368 HEREDITY AND ENVIRONMENT 



to determine the degree of change which has 

 been brought about under human guidance. 

 In other cases where the original wild species 

 are unknown it is possible to determine the 

 amount of modification which has taken place 

 within relatively recent times. DeCandolle 

 recognizes 247 species of cultivated plants, 193 

 of which still exist in the wild state. 



The degree of change which has taken place 

 under human guidance is very remarkable. 

 In some cases dozens and even hundreds of 

 races have been formed, showing the most re- 

 markable differences in size, structure and 

 proportions of parts, as well as in functions, 

 instincts and behavior. The extent to which 

 heredity may be guided by man is forcibly il- 

 lustrated by our present races of domestic 

 pigeons which Darwin said would be classed 

 by any naturalist who did not know their ori- 

 gin in not less than twenty different species 

 and three different genera, though all of them 

 have descended from the wild rock pigeon 

 (Figs. 84, 85) ; or by the numerous races of 

 dogs varying in size from a toy dog to a Great 

 Dane or St. Bernard and showing almost un- 



