189.3 EVIDENCE FOE PHYSIOLOGICAL SELECTION 331 



logical Selection,' what I maintain is that the origin 

 of species is in all cases due to isolation of some hind, 

 but that only in the case of differential fertility can 

 physo. sel. have been the kind of isolation at work. 

 Therefore, it would be fatal to my views if all species 

 were cross-sterile, because this would prove vastly 

 too much. What the theory of phy. sel. requires 

 is exactly what occurs, viz. cross-sterility between 

 allied species in nearly all cases where species have 

 been differentiated on common areas or identical 

 stations, and more or less complete cross-fertility 

 where they have been differentiated on different (dis- 

 continuous) areas, or else prevented from intercrossing 

 by yet some other means of isolation. 



I have collected a quantity of evidence in favour 

 of both these otherwise inexplicable correlations. 

 But I should like to know the species of wild fowl 

 which you have found to be hybridisable or cross- 

 fertile, so that I may ascertain whether their natural 

 breeding areas are, or are not, identical. Of course I 

 should expect them not to be. 



I have been told to save my eyes as much as 

 possible, and therefore conduct most of my corre- 

 spondence by dictation. But not being used to this 

 process, I find it even more difficult than before to 

 express my meaning with clearness, so I will tackle 

 with my own hand what you. say about Aquilegias. 



I have looked up the group, and find that, with 

 the exception of vulgaris (common columbine), all 

 the European species seem to occupy restricted areas, 

 or else well-isolated stations. Also, that the same 

 seems to apply as a very general rule to other species 



