RESEMBLANCES AMONG- ANIMALS. 67 



temperate into an arctic district, the conditions are 

 changed. During a large portion of the year, and 

 just when the struggle for existence is most severe, 

 white is the prevailing tint of nature, and dark colours 

 will be the most conspicuous. The white varieties will 

 now have an advantage ; they will escape from their 

 enemies or will secure food, while their brown com- 

 panions will be devoured or will starve ; and as " like 

 produces like" is the established rule in nature, the 

 white race will become permanently established, and 

 dark varieties, when they occasionally appear, will soon 

 die out from their want of adaptation to their environ- 

 ment. In each case the fittest will survive, and a race 

 will be eventually produced adapted to the conditions 

 in which it lives. 



We have here an illustration of the simple and effec- 

 tual means by which animals are brought into harmony 

 with the rest of nature. That slight amount of varia- 

 bility in every species, which we often look upon as 

 something accidental or abnormal, or so insignificant as 

 to be hardly worthy of notice, is yet the foundation of 

 all those wonderful and harmonious resemblances which 

 play such an important part in the economy of nature. 

 Variation is generally very small in amount, but it 

 is all that is required, because the change in the 

 external conditions to which an animal is subject is 

 generally very slow and intermittent. When these 

 changes have taken place too rapidly, the result has 

 often been the extinction of species ; but the general 

 rule is, that climatal and geological changes go on 



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