the hot valleys of the equatorial Cordillera sheep are not fully
fecund;[389] and according to Lord Somerville,[390] the merino-sheep
which he imported from Spain were not at first perfectly fertile. It is
said[391] that mares brought up on dry food in the stable, and turned
out to grass, do not at first breed. The peahen, as we have seen, is
said not to lay so many eggs in England as in India. It was long before
the canary-bird was fully fertile, and even now first-rate breeding
birds are not common.[392] In the hot and dry province of Delhi, the
eggs of the turkey, as I hear from Dr. Falconer, though placed under a
hen, are extremely liable to fail. According to Roulin, geese taken
within a recent period to the lofty plateau of Bogota, at first laid
seldom, and then only a few eggs; of these scarcely a fourth were
hatched, and half the young birds died: in the second generation they
were more fertile; and when Roulin wrote they were becoming as {162}
fertile as our geese in Europe. In the Philippine Archipelago the
goose, it is asserted, will not breed or even lay eggs.[393] A more
curious case is that of the fowl, which, according to Roulin, when
first introduced would not breed at Cusco in Bolivia, but subsequently
became quite fertile; and the English Game fowl, lately introduced, had
not as yet arrived a its full fertility, for to raise two or three
chickens from a nest of eggs was thought fortunate. In Europe close
confinement has a marked effect on the fertility of the fowl: it has
been found in France that with fowls allowed considerable freedom only
twenty per cent. of the eggs failed; when allowed less freedom forty
per cent. failed; and in close confinement sixty out of the hundred
were not hatched.[394] So we see that unnatural and changed conditions
of life produce some effect on the fertility of our most thoroughly
domesticated animals, in the same manner, though in a far less degree,
as with captive wild animals.

It is by no means rare to find certain males and females which will not
breed together, though both are known to be perfectly fertile with
other males and females. We have no reason to suppose that this is
caused by these animals having been subjected to any change in their
habits of life; therefore such cases are hardly related to our present
subject. The c