THE ORIGIN OF SPECIES 41 



other well-determined forms (which gives a fauna 

 its characteristic appearance) cannot be explained 

 either, except through actual blood relationship. 



The various barriers or obstacles to migration are a 

 very important factor. For instance, in islands lo- 

 cated at a certain distance from a continent, certain 

 genera of animals may be lacking entirely. No is- 

 land located over 300 miles away from a continent 

 counts in its indigenous fauna batrachians or land 

 mammals. Generally speaking, the fauna of islands 

 can give, in regard to descent, very valuable indica- 

 tions. Thus it is observable that the animal population 

 of an island, while having its peculiar characteristics, 

 resembles that of the nearest continent; also that the 

 closer the islands are to one another the greater is 

 the similarity between their faunas. 



The most conclusive evidence in favour of the 

 theory of descent, however, is^Jn^Qarwin's estima- 

 tion, that furnished by embryology. The most salient 

 fact instanced is the resemblance betweenjthe embryos 

 of -various animals, which is much closer than the re- 



semblance between the adult animals themselves. 

 The embryos of a mammal, of a bird and of a snake, 

 for example, cannot be distinguished from one another 

 at the primary stage of their development. Another 

 important point is the similarity, in the various em- 

 bryos, of homologous parts destined to become dif- 

 ferentiated at a later stage. 



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