CHAPTER VI
THE PROBLEM OF CLASSIFICATION
One who has studied attentively the skeleton of reptiles cannot fail to be impressed with the fact that similar or even apparently identical structures have arisen in different orders. Procoelous vertebrae, for instance, occur in crocodiles, pterodactyls, lizards, and frogs when it seems impossible that all should have been evolved from the same common ancestor with procoelous vertebrae. Snakes, some lizards, and certain Stegocephalia have a peculiar mode of articulation of the vertebrae, called zygosphenal, but their evolution from a common ancestor is impossible. For such resemblances the convenient term homoplasy has been proposed. Did they occur rarely in organisms they would not trouble us much; but they are everywhere in nature, and the problem of all classification is to distinguish between them and those characters due to heredity. Until we have learned to distinguish them our classification must remain more or less artificial.
The true end of all classification is genealogy. Some time in the Carboniferous period there was but a single kind of reptile, differing very slightly from its ancestors, and from this reptile has descended all the kinds that have ever lived. In the adaptation of its progeny to various provinces and modes of life they have divided into innumerable branches. Many of these branches were feeble and of short duration; others have continued to modern times, but none has ever reunited with another branch, even though small. Our object in classification is to determine these branches, and especially the early or primary ones. The twigs we call species, the lesser branches genera and families, the limbs orders, and the main boughs subclasses. It is easy enough relatively to distinguish the twigs and smaller branches, but it is often very difficult to determine where the limbs united with the boughs and where the boughs joined the trunk. A perfect classification would be dichotomous, each bough, limb, and branch dividing first into two, and each division again into two; but an approximation even to such a classification cannot be attained, and we must often treat groups of organisms as though
