380 ME. G. J. EOMAKES ON PHYSIOLOGICAL SELECTION. 



And here we catch sight of another respect in which physiolo- 

 gical selection probably cooperates with natural selection. As 

 previously remarked, Mr. Darwin felt profoundly the strength of 

 this objection from sterility between species, and, I may now 

 add, he tried to imagine some way in which the general fact of 

 such sterility might be reasonably attributed to natural selection. 

 If he could have done this, of course he would have mitigated the 

 diflSculty ; for, as he writes, "it would clearly be advantageous to 

 two varieties or incipient species if they could be kept from 

 blending, on the same principle that, when man is selecting at the 

 same time two varieties, it is necessary that he should keep them 

 separate." But, as the result of his discussion, he concludes: — 

 " In considering the probability of natural selection having come 

 into action in rendering species mutually sterile, the greatest diffi- 

 culty will be found to be in the existence of many graduated 

 steps from slightly lessened fertility to absolute sterility. It may 

 be admitted that it would profit an incipient species, even if it 

 were rendered in some slight degree sterile when crossed with 

 its parent-form or with some other variety ; for thus fewer 

 bastardized and deteriorated oflFspring would be produced to com- 

 mingle their blood with the new species in process of formation. 

 But he who will take the trouble to reflect on the steps by which 

 this first degree of sterility could be increased through natural 

 selection to that high degree which is common with so many 

 species, will find the subject extraordinarily complex. After 

 mature reflection it appears to me that this could not have been 

 effected through natural selection." 



Now, with this conclusion I fully agree ; but it will by this 

 time be clearly seen that what cannot be efi'ected by natural selection 

 may well be effected by physiological selection. For both the con- 

 siderations whichMr. Darwin here candidly adduces as insuperable 

 difficulties in the way of supposing sterility due to natural selection , 

 are just theconsiderations which most strongly favour thehypothesis 

 of physiological selection. These two considerations are, first," the 

 many graduated steps from slightly lessened fertility to absolute 

 sterility," and, second, " the steps by which this first degree of 

 sterility would be increased." Now, as already shown in a previous 

 part of this paper, these " many graduated steps " are just what 

 we might expect to find on the theory of physiological selection ; 

 while, upon this theory, there is no need to suppose that "the 

 first degree of sterility " must necessarily go on increasing. In 



