372 THE HISTOEY OF CEEATION. 



only in the simplest non-sexual manner. Even now all 

 Protista, as well as all the countless forms of cells, which 

 constitute the body of higher organisms, multiply themselves 

 only by non-sexual generation. And yet there arise here 

 " new species " by differentiation in consequence of natural 

 selection. 



But even if we were to take into consideration the animal 

 and vegetable species with separate sexes, in this case too 

 we should have to oppose Wagner's chief proposition, that 

 " the migration of organisms and their formation of colonies 

 is the necessary condition of natural selection." August 

 Weismann, in his treatise on the "Influence of Isolation 

 upon the Formation of Species," ^* has already sufficiently 

 refuted that proposition, and has shown that even in one 

 and the same district one bi-sexual species may divide itself 

 into several species by natural selection. In relation to this 

 question, I must again, call to mind the great influence 

 which division of labour, or differentiation, possesses, being 

 one of the necessary results of natural selection. All 

 the different kinds of cells constituting the body of the 

 higher organisms, the nerve cells, muscle cells, gland cells, 

 etc., all these " good species," these " bonse species " of 

 elementary organisms, have arisen solely by division of 

 labour, in consequence of natural selection, although they 

 not only never were locally isolated, but ever since their 

 origin have always existed in the closest local relations one 

 with another. Now, the same reasoning that applies to these 

 elementary organisms, or " individuals of the first order," 

 applies also to the many-celled organisms of a higher order 

 which only at a later date have arisen as " good species " 

 from among their fellows. 



