EVOLUTION OF THE COLORS OF BIRDS. 113 



local variation which is concerned only with the sexual 

 system. Why, then, should we suppose that it differs 

 from a local variation taking place in any other part of 

 the organism? Why should we suppose that, unlike all 

 other such variations, it can never be independent, but 

 must always be superinduced as a secondary result of 

 changes taking place elsewhere?"* 



After stating at length his reasons for believing that 

 variations in the reproductive system may arise inde- 

 pendently of variations in the organism, Mr. Romanes 

 qualifies this by admitting that in some cases the varia- 

 tion in the sexual organs may be correlated with other 

 variations in the system, and may have been primarily 

 caused by natural selection; but in granting this he 

 shows that natural selection simply becomes one of the 

 causes determining physiological selection. " If," he 

 says, "we thus regard sterility between species as the 

 result of what I have called a local variation, or a varia- 

 tion arising only in the reproductive system — whether 

 this be induced by changes taking place in other parts of 

 the organism, to changes in the conditions of life, or to 

 changes inherent in the reproductive system itself — we 

 can understand why such sterility rarely, though some- 

 times, occurs in our domesticated productions; why it so 

 generally occurs in some degree between species; and 

 why as between species it occurs in all degrees." 



As to the evidence of this infertility between indi- 

 viduals of the same species, which is a necessary 

 assumption of the theory, many cases of failure to 

 interbreed have been adduced by Darwin and others. 

 Or the time of flowering or mating may be accelerated 

 or retarded in certain individuals, thus isolating them 

 from the rest of the race. 



* Nature xxxiv, pp. 338-339. 

 8 



