Darwinism (1889): An exposition of the theory of natural selection, with some of its applications — John Shaqi
Darwinism (1889): An exposition of the theory of natural selection, with some of its applicationsWallace, Alfred Russel
Science
Darwinism (1889): An exposition of the theory of natural selection, with some of its applications
Wallace, Alfred Russel
Evolution; Natural selection
ever-present war, though by its means alone, perhaps, we are saved from
famine and pestilence.
Let us now consider a less extreme and more familiar case. We possess a
considerable number of birds which, like the redbreast, sparrow, the
four common titmice, the thrush, and the blackbird, stay with us all the
year round These lay on an average six eggs, but, as several of them
have two or more broods a year, ten will be below the average of the
year's increase. Such birds as these often live from fifteen to twenty
years in confinement, and we cannot suppose them to live shorter lives
in a state of nature, if unmolested; but to avoid possible exaggeration
we will take only ten years as the average duration of their lives. Now,
if we start with a single pair, and these are allowed to live and breed,
unmolested, till they die at the end of ten years,--as they might do if
turned loose into a good-sized island with ample vegetable and insect
food, but no other competing or destructive birds or quadrupeds--their
numbers would amount to more than twenty millions. But we know very well
that our bird population is no greater, on the average, now than it was
ten years ago. Year by year it may fluctuate a little according as the
winters are more or less severe, or from other causes, but on the whole
there is no increase. What, then, becomes of the enormous surplus
population annually produced? It is evident they must all die or be
killed, somehow; and as the increase is, on the average, about five to
one, it follows that, if the average number of birds of all kinds in our
islands is taken at ten millions--and this is probably far under the
mark--then about fifty millions of birds, including eggs as possible
birds, must annually die or be destroyed. Yet we see nothing, or almost
nothing, of this tremendous slaughter of the innocents going on all
around us. In severe winters a few birds are found dead, and a few
feathers or mangled remains show us where a wood-pigeon or some other
bird has been destroyed by a hawk, but no one would imagine that five
times as many birds as the total number in the country in early spring
die every year. No doubt a considerable proportion of these do not die
here but during or after migration to other countries, but others which
are bred in distant countries come here, and thus balance the account.
Again, as the average number of young produced is four or five times
that of the parents, we ought to have at least five times as many birds
in the country at the end of summer as at the beginning, and there is
certainly no such enormous disproportion as this. The fact is, that the
destruction commences, and is probably most severe, with nestling birds,
which are often killed by heavy rains or blown away by severe storms, or
left to die of hunger if either of 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
Public-domain text, read in full here on John Shaqi.
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