208 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 
the field mouse, which is thus an enemy 
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 
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 
Interdependence 
of species. 
The arctic birch. 
