140 THE WOELD OF LIFE 



which Darwin thinks they w^ere nndonbtedly derived. Their 

 numerous enemies would at first capture the larger, more bulky, 

 and slower-moving individuals, then the white or black speci- 

 mens, who would be more easily seen and pounced upon. This 

 process, continuously acting for a few generations, would result 

 in a smaller and more dusky-coloured race. The continuous 

 attack persisting, the size would be again reduced, and the most 

 agile and rapid in movement would alone survive. There- 

 after, the nocturnal habit would be acquired by the day-feeders 

 being almost exterminated, and owls would probably alone 

 remain as formidable enemies. Lastly, the extreme wildness, 

 sensitiveness to danger, perhaps to noise or movement of any 

 kind, would be developed, while the reduction of the supra- 

 orbital 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 con- 

 nected 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 (1) 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 ISTorth America, the lemming in Scan- 

 dinavia, 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 



