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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