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 identi- 

 cal 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 dis- 

 tinguish 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 hfe they have divided into innu- 

 merable 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 famihes, the limbs orders, and the main boughs sub- 

 classes. 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 



