I grant frankly that I do not believe that the forms which I have selected represent exactly the ancestors of man.  They have all been more or less modified.  I claim only that in the balance and relative development of their organic systems—­muscular, digestive, nervous, etc.—­they give us a very fair idea of what our ancestor at each stage must have been.  But it is on this balance and relative development of the different systems, that is, whether an animal is more reproductive, digestive, or nervous, that my argument will in the main be based.

But if the older ancestors have so generally disappeared, and their surviving relatives have been so greatly modified, how can we make even a shrewd guess at the ancestry of higher forms?  The genealogy of the animal kingdom has been really the study of centuries, although the earlier zooelogists did not know that this was to be the result of their labors.  The first work of the naturalist was necessarily to classify the plants and animals which he found, and catalogue and tabulate them so that they might be easily recognized, and that later discovered forms might readily find a place in the system.  Hypotheses and theories were looked upon with suspicion.  “Even Linnaeus,” says Romanes, “was express in his limitations of true scientific work in natural history to the collecting and arranging of species of plants and animals.”  The question, “What is it?” came first; then, “How did it come to be what it is?” We are just awakening to the question, “Why this progressive system of forms, and what does it all mean?”

Let us experiment a little in forming our own classification of a few vertebrates.  We see a bat flying through the air.  We mistake it for a bird.  But a glance at it shows that it is a mammal.  It is covered with hair.  It has fore and hind legs.  Its wings are membranes stretched between the fingers and along the sides of the body.  It has teeth.  It suckles its young.  In all these respects it differs from birds.  It differs from mammals only in its wings.  But we remember that flying squirrels have a membrane stretching along the sides of the body and serving as a parachute, though not as wings.  We naturally consider the wings as a sort of after-thought superinduced on the mammalian structure.  We do not hesitate to call it a mammal.

The whale makes us more trouble; it certainly looks remarkably like a fish.  But the fin of its tail is horizontal, not vertical.  Its front flippers differ altogether from the corresponding fins of fish; their bones are the same as those occurring in the forelegs of mammals, only shorter and more crowded together.  Later we find that it has lungs, and a heart with four chambers instead of only two, as in fish.  The vertebrae of its backbone are not biconcave, but flat in front and behind.  And, finally, we discover that it suckles its young.  It, too, is in all its deep-seated characteristics a mammal.  It is fish-like only in characteristics which it might easily have acquired in adaptation to its aquatic life.  And there are other aquatic mammals, like the seals, in which these characteristics are much less marked.  Their adaptation has evidently not gone so far.