EVOLUTION OF THE COLORS OF BIRDS. 121 



certain points, it may be well to pass on now to a brief 

 survey of his objections. The central idea of his argu- 

 ment is that variations are never, spontaneous, but 

 always due to a definite cause, so that "the inevitable 

 new species will be produced even in defiance of 

 increased fertility between the diverging forms." He 

 lays great stress upon geographical isolation, however, 

 and doubts that any new species of bird has arisen with- 

 out the aid of this factor. He propounds a rather novel 

 explanation of the mutual infertility of natural species 

 as compared with the fertility of domesticated breeds. 

 Briefly stated it is this: Domestic animals are species 

 which from some unknown reason are capable of adapt- 

 ing themselves to a great variety of circumstances. 

 Just as the individual as a whole is capable of surviving 

 in various environments, so also, it may be supposed, 

 the germ cell of the male of such individuals would have 

 a greater vitality, and would be capable of surviving in 

 an unusual ovarian environment. A proportion is thus 

 established between the individual and its seed — as the 

 stable individual is to the plastic individual so is the 

 reproductive element stable or plastic. He calls atten- 

 tion to the fact that in a state of nature fertile hybrids 

 occur most frequently among the Phasianidcv and 

 Anutidce, " the very families to which most of our do- 

 mestic birds belong." This is certainly a very plausible 

 and ingenious theory, and I am not familiar with any 

 attempted refutation. 



Unlike Mr. Wallace, Mr. Seebohm freely grants that 

 " specific differences are frequently, if not usually, with- 

 out utilitarian significance," but he considers that this is 

 due to the fact that they have been brought about by 

 definite variation without reference either to natural or 

 physiological selection. Upon one point, however, he 

 and Mr. Wallace agree, namely, that granting all that 



