II 
THE STRUGGLE FOR EXISTENCE 
27 
the parents is killed ; while they offer a defenceless prey to 
jackdaws, jays, and magpies, and not a few are ejected from 
their nests by their foster-brothers the cuckoos. As soon as 
they are fledged and begin to leave the nest great numbers 
are destroyed by buzzards, sparrow-hawks, and shrikes. Of 
those which migrate in autumn a considerable proportion are 
probably lost at sea or otherwise destroyed before they reach a 
place of safety ; while those which remain with us are greatly 
thinned by cold and starvation during severe winters. Exactly 
the same thing goes on with every species of wild animal and 
plant from the lowest to the highest. All breed at such a rate, 
that in a few years the progeny of any one species would, if 
allowed to increase unchecked, alone monopolise the land ; 
but all alike are kept within bounds by various destructive 
agencies, so that, though the numbers of each may fluctuate, 
they can never permanently increase except at the expense of 
some others, which must proportionately decrease. 
Cases showing the Great Powers of Increase of Animals. 
As the facts now stated are the very foundation of the 
theory we are considering, and the enormous increase and 
perpetual destruction continually going on require to be kept 
ever present in the mind, some direct evidence of actual cases 
of increase must be adduced. That even the larger animals, 
which breed comparatively slowly, increase enormously when 
placed under favourable conditions in new countries, is shown 
by the rapid spread of cattle and horses in America. 
Columbus, in his second voyage, left a few black cattle at St. 
Domingo, and these ran wild and increased so much that, 
twenty-seven years afterwards, herds of from 4000 to 8000 
head were not uncommon. Cattle were afterwards taken 
from this island to Mexico and to other parts of America, and 
in 1587, sixty-five years after the conquest of Mexico, the 
Spaniards exported G4,350 hides from that country and 
35,444 from St. Domingo, an indication of the vast numbers 
of these animals which must then have existed there, since 
those captured and killed could have been only a small portion 
of the whole. In the pampas of Buenos Ayres there were, at 
the end of the last century, about twelve million cows and 
three million horses, besides great numbers in all other parts 
