130 THE WORLD OF LIFE CHAP. 



formidable enemies. Lastly, the extreme wildness, sensitive- 

 ness to danger, perhaps to noise or movement of any kind, 

 would be developed, while the reduction of the supraorbital 

 process may perhaps have been beneficial by reducing the 

 width of the head, and thus allowing them to enter small 

 holes in the rocks more rapidly ; or it may possibly be 

 connected with the more nocturnal habits. We thus see 

 that all the changes that have occurred in this interesting 

 animal have no relation whatever to mere "isolation," which 

 many writers still persist in claiming as a vera causa of 

 specific change, but are all clearly traceable as the results of 

 (i) rapid powers of multiplication; (2) that small amount 

 of variability which we know occurs in all such animals ; 

 and (3) rigid selection through diurnal and nocturnal birds 

 of prey, which we have seen to play so large a part in 

 keeping down the numbers of the passenger pigeons in 

 North America, the lemming in Scandinavia, and the mice 

 in La Plata. 



The two cases now adduced, showing how nature actually 

 works in the production of slightly modified forms through 

 " variation " and " survival of the fittest," will, I think, render 

 the process of species -formation sufficiently intelligible. 

 Very slight inorganic agencies have here been seen at 

 work in one case a single severe storm, in the other a 

 change to an isolated habitat where slightly new conditions 

 prevailed. But when in the course of those periods when 

 geological changes were most actively at work, larger and 

 more permanent climatic changes occurred, or when more 

 marked diversities of soil and vegetation, with exposure to 

 more severe competition, were brought about, those modifica- 

 tions of the environment would inevitably result in more 

 marked and more varied adaptations of form, structure, or 

 habits, bringing about what we everywhere recognise as 

 perfectly distinct species. 



In the present work I do not propose to go farther into 

 this matter, which has been treated with sufficient detail and 

 with copious illustrations in my Darwinism and other works, 

 as well as in Darwin's classical volumes, The Origin of 

 Species and Animals, and Plants under Domestication. I 

 will therefore now proceed to an account of some of those 



