Chap. III. 
CHECKS TO INCREASE. 
67 
its life; that heavy destruction inevitably falls either 
on the young or old, during each generation or at 
recurrent intervals. Lighten any check, mitigate the 
destruction ever so little, and the number of the species 
will almost instantaneously increase to any amount. 
The causes which check the natural tendency of each 
species to increase in number are most obscure. Look 
at the most vigorous species; by as much as it swarms 
in numbers, by so much will its tendency to increase be 
still further increased. We know not exactly what the 
checks are in even one single instance. Nor will this 
surprise any one who reflects how ignorant we are on 
this head, even in regard to mankind, so incomparably 
better known than any other animal. This subject has 
been ably treated by several authors, and I shall, in my 
future work, discuss some of the checks at considerable 
length, more especially in regard to the feral animals of 
South America. Here I will make only a few remarks, 
just to recall to the reader’s mind some of the chief 
points. Eggs or very young animals seem generally to 
suffer most, but this is not invariably the case. With 
plants there is a vast destruction of seeds, but, from 
some observations which I have made, I believe that it 
is the seedlings which suffer most from germinating 
in ground already thickly stocked with other plants. 
Seedlings, also, are destroyed in vast numbers by various 
enemies; for instance, on a piece of ground three feet 
long and two wide, dug and cleared, and where there 
could be no choking from other plants, I marked all the 
seedlings of our native weeds as they came up, and out 
of the 357 no less than 295 were destroyed, chiefly by 
slugs and insects. If turf which has long been mown, 
and the case would be the same with turf closely browsed 
by quadrupeds, be let to grow, the more vigorous plants 
