EVOLUTION OF THE COLORS OF BIRDS. 123 



may be due to the change in season of flowering or pair- 

 ing, or to difference in food or climate, or they may even 

 be originated by natural selection itself. Indeed, Mr. 

 Wallace has suggested as an " alternative hypothesis" 

 to physiological selection, that specific sterility is due to 

 a relation existing between the external coloring and 

 sexual compatibility, so that whenever a change in color 

 is produced, the individuals so changed will not be fer- 

 tile with the parent stock. Mr. Romanes clearly shows 

 that instead of being an alternative hypothesis, if 

 tenable at all, it merely furnishes us with an additional 

 indirect cause of the infertility demanded by physio- 

 logical selection. Mr. Romanes gives a number of 

 forcible reasons for not granting this theory the promi- 

 nent function attributed to it by Mr. Wallace. Thus he 

 asserts that "many species which are mutually sterile 

 differ very little in color," while " most species which 

 are mutually fertile differ considerably " in this respect. 

 Furthermore, "in. the case of natural species, it often 

 happens that a great difference in respect of fertility 

 occurs, according to which has acted as the male and 

 which as the female, yet in both these crosses the colour 

 of each species is, of course, the same." 



The controversy which Mr. Romanes had with Mr. 

 W. Thiselton-Dyer, in the columns of Nature in 1889, 

 respecting physiological selection, did not serve to eluci- 

 date many new points. Mr. Dyer's objections to the 

 terms physiological selection and the segregation of 

 the fit, need not here detain us, for Mr. Romanes had 

 previously justified their use. The discussion centered 

 upon the relation between natural and physiological se- 

 lection. Mr. Romanes has so clearly expressed this in 

 an admirable figure that to quote it, will be a sufficient 

 epitome of the argument. He says: "In short, species 

 are like leaves, successive and transient crops of which 



