BIRDS AND NATURE 
15 
combined flock is that composed of several species of tit- 
mouse, nuthatches, small woodpeckers and tree-creepers, 
which, keeping together, systematically clean every part of 
the trees, dividing the labour in accordance with the peculiar 
structure of each several bird. 
Hitherto we have treated birds only in their relation to 
intact Nature. 
This relation changes, the moment man, in his own inter- 
ests, interferes with Nature's work or, as we are wont to 
say, makes Nature his slave, at the same time producing 
essential changes in her progress. 
By breaking up the virgin earth to sow it with cereals 
or plant herbs therein; by rooting out woods to use their 
trees or cultivate the territory they cover; by the regulation 
or draining off of waters, thus making essential changes 
in the relation of land to water — by all these and in 
similar ways man changes the condition of life of plants 
and animals. 
By rotatory cultivation, carried out on a large scale, man 
multiplies lower-grade animals, particularly insects, that are 
dependent on the products of the earth, supplying them 
with favourable conditions of subsistence, the natural result 
of which is increase in numbers. And it is this which, from 
a human point of view, involves damage, against which 
man is bound to defend himself as best he can. 
The surest and simplest means of defence for man to 
adopt would be to restore the natural state; as, however, in 
the interest of his own subsistence, he is unable to do that, 
he is obliged to defend himself by measures that will ward 
off or at least diminish the damage. And that means that 
he must artificially replace those conditions which, by disturb- 
ing the natural state, he has done away with or essentially 
altered. 
