22 Gulick — Divergent Evolution and the Darwinian Theory. 



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 modi- 

 fied in different ways. Again, he speaks of ' " the divergent 

 tendency of natural selection," resulting from " the principle 

 of benefit being derived from divergence of character," as ex- 

 plaining divergence of character in the members of one species 

 competing with each other on a common area. How the con- 

 tradictions in the two statements are to be reconciled, 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 diver- 

 gent evolution, and as it is specially desirable to get as clear an 

 understanding as possible of Darwin's method of explanation, 

 I shall consider his reasoning somewhat fully. 



Some degree of Local Separation under Different Environments. 



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 Origin of Species, it will 

 be seenihat 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 state- 

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

 be one that has been formed by monotypic transformation, the 

 old form disappearing with the production of the new, or it 

 may be one that has arisen through polytypic transformation 

 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 neces- 

 sary that the species or 2j\y part of it should enter a new 

 environment, or that crossing should be prevented. But did 

 he not consider both these conditions necessary for the forma- 

 tion of two or more species from one original species ? 



He says " Intercrossing will affect those animals most which 

 unite for each birth and wander much, and which do not breed 

 at a very quick rate. Hence with animals of this nature, for 

 instance birds, varieties will generally be confined to different 

 countries ; and this I find to be the case. With hermaphro- 

 dite organisms which cross only occasionally, and likewise with 

 animals which unite for each birth, but which wander little 

 and can increase at a very rapid rate, a new and improved 

 variety might be quickly formed on any one spot, and might 

 there maintain itself in a body and afterward spread, so that 



