MODIFICATION OF SPECIES 367 



are thought to have come mostly from the former, while table birds 

 have inherited more from the latter. The various breeds of pigs are 

 all regarded as descendants of two wild boar species, one from Europe 

 and Africa, the other from India. Dogs probably have a somewhat 

 greater variety of wild ancestry, since their characteristics indicate con- 

 tributions from the timber wolf of Russia, the jackal of Europe, the 

 coj^ote of North America, and the dingo of Australia. Sea island cotton 

 is probably derived from two wild species, upland cotton from at least 

 three. Corn has an obvious relative in wild teosinte, but it is likely that 

 other species of grasses are also ancestral to it. 



All this modification of breeds is evolution of a sort. That Darwin 

 was justified in concluding from it that selection has been likewise the 

 guiding factor in nature, some biologists have doubted. For domestic 

 breeds exhibit one important quality which is uncommon in natural 

 species; they are generally interfertile. The several kinds of dogs differ 

 from one another structurally quite as much as wild species do; but they 

 can be crossed, while wild species usually cannot be. It has often been 

 argued that if selection were responsible for species formation in nature, 

 these species should be as fertile with one another as are domestic varie- 

 ties. This criticism overlooks one difference between the selecting agents. 

 Man is vitally interested in maintaining interfertility of" his stocks, for 

 his method requires that he cross them. If sterility had arisen between 

 individuals, because of chromosome aberrations or for any other reason, 

 those individuals would have been rejected. In nature, such individuals 

 would have survived if lucky and if otherwise fit. By keeping his stocks 

 fertile among themselves, and by crossing them frequently, man has 

 speeded up the process of change far beyond any rate that might have 

 occurred naturally. Man's goals have also been very different from 

 those to which natural selection leads. But in no other important respect 

 have the two processes been unlike. 



Evolution of Man. — The fossil evidence of man's origin was briefly 

 outlined in the preceding chapter. Whether there has been any impor- 

 tant evolution in man since he attained the capacities of Cro-Magnon 

 man, for example, is uncertain. There is no historical evidence of such 

 change. It is often said that man has made no progress in physical or 

 mental qualities in the last 10,000 years. This statement may be true, 

 but there is no way to know. It would be expected that there had been 

 some evolution during that time. Man is extraordinarily heterozygous, 

 and there is much hybridization between stocks. Presumably also muta- 

 tions arise in man. Unless all individuals survive, and all are equally 

 fertile, it is difficult to see how evolution can fail to occur. Whether 

 that evolution is progress upward or not is another matter. 



Since man has guided the evolution of his flocks and herds, it would 



