1893 EVIDENCE FOE PHYSIOLOGICAL SELECTION 305 



you at the Athenseum, he undertook some two or three 

 years ago on my behalf to raise discussions in the 

 papers, to which he alludes. Since that time he has 

 sent me, I believe, copies of all the numberless letters 

 which have been published in consequence. The 

 result of our inquiry has been to confirm the opinion 

 which he gave me at the first, and also to form my 

 own in the same direction. (See my article in 

 answer to Herbert Spencer in the ' Contemporary 

 Review ' for April.^) 



As regards the isolation of species I do not 

 understand why you should suppose that the facts of 

 hybridisation to which you allude should in any way 

 modify my ' belief.' As fully set forth in ' Physio- 

 logical Selection,' what I maintain is that the origin 

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

 but that only in the case of difierential 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 

 alhed 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. 



^ Mr. Herbert Spencer on ' Natural Selection,' Contemporary Beview, 

 April 1893. 



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