2o8 FOOT-NOTES TO EVOLUTION. 



illustration given us by Mr. Darwin. The clover depends 

 on the bumble-bee for the fertilization of its pods. The 



nests of the bumblebee are destroyed by 

 n er epen ence ^.j^^ ^^^j^ mouse, which is thus an enemy 

 of species. 



of the clover. The balance is restored 



by the work of the cat, who captures the mouse and pre- 

 vents its ravages on the nests of the bee. The old nur- 

 sery jingle of " the cow that tossed the dog that worried 

 the cat that killed the rat " is repeated throughout Na- 

 ture. With any change in any of the elements in this 

 series the whole equilibrium of Nature is interrupted. 

 For this equilibrium is apparent only — a sort of armed 

 neutrality, an established order of things which the 

 superficial observer mistakes for real peace and per- 

 manence. 



In some groups we find evidence of a progressive 

 adaptation of individuals to circumstances — for example, 



to climate, ending in the formation of 

 The arctic birch. , • , t i • 



new species to accord with changed con- « 



ditions of temperature. We may illustrate this by 

 means of the arctic birches. In Norway, as in most 

 northern regions with a moist climate, there are large 

 forests of birches. In the valleys, where the summers 

 are warm and reasonably long, the birches of different 

 species grow to be considerable trees. Farther to the 

 north, or higher up the mountains, the summer is too 

 short for the growth of birch trees and their place is 

 taken by birches which never pass beyond the size of 

 small bushes. Still higher up there are birches even 

 where snow falls every month of the year, and the dis- 

 tant sun gives only a glimpse of summer in July. Com- 

 petition with other plants is, of course, not severe in 

 such regions, but the birches must struggle against the 

 weather. They can live and multiply if only they can 

 adjust themselves to the conditions of life. They must 



