GEOGRAPHICAL DISTRIBUTION 417 



life; in the toad, lizard, robin, rabbit, and related species, 

 this fish-like condition gradually disappears. 



From their development, as well as from evidence sup- 

 plied by comparative anatomy and fossil remains, it is 

 very evident that modern vertebrates are the descendants 

 of primitive backboned gill-bearing ancestors whose 

 evolution has resulted in the thousands of modern species. 

 Broadly speaking, each vertebrate, like other animals as 

 well as plants, in its development as an individual repeats 

 its ancestral history. 



Geographical Distribution. — Although the broader 

 features of the subject will be discussed in another chap- 

 ter, the evidence supplied from this field in support of 

 evolution demands a preliminary statement. In the first 

 place, it is a most significant fact that although the ani- 

 mals and plants of oceanic islands are distinct, they never- 

 theless bear an unmistakable resemblance to those on the 

 adjacent continent. In some instances, the islands, to- 

 gether with their imprisoned inhabitants, were cut off 

 from the mainland, by means of developing channels; 

 in other cases they were populated by migrants. In 

 either event, the isolated colonies underwent evolu- 

 tionary changes and gradually became distinct. The 

 longer they remained distinct the more unlike they be- 

 came. The great differences between the animals and 

 plants of Africa and Madagascar or between those of 

 Asia and Australia, for example, are explained on the 

 basis of a separation of great antiquity. 



As a result of this evolution within a district, hedged 

 about by barriers, several closely related species may 

 evolve. Indeed it is generally true that the most closely 

 related species are found in neighboring areas. For 

 example, there are over four hundred species of humming 

 birds, all confined to the New World ; kangaroos of several 

 species are peculiar to Australia, and the adjacent islands; 

 the Old World monkeys are very distinct from those of 

 the Western Hemisphere. Where related species are 



