Chap. III. MUTUxiL CHECKS TO INCREASE. 75 
yirgin forests. What a struggle between the several 
kinds of trees must here have gone on during long cen¬ 
turies, each annually scattering its seeds by the thousand; 
what war between insect and insect—between insects, 
snails, and other animals with birds and beasts of prey—- 
all striving to increase, and all feeding on each other or 
on the trees or their seeds and seedlings, or on the other 
plants which first clothed the ground and thus checked 
the growth of the trees! Throw up a handful of feathers, 
and all must fall to the ground according to definite 
laws; but how simple is this problem compared to the 
action and reaction of the innumerable plants and 
animals which have determined, in the course of cen¬ 
turies, the proportional numbers and kinds of trees now 
growing on the old Indian ruins! 
The dependency of one organic being on another, as 
of a parasite on its prey, lies generally between beings 
remote in the scale of nature. This is often the case 
with those which may strictly be said to struggle witli 
each other for existence, as in the case of locusts and 
grass-feeding quadrupeds. But the struggle almost in¬ 
variably will be most severe between the individuals of 
the same species, for they frequent the same districts, 
require the same food, and are exposed to the same 
dangers. In the case of varieties of the same species, 
the struggle will generally be almost equally severe, 
and we sometimes see the contest soon decided : for 
instance, if several varieties of wheat be sown together, 
and the mixed seed be resown, some of the varieties 
which best suit the soil or climate, or are naturally the 
most fertile, will beat the others and so yield more 
seed, and will consequently in a few years quite sup¬ 
plant the other varieties. To keep up a mixed stock 
of even such extremely close varieties as the variously 
E 2 
