14 GENETICS AND EUGENICS 



simpler and less specialized than those of existing horses. 

 The general distribution of organisms throughout the geo- 

 logical strata agrees, moreover, in a remarkable way with 

 what is to be expected on the evolution theory. 

 ^ "6. Changes under Domestication. Among domesticated 

 animals and plants we know of numerous cases in which the 

 actual origin of new forms has been observed. These have 

 often differed from their predecessors by amounts quite com- 

 parable with the differences by which natural species or even 

 genera are separated. A notable example of this process is 

 afforded by the numerous breeds of pigeons known to have 

 arisen under domestication from a single wild species. We 

 have no reason whatever for supposing that domesticated 

 species are more mutable than wild species, and there is con- 

 sequently every reason to believe that changes of a similar 

 character take place in Nature. The conditions of domesti- 

 cation, of course, afford much better opportunities of observ- 

 ing such phenomena. 



\ " 7. The Observed Facts of Mutation. Nevertheless, indi- 

 vidual specimens of particular wild species are frequently 

 found showing modifications which, if they occurred con- 

 stantly in an isolated group, would afford a basis for the 

 description of new species. In a few cases the actual occur- 

 rence of similar changes has been observed in wild species of 

 plants. 



" We see, therefore, that the evidence in favour of the 

 existing species of animals and plants, having arisen by a 

 process of evolution, is of a most ample and convincing kind." 



How some of these evidences first presented themselves 

 to Darwin's mind and how he came later to value them, Dar- 

 win states in the closing pages of the Introduction to his 

 Variation of Animals and Plants under Domestication, 



WTien I visited, during the voyage of H. M. S. Beagle, the Galapagos 

 Archipelago, situated in the Pacific Ocean about five hundred miles from 

 South America, I found myself surrounded by peculiar species of birds, 

 reptiles, and plants, existing nowhere else in the world. Yet they nearly 

 all bore an American stamp. In the song of the mocking-thrush, in the 

 harsh cry of the carrion-hawk, in the great candlestick-like opuntias, I 



