Appendix C. 173 



disconnected. They may both have originated on the same 

 area; or one may have diverged from the other before it 

 migrated from that area ; or even if, when it migrated, it was 

 unchanged, and if in its new home it afterwards split into two 

 species by physiological selection, the newer species would 

 probably prove infertile, not only with its parent type, but 

 also with its grand-parent in any other part of the world. 



Seebohm on Isolation. Seebohm is so strongly influenced 

 by the difficulty from " the swamping effects of free intercross- 

 ing," that he is driven by it to adopt Asa Gray's hypothesis 

 of variations as Ideological. Indeed, he goes as far as 

 Wagner, for he maintains that in no case can there be 

 divergence or multiplication of species without isolation. 

 He makes the important statement that "the more the 

 geographical distribution of birds is studied, the more doubtful 

 it seems to be that any species of bird has ever been differen- 

 tiated without the aid of geographical isolation" (Charadriidae, 

 p. 17). If this is true, it makes in favour of physiological 

 selection by showing the paramount importance of the 

 swamping effects of intercrossing, and consequent impor- 

 tance of isolation. But it makes against physiological 

 selection by showing that the geographical form of isolation 

 is sufficient to explain all the cases of specific differentiation 

 in birds. But I must remember that the latter point rests 

 largely on negative inference, and that birds, owing to 

 their highly locomotive habits, are the class of animals where 

 physiological selection is likely to be most handicapped. 



Herbert on Hybridization. Herbert tells us that when he 

 first astonished ihe Horticultural Society by laying before them 

 the results of his experiments on hybridization, his brother 

 botanists took serious alarm. For it appeared to them that 

 this " intermixture of species would confuse the labours 

 of botanists, and force them to work their way through 

 a wilderness of uncertainty." Therefore he was bluntly told 



