878 TEXTBOOK OF ZOOLOGY 



center of the range of the genus, and the more specialized species 

 are found in the scattered outskirts of the range. 



From the preceding statements concerning distribution it seems 

 that any given species originates in a definite locality, that it multi- 

 plies there and migrates in all possible directions. It modifies as 

 it goes in response to the various new conditions prevailing and 

 becomes divided into local varieties which in the course of time 

 become species. Thus the working method of animal distribution, as 

 it has been presented, is the principle of descent from preceding 

 generations with modification. 



Morphological Evidence. — Classification of the animals shows in 

 fact something of the morphological evidence, since current classi- 

 fication is based chiefly on anatomical features and comparative 

 anatomy. The groups of the classification are established largely 

 on anatomical similarities. The differences existing among the rep- 

 resentatives of all the classes of vertebrates are relatively slight 

 when set over against the fundamental similarities. Closely related 

 groups show numerous similarities in the form of homologies. The 

 flipper of the whale, the wing of the bat or the bird, the foreleg of 

 the cat and the arm of man all show the same general type of 

 structure in spite of certain specific differences. There is seldom 

 any question of their phylogenetic relationship. 



The presence of series of similar parts on different segments of 

 the same animal and the various specializations of these parts show 

 a progressive development. The highly specialized walking legs, 

 uropod, claws, and antennae of crayfish, for example, have all devel- 

 oped from the simple swimmeret type of appendage. They form a 

 serial homology and are also homologous to the appendages of all 

 other Crustacea as well. 



In higher forms of animals, such as man, there are numerous struc- 

 tures which seem to be useless and are even harmful in some in- 

 stances. These are spoken of as vestigial structures. Such parts 

 correspond in structure and plan to functional parts in other re- 

 lated forms, but are reduced morphologically and without the origi- 

 nal function. In man, one probably thinks first of the vermiform 

 appendix of the colon as such a structure. This is apparently func- 

 tionless in human beings and can be removed with no loss, but this 

 same organ in rabbits, some birds, and other animals is an extensive 

 and functional digestive organ. Man has a coccyx or vestigial tail, 



