anent increase, except in restricted
localities, is almost impossible. For example, our own observation must
convince us that birds do not go on increasing every year in a
geometrical ratio, as they would do, were there not some powerful check
to their natural increase. Very few birds produce less than two young
ones each year, while many have six, eight, or ten; four will certainly
be below the average; and if we suppose that each pair produce young
only four times in their life, that will also be below the average,
supposing them not to die either by violence or want of food. Yet at
this rate how tremendous would be the increase in a few years from a
single pair! A simple calculation will show that in fifteen years each
pair of birds would have increased to nearly ten millions! whereas we
have no reason to believe that the number of the birds of any country
increases at all in fifteen or in one hundred and fifty years. With such
powers of increase the population must have reached its limits, and have
become stationary, in a very low years after the origin of each species.
It is evident, therefore, that each year an immense number of birds must
perish--as many in fact as are born; and as on the lowest calculation
the progeny are each year twice as numerous as their parents, it follows
that, whatever be the average number of individuals existing in any
given country, _twice that number must perish annually_,--a striking
result, but one which seems at least highly probable, and is perhaps
under rather than over the truth. It would therefore appear that, as far
as the continuance of the species and the keeping up the average number
of individuals are concerned, large broods are superfluous. On the
average all above _one_ become food for hawks and kites, wild cats and
weasels, or perish of cold and hunger as winter comes on. This is
strikingly proved by the case of particular species; for we find that
their abundance in individuals bears no relation whatever to their
fertility in producing offspring. Perhaps the most remarkable instance
of an immense bird population is that of the passenger pigeon of the
United States, which lays only one, or at most two eggs, and is said to
rear generally but one young one. Why is this bird so extraordinarily
abundant, while others producing two or three times as many young are
much less plentiful? The explanation is not difficult. The food most
congenial to this species, and on which it th