566 APPLIED BIOLOGY 



that were easily accessible to the descendants of their com- 

 mon ancestors. The animals and plants on islands near con- 

 tinents are very like those of the mainland. Within any 

 continent, the species show a correlation with the possibility 

 of distribution. For example, the crayfishes of eastern North 

 America, though of many species, belong to one genus. On 

 the west coast there is another genus. As might be expected, 

 the high mountains have kept the two genera apart. There 

 are thousands of known cases in which mountains, rivers, and 

 seas have been barriers confining groups of animals or plants 

 to a given territory, with the result that their descendants 

 show close relationship. 



The fact that there are many species of both animals and 

 plants which occur in both Asia and America and in Europe 

 and America suggests that there was once some land con- 

 nection over which the ancestors of existing forms might have 

 passed to America. That such a connection once existed 

 in the region of Behring Sea seems probable to geologists. 



Another interesting series of facts of distribution which 

 point to evolution are those relating to introduction of new 

 species into regions where they do not live naturally. In 

 America we have numerous weeds, insects, English sparrows 

 and other Old World organisms which are certainly well 

 adapted to the conditions here. Why then were they not here 

 naturally? The answer is that they were developed in 

 limited geographical territory where their direct ancestors 

 and near allies lived. This is why hundreds of species of 

 organisms occur only in one part of the world, often on a single 

 island, in a valley, river, or other region isolated by natural 

 barriers. 



498. Experimental Evidences. The most convincing 

 evidence of descent is obtained from study of domesticated 

 animals and plants, with which for thousands of years man 

 has, consciously and unconsciously, been experimenting. 

 That organisms are changeable is proved beyond doubt by 



