DARWIN 119 



cance. Then occurs a slow change of the envi- 

 ronment. The cHmate becomes colder. Perhaps 

 an ice-age sets in, such as our earth seems to have 

 passed through many times. There are two alter- 

 natives. A very hard winter may set in at once 

 and all the sheep perish, because their woolly coat 

 is too thin in all cases. That would mean the 

 extinction of a whole species. But the severe 

 cold may come on gradually. The winters are 

 more trying. So many sheep perish in the first 

 winters ; but so many others survive. Which will 

 survive ? Naturally, those that happened to have 

 the thicker coats. Those alone live on to the 

 spring, and reproduce. The following year the coat 

 is thicker all round, as the lambs all came from 

 relatively thick-coated parents. The winter 

 decimates them again, and the thickest coated 

 survive once more, and so on. The pressure of 

 external conditions, the '^ struggle for life," selects 

 just as man does. Only the best adapted indivi- 

 duals survive and reproduce. 



The whole earth is a vast field of splendid 

 adaptations. The tree-frogs are green because 

 only green frogs are preserved ; all the others are 

 destroyed. The arctic hare is white on the snow, 

 the desert-fox yellow. For a thousand reasons 

 in the course of the earth's development these 

 backgrounds — white, yellow, green ; snow, desert, 

 forest, &c. — have themselves been constantly 

 changing under the action of Lyell's changes in 

 the crust of the earth. Hence constantly fresh 

 adaptations, with a certain percentage of complete 



