect-eating forms to which armadillos and ant-bears belong. Of greater interest are the pouched mammals like the kangaroo and opossums, which live almost exclusively in the Australian realm. The kangaroo is endowed with a head somewhat like that of a goat, and well-developed hind legs that enable it to make leaps of astonishing length. Some of its relatives, such as the bandicoot, are like rats, or like bears, as in the case of the wombat. The Tasmanian wolf is another true marsupial, even though divergent adaptation has brought it to resemble the carnivora of the dog tribe in general appearance and in special structures like the teeth. Finally at the very bottom of the mammalian scale are two small forms living in the Australian faunal region. The duckbill or Ornithorhynchus is the better known animal, with its close fur, webbed feet, and flattened ducklike beak, while its only other near relative, the Echidna, is somewhat similar to the spiny hedgehog in external appearance. A unique peculiarity of these two forms is that they produce eggs much like those of reptiles and birds, and this fact, together with others of a structural nature, brings the whole group of mammals near to the lower classes of the Vertebrata.

Looking back on the several orders of mammals, it will be seen that the last mentioned are much less differentiated or specialized in their general organization. Above the level of the egg-layers and the pouched mammals, the higher orders branch out in different directions and reach up to various levels of the scale of animal organization.

The foregoing structural evidences of organic transformation in the past histories of cats and seals and whales insistently recall the analogies of the locomotive and the ship employed at the outset. All these animals, like the mechanical examples, have come to differ in their derivation from the same original parents, and their lines of descent have diverged so as to fit the products of evolutionary modification to diverse circumstances. Even the vestigial organs of animals have their counterparts in the machines. The cowcatcher was a large and important structure in the early days of railroading, but it has become relatively useless with the decrease of grade crossings and the construction of more complete lines of fence. The structure still persists, sometimes in a greatly reduced form. Even more obvious is the change of structure in the case of masts of vessels, which originally bore the sails for propelling the ship. When steam engines were employed to give motive power, masts did not disappear. They now provide the derrick supports of trading steamers; in battleships their function is changed to that of fighting tops and signal yards. Even the poles carried by canal boats to bear windmills must be regarded as the reduced vestiges of masts originally constructed to carry sails; and their adaptive evolution, like that of countless structures in animals, has been accomplished by degeneration.

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