CHAPTEE III. 



THE DIFFERENTIATION OF SPECIES. 



Evolution 

 may proceed 

 without dif- 

 ferentiation. 



Geographi- 

 cal Isolation. 



In the preceding chapter we discussed the swamping effects of interbreeding, and assumed 

 that it must have prevented the perpetuation of isolated instances of new beneficial varia- 

 tions. In order to accumulate a very great number of very small variations, which appears 

 to be the only way in which, for example, man can have been produced from the ancestral 

 ape, it seems as if the slight modifications of each successive generation must have been in 

 a definite direction, and must have occurred amongst a great number of individuals, at the 

 same time, in the same place, and for a great number of generations. 



The origin of species is, however, still unaccounted for. There is no reason why 

 Evolution should not go on indefinitely modifying a species from generation to generation 

 until a preglacial monkey becomes a man, and yet no second contemporary species be 

 originated. The origination of a second species is prevented by interbreeding. So long as 

 the area of distribution of the species is continuous and not too large, the constant inter- 

 marriage which takes place between the males of one family and the females of another 

 distributes the inherited and transmittable modifications throughout the race or species ; 

 which may advance or retrograde according to circumstances, but is prevented by inter- 

 breeding from originating a second species. 



Variations of this nature, which for want of a better name we call definite variations, 

 are amply sufficient to account for all the known facts of Evolution, which may proceed 

 slowly but surely, and we believe has thus proceeded from time immemorial, the rapidity 

 of its progress constantly retarded by the swamping effects of interbreeding, and constantly 

 accelerated by Natural Selection, or the weeding out of the least fit in the struggle for 

 life. 



In order to originate a second species it is necessary to counteract the levelling effects 

 of interbreeding by isolating some of the individuals comprising the species, so that there 

 may be two colonies, which are unable to communicate with each other, and consequently 

 unable to interbreed. It is not necessary to insist upon the importance of Isolation as the 

 only agent by which contemporary species can be multiplied. It is recognized by Darwin, 



