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![E] 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 few 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 or 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.

+--------------------------------------------------------------+
| [E] This is under estimated. The number would really amount |
| to more than two thousand millions! |
+--------------------------------------------------------------+

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