Isolation. 7 



sists in the intentional mating of like with like to the 

 exclusion of unlike. 



The point, then, which in the first instance must be 

 firmly fastened in our minds is this : so long as there 

 is free intercrossing, heredity cancels variability, and 

 makes in favour of fixity of type. Only when as- 

 sisted by some form of discriminate isolation, which 

 determines the exclusive breeding of like with like, 

 can heredity make in favour of change of type, or 

 lead to what we understand by organic evolution. 



Now the forms of discriminate isolation, or homo- 

 gamy, are very numerous. When, for example, any 

 section of a species adopts somewhat different habits 

 of life, or occupies a somewhat different station in 

 the economy of nature, homogamy arises within that 

 section. There are forms of homogamy on which 

 Darwin has laid great stress, as we shall presently 

 find. Again, when for these or any other reasons a 

 section of a species becomes in any small degree 

 modified as to form or colour, if the species happens 

 to be one where any psychological preference in 

 pairing can be exercised as is very generally the 

 case among the higher animals exclusive breeding 

 is apt to ensue as a result of such preference ; for 

 there is abundant evidence to show that, both in birds 

 and mammals, sexual selection is usually opposed to 

 the intercrossing of dissimilar varieties. Once more, 

 in the case of plants, intercrossing of dissimilar 

 varieties may be prevented by any slight difference in 

 their seasons of flowering, of topographical stations ; 

 or even, in the case of flowers which depend on 

 insects for their fertilization, by differences in the 

 instincts and preferences of their visitors. 



