364 MB. G. J. EOMANF.S ON PHYSIOLOGICAL SELECTION'. 



Boimiilly occurs withiB each pareut form. When, for instance 

 we are told by Gartner that the yellow and white varieties of one 

 species of Veriascum are considerably more fertile with the simi- 

 laily coloured varieties of distinct species than they are with the 

 differently coloured varieties of the same species, we can only 

 conclude that the state of the reproductive system is such that 

 there is a higher degree of sterility — or a lesser degree of sexual 

 affinity — within the limits of the parent form, than there is 

 between it and another variety so far changed as to constitute a 

 distinct species. I do not, of course, pretend that in these cases 

 the species towards which the increased fertility is exhibited has 

 been separated from the other by physiological selection. Indeed 

 to do this would be to prove too much, because if the separation 

 had been effected by physiological selection, there ought as a 

 result to be increased sterility, and not increased fertility between 

 these two particular specific forms. But I adduce these facts as 

 forcible evidence of physiological selection, because they show, 

 in the strongest imaginable way, that the conditions of sexual 

 affinity which are required for physiological selection are to be 

 found even between varieties so widely separated as to constitute 

 true species. For if these conditions of sexual affinity may be 

 such that an organism is actually more fertile with members of 

 a distinct species than it is with members of its own species, 

 much more may an organism which has become infertile with its 

 parent form continue fertile with itself. In the cases mentioned 

 the individual sexual organs may be regarded as relatively sterile 

 towards their parent, i. e. their own specific form, while relatively 

 fertile towards another specific form. Much more then may an 

 individual be relatively sterile towards its pareut form, while 

 relatively fertile towards its own varietal form. 



The same argument may be adduced from the case of animals. 

 There are many recorded instances of both birds and mammals 

 which, when under confinement, have proved themselves more 

 fertile with members of different species than with members of 

 their own. Now, whether this state of matters be supposed to 

 be normal or superinduced by changes in the conditions of life, 

 in either case we have organisms which are relatively sterile 

 towards their own parent form, or relatively fertile towards 

 another varietal form so different as to constitute a distinct 

 species. As in the similar case of the plants above mentioned, 

 th erefore, we may here repeat how much more probable than 



