26 GENERAL EVOLUTION. 



from the raptores, yet tlie same hook and dentate bill reappears in 

 them, as adapted to flesh-eating habits. 



Among the reptiles, which were no doubt originally land ani- 

 mals, and derived from batrachians, we haye a large number 

 adapted to swimming in the ocean, and these not all of the same 

 stock. Thus the Plesiosauri are crocodilian in relationship, while 

 the Pythonomorpha of America were of the same that produced 

 the snake and lizard. Again, the same modification appears in the 

 Cetaceans, or whale, etc., among mammals which are primarily a 

 terrestrial division. 



It is a nice point of phylogeny (or the science of genealogy) to 

 ascertain whether adaptive or strictly ^'bathmic" (or embryonic 

 grade) characters came first in a time in a given group. Among 

 frogs we have four divisions. One has an embryonic mouth and 

 embryonic breast-bones ; another, embryonic mouth and complete 

 breast-bones ; the third, embryonic breast - bone but complete 

 mouth; and the fourth, with both complete. The first is the lowest 

 and probably the oldest in time ; agreeably to this supposition, it 

 is distributed over the whole earth. The second is East Indian 

 and African ; the third and most extensive is Australian and 

 American ; while the fourth is confined to the Old World and 

 JSTorth America. The second, third, and fourth divisions possess 

 corresponding series of genera of different structure adapted to 

 different modes of life. Some have shovels for burrowing, some 

 webs for swimming, and some palettes for adhering to branches 

 and leaves of trees. If these characters were first fixed, then those 

 with the more perfect breast-bone and mouth are descendants of 

 those with the less perfect ; if, on the other hand, the conditions 

 of mouth and breast-bone were first fixed, then each division thus 

 defined in its special region was modified into the subdivisions, 

 each adapted to a special mode of life. I have called these parallel 

 divisions "homologous groups," and probably the origin of the 

 embryonic modifications has sometimes come first, and sometimes 

 the adaptive structures have preceded. Nevertheless, the lowest or 

 most embryonic division will often have developed its own adapt- 

 ive divisions, and each of these will agree in 'producing descend- 

 ants which have advanced in the embryonic scale, and so produce 

 homologous groups. 



For these and other reasons it is concluded that the useful 

 characters, defining natural divisions of animals, have been pro- 

 duced by the special "location of growth-force" by use. Useless 



