n, "in order that they may not be devoured by field-mice;"[568] so we
see the use of the bitter principle in almonds.

Other slight differences, which would be thought quite unimportant, are no
doubt sometimes of great service both to plants and animals. The
Whitesmith's gooseberry, as formerly stated, produces its leaves later than
other varieties, and, as the flowers are thus left unprotected, the fruit
often fails. In one variety of the cherry, according to Mr. Rivers,[569]
the petals are much curled backwards, and in consequence of this the
stigmas were observed to be killed by a severe frost; whilst at the same
time, in another variety with incurved petals, the stigmas were not in the
least injured. The straw of the Fenton wheat is remarkably unequal in
height; and a competent observer believes that this variety is highly
productive, partly because the ears, from being distributed at various
heights above the ground, {233} are less crowded together. The same
observer maintains that in the upright varieties the divergent awns are
serviceable by breaking the shocks when the ears are dashed together by the
wind.[570] If several varieties of a plant are grown together, and the seed
is indiscriminately harvested, it is clear that the hardier and more
productive kinds will, by a sort of natural selection, gradually prevail
over the others; this takes place, as Colonel Le Couteur believes,[571] in
our wheat-fields, for, as formerly shown, no variety is quite uniform in
character. The same thing, as I am assured by nurserymen, would take place
in our flower-gardens, if the seed of the different varieties were not
separately saved. When the eggs of the wild and tame duck are hatched
together, the young wild ducks almost invariably perish, from being of
smaller size and not getting their fair share of food.[572]

Facts in sufficient number have now been given showing that natural
selection often checks, but occasionally favours, man's power of selection.
These facts teach us, in addition, a valuable lesson, namely, that we ought
to be extremely cautious in judging what characters are of importance in a
state of nature to animals and plants, which have to struggle from the hour
of their birth to that of their death for existence,--their existence
depending on conditions, about which we are profoundly ignorant.

_Circumstances favourable to Selection by Man._

The possibility of selection rests on variability, and this, as we