ants of the same species (_Formica rufa_) from one ant-hill to another,
inhabited apparently by tens of thousands of ants; but the strangers were
instantly detected and killed. I then put some ants taken from a very large
nest into a bottle strongly perfumed with assafoetida, and after an
interval of twenty-four hours returned them to their home; they were at
first threatened by their fellows, but were soon recognised and allowed to
pass. Hence each ant certainly recognises, independently of odour, its
fellow; and if all the ants of the same community have not some countersign
or watchword, they must present to each other's senses some distinguishable
character.

{252}

The dissimilarity of brothers or sisters of the same family, and of
seedlings from the same capsule, may be in part accounted for by the
unequal blending of the characters of the two parents, and by the more or
less complete recovery through reversion of ancestral characters on either
side; but we thus only push the difficulty further back in time, for what
made the parents or their progenitors different? Hence the belief[604] that
an innate tendency to vary exists, independently of external conditions,
seems at first sight probable. But even the seeds nurtured in the same
capsule are not subjected to absolutely uniform conditions, as they draw
their nourishment from different points; and we shall see in a future
chapter that this difference sometimes suffices greatly to affect the
character of the future plant. The less close similarity of the successive
children of the same family in comparison with human twins, which often
resemble each other in external appearance, mental disposition, and
constitution, in so extraordinary a manner, apparently proves that the
state of the parents at the exact period of conception, or the nature of
the subsequent embryonic development, has a direct and powerful influence
on the character of the offspring. Nevertheless, when we reflect on the
{253} individual differences between organic beings in a state of nature,
as shown by every wild animal knowing its mate; and when we reflect on the
infinite diversity of the many varieties of our domesticated productions,
we may well be inclined to exclaim, though falsely as I believe, that
Variability must be looked at as an ultimate fact, necessarily contingent
on reproduction.

Those authors who adopt this latter view would probably deny that each
separate variation has its