2o6 THE OSTEOLOGY OF THE REPTILES 



radiating from a common center. And it is also evident that such 

 divisions occurred rapidly. Many of the first groups of species that 

 branched as twigs from the common stem were the ancestors of or- 

 ders, for they held, all of them, possibilities of great developments; 

 succeeding species became more and more restricted in their 

 potentialities. 



Our chief object, then, in classification is to trace the history of 

 each species, genus, family, and order to its separation from allied 

 forms, and to give to each minor and major group a name and place. 

 And our chief difiiculty in doing this is to determine whether the re- 

 semblances that they show to each other have been due to descent 

 and common heritage, or have been the result of common environ- 

 mental influences. The problems are hard and always will be hard 

 because actual proofs of heredity must ultimately rest on the facts of 

 paleontology, and paleontological history is and always will be im- 

 perfect. In all probability the earth since remote ages has always 

 been as densely populated with living organisms as it is at the present 

 time, and rapidly or slowly in different kinds of organisms evolution 

 and extinction have replaced the faunas and floras many times. There 

 are to-day living upon the earth about twenty thousand species of 

 air-breathing vertebrate animals, and doubtless there has been no 

 time since the first general invasion of land by air-breathers that the 

 number has been less ; it may have been greater, since man has ex- 

 erted a powerful influence upon them. As the only air-breathers of 

 paleozoic times were amphibians and reptiles, there must have been, 

 during the time that they reigned supreme, — from the Mississippian 

 to the Jurassic, millions of years, — scores of thousands of their kinds; 

 we know but a few hundreds. Had we records of all that have lived, 

 the major problems would be much easier, the minor ones greatly 

 increased. 



Nevertheless, in tracing the genealogies of organisms, that is, in 

 classifying them, we are aided by general laws which have obtained 

 recognition among students of extinct animals. First of all, by the 

 law that evolution is irreversible, that organs or functions once lost 

 can never be regained by descendants ; similar organs or similar func- 

 tions often, but never the original ones. By the general law that 

 there has been a continuous loss of parts; we can trace, for instance, 

 probably every bone of the human skull back to the primitive rep- 



