use the moths
could get all the nectar without forcing their trunks down to the very
base. The latter would be well fertilized, and the longest would on the
average be the best fertilized of all. By this process alone the average
length of the nectary would annually increase, because, the
short-nectaried flowers being sterile and the long ones having abundant
offspring, exactly the same effect would be produced as if a gardener
destroyed the short ones and sowed the seed of the long ones only; and
this we know by experience would produce a regular increase of length,
since it is this very process which has increased the size and changed
the form of our cultivated fruits and flowers.

But this would lead in time to such an increased length of the nectary
that many of the moths could only just reach the surface of the nectar,
and only the few with exceptionally long trunks be able to suck up a
considerable portion.

This would cause many moths to neglect these flowers because they could
not get a satisfying supply of nectar, and if these were the only moths
in the country the flowers would undoubtedly suffer, and the further
growth of the nectary be checked by exactly the same process which had
led to its increase. But there are an immense variety of moths, of
various lengths of proboscis, and as the nectary became longer, other
and larger species would become the fertilizers, and would carry on the
process till the largest moths became the sole agents. Now, if not
before, the moth would also be affected, for those with the longest
probosces would get most food, would be the strongest and most vigorous,
would visit and fertilize the greatest number of flowers, and would
leave the largest number of descendants. The flowers most completely
fertilized by these moths being those which had the longest nectaries,
there would in each generation be on the average an increase in the
length of the nectaries, and also an average increase in the length of
the probosces of the moths; and this would be a _necessary result_ from
the fact that nature ever fluctuates about a mean, or that in every
generation there would be flowers with longer and shorter nectaries, and
moths with longer and shorter probosces than the average. No doubt there
are a hundred causes that might have checked this process before it had
reached the point of development at which we find it. If, for instance,
the variation in the quantity of nectar had been at any