376 HEREDITY AND ENVIRONMENT 



mestic forms came are known and it is possible 

 to compare them with their modified descend- 

 ants and thus 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 recent times. 



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 

 proportion 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- 



