86 EVOLUTION OF TO-DAY. 



together ; while on the other hand a nascent organ 

 would probably be very different in many respects. 

 Again, an atrophied organ would always be func- 

 tionless, while a nascent organ would have some 

 function, though frequently a different one from 

 that possessed by its developed homologue. Thus 

 it is easy to decide that the rudimentary leg of the 

 snake is an atrophied organ, while the fin of a fish is 

 probably to be considered as a nascent structure. 

 But although it may not always be possible to dis- 

 tinguish into which category to place a given organ, 

 it must be recognized that the theory of descent as 

 well as the theory of types comprehends and ex- 

 plains both. 



Summary. 



A careful study of living things has been slowly 

 but persistently forcing naturalists to the conclusion 

 that the organic world forms a unit. Species are 

 found to graduate into each other, and so, too, do 

 genera show connecting links. If we take into cog- 

 nizance the discoveries of paleontology, we find that 

 the larger groups of families and orders exhibit the 

 same tendency toward convergence. And when, 

 finally, we consider the evidence of embryology, the 

 great sub-kingdoms, and even the kingdoms of ani- 

 mals and plants, come together. The whole organic 

 world is thus found to be related in groups sub- 

 ordinate to groups, a relation which is diagrammat- 

 ically represented by a highly branching tree, each 

 twig of which is related either closely or distantly to 

 every other. And that such a conception of the re- 



