CHAPTEK XXVII 



Artificial selection Continuous and single selection The mutation theory 

 of the origin of species Mutual adaptation Unit characters 

 Isolation Physiological selection Non-adaptive characters The 

 evolution of man. 



IT has long been recognized that much light may be thrown 

 upon the problem of the origin of species by the careful 

 study of the methods which mankind has adopted for the 

 improvement of the various races of cultivated plants and 

 domesticated animals. Many such races have been so greatly 

 modified that, did they occur in what is commonly called a state 

 of nature, we should be obliged to regard them as distinct species. 

 The history of some of these is lost in antiquity and we have no 

 positive knowledge of the methods by which the improvement of 

 wild species was first effected. We may assume, however, with 

 some degree of confidence, that the earliest breeders and 

 cultivators would select for cultivation and propagation those 

 individuals which offered them the most valuable qualities, and 

 that they would reject such as exhibited marked signs of 

 inferiority. This process, repeated from generation to generation 

 through thousands of years, and aided in each generation by the 

 direct^ effects of cultivation, could not fail to bring about con- 

 spicuous results. For an account of what has been effected 

 in this manner the student should consult Charles Darwin's 

 classical work on the " Variation of Animals and Plants under 

 Domestication." 



The almost unconscious efforts of our ancestors have given 

 place in modern times to deliberate and systematic attempts to 

 discover the principles upon which the improvement of cultivated 

 races, both of plants and animals, should be based. 



Perhaps no species of plants have been more improved by 

 man than the various cereals upon which he relies so largely for 

 his food supply. Professor de Vries, in his interesting book on 

 " Plant-Breeding," 1 describes how such improvement has been 



J Kegan Paul, Trench, Trubner & Co., London, 1907. 



