142 HEREDITY AND SELECTION IN SOCIOLOGY 



it ; and the question wlietlier, were life differently constituted, 

 it would be feasible for selection to adapt other forms of life 

 to other conditions is an idle one. 



C. — Influence of Isolation. 



Isolation necessarily plays an important role in the evolution 

 of species. Darwin laid particular stress on the influence which 

 species sharing the same habitat exert on one another. Thus, 

 the vegetation of Paraguay would be considerably different from 

 what it is at present if cattle or horses existed there in a 

 feral state ; but the absence of cattle and horses is due to the 

 prevalence of a certain fly which lays its egg in the navels of 

 these animals when first bom. The number of these flies depends 

 on that of other insects, and they, in turn, depend upon the 

 existence of certain insectivorous birds. In the same way, in 

 England, humble-bees are the only bees which visit red clover, 

 as other bees cannot reach the nectar. But the number of 

 humble-bees in any district depends, in great measure, upon 

 the number of field-mice ; and the number of field-mice is de- 

 pendent on the number of cats ; so that, eventually, the amount 

 of red clover in a district is intricately bound up with the number 

 of cats in that district.^ These well-known examples which 

 Darwin gives of the mutual influence exerted by species having 

 the same habitat suffice to give an illustration of what we 

 mean. 



An acquaintance with these phenomena prepares us to expect 

 their corollary, that any change in the flora or fauna of a given 

 district must necessarily react on all the other species in- 

 habiting that district, probably causing them to vary in a greater 

 or lesser degree. In the same way, when the habitat of a species 

 is changed, when it finds itself transported into new conditions, 

 a variation will set in ; for the species will have to undergo 

 * The Origin of Species, pp. 89, 90. 



