ange of food cannot be the chief cause; for ostriches, ducks,
and many other animals, which must have undergone a great change in this
respect, breed freely. Carnivorous birds when confined are extremely
sterile; whilst most carnivorous mammals, except plantigrades, are
moderately fertile. Nor can the amount of food be the cause; for a
sufficient supply will certainly be given to valuable animals; and there is
no reason to suppose that much more food would be given to them, than to
our choice domestic productions which retain their full fertility. Lastly,
we may infer from the case of the elephant, chetah, various hawks, and of
many animals which are allowed to lead an almost free life in their native
land, that want of exercise is not the sole cause.

It would appear that any change in the habits of life, whatever these
habits may be, if great enough, tends to affect in an inexplicable manner
the powers of reproduction. The result {160} depends more on the
constitution of the species than on the nature of the change; for certain
whole groups are affected more than others; but exceptions always occur,
for some species in the most fertile groups refuse to breed, and some in
the most sterile groups breed freely. Those animals which usually breed
freely under confinement, rarely breed, as I was assured, in the Zoological
Gardens, within a year or two after their first importation. When an animal
which is generally sterile under confinement happens to breed, the young
apparently do not inherit this power; for had this been the case, various
quadrupeds and birds, which are valuable for exhibition, would have become
common. Dr. Broca even affirms[385] that many animals in the Jardin des
Plantes, after having produced young for three or four successive
generations, become sterile; but this may be the result of too close
interbreeding. It is a remarkable circumstance that many mammals and birds
have produced hybrids under confinement quite as readily as, or even more
readily than, they have procreated their own kind. Of this fact many
instances have been given;[386] and we are thus reminded of those plants
which when cultivated refuse to be fertilised by their own pollen, but can
easily be fertilised by that of a distinct species. Finally, we must
conclude, limited as the conclusion is, that changed conditions of life
have an especial power of acting injuriously on the reproductive system.
The whole case is quite peculiar, for th