EVOLUTION OF THE COLORS OF BIRDS. 59 



parental forms. If then the union of sperm and ovum 

 be the source of new or more favorable variations other 

 than, or stronger than those of either parent, this must 

 be due to the fact that the hereditary tendencies not 

 merely commingle, but under favorable conditions com- 

 bine, in some way different indeed from, but perhaps 

 analagous to, that exemplified in chemical combina- 

 tion." 



According to this theory it is of course possible that 

 new variations could be originated by sexual admixture. 

 It would also help to explain the presence of useless 

 specific characters, which would, according to the old 

 view, be obliterated by the swamping effects of inter- 

 crossing. "If, however," says. Morgan,* "on the 

 hypothesis of combination, we have definite organic 

 compounds, instead of, or as well as, mere hereditary 

 mixtures, if, in other words, variations take definite lines 

 determined by the laws of organic combination (as the 

 nature and properties of chemical compounds are deter- 

 mined by the laws of chemical combination), then this 

 difficulty disappears." It would account also for the 

 introduction of new parts or organs, and for their pres- 

 ervation before they had reached a point of usefulness. 

 The chief objection to this view, aside from its purely 

 hypothetical nature, which the author freely concedes, 

 is the fact that it appears to prove too much. There 

 are three general hypotheses concerning modifications: 

 (1.) The variations may be "spontaneous," or due to 

 some unknown cause, and occurring in all directions 

 indiscriminately, as Wallace and other Neo-Darwinians 

 claim, so there would be ample material for natural 

 selection to work upon. According to this view external 

 influences have nothing to do with variations. (2.) 



*1. c., p. 152. 



