and the Darwinian Theory. 157 



original stock are subjected to different forms of selection, 

 while at the same time some cause prevents free crossing 

 between the different sections. We now inquire whether 

 Darwin has made us acquainted with any cause or combina- 

 tion of causes that, without the aid of man, produces diversity 

 of selection and at the same time the independent generation 

 of the different classes of variations thus preserved. 



Darwin discusses the causes of natural selection more fully 

 than the causes of diversity of natural selection. He does not 

 speak of uniformity and diversity of natural selection, but of 

 the individuals of the same species living under the same ex- 

 ternal conditions as being modified in the same way, and of 

 those living under dissimilar external conditions as being 

 modified in different ways. Again, he speaks of " the diver- 

 gent tendency of natural selection," resulting from " the 

 principle of benefit being derived from divergence of cha- 

 racter," as explaining divergence of character in the members 

 of one species competing with each other on a common area. 

 How the contradictions in the two statements are to be recon- 

 ciled, and how, in the second case, the unifying influence of 

 free crossing is prevented, he does not show, so far as I can 

 discover. As the subject is of the highest importance in the 

 explanation of divergent evolution, and as it is specially 

 desirable to get as clear an understanding as possible of Dar- 

 win's method of explanation, I shall consider his reasoning 

 somewhat fully. 



Same Degree of Local Separation under Different Environ- 

 ments. 



Darwin often speaks of the influence of crossing in retarding 

 or preventing the formation of new races and species ; but, 

 from the following extracts from his l Origin of Species,' it 

 will be seen that it is not quite so clear what combination of 

 causes he considered necessary for the production of two or 

 more species from one original species. The obscurity in his 

 statements results, I think, from the fact that " a new species " 

 may be one that has been formed by monotypic transforma- 

 tion, the old form disappearing with the production of the new, 

 or it may be one that has arisen through polytypic transform- 

 ation, which is the modification of one branch of the species, 

 while other branches remain either unmodified or modified in 

 other ways. For the formation of a new species, in the 

 former meaning of the word, he evidently did not consider it 

 necessary that the species or any part of it should enter a new 



Ann. & Mag. N. Hist. Ser. 6. Vol. v. 12 



