busily some insects have been shown to be
engaged in conveying anther-dust from flower to flower, especially bees,
flower-eating beetles, and the like, it seems a most enigmatical problem
how it can happen that promiscuous alliances between distinct species
are not perpetually occurring.

How continually do we observe the bees diligently employed in collecting
the red and yellow powder by which the stamens of flowers are covered,
loading it on their hind legs, and carrying it to their hive for the
purpose of feeding their young! In thus providing for their own progeny,
these insects assist materially the process of fructification.[830] Few
persons need be reminded that the stamens in certain plants grow on
different blossoms from the pistils; and unless the summit of the pistil
be touched with the fertilizing dust, the fruit does not swell, nor the
seed arrive at maturity. It is by the help of bees chiefly, that the
development of the fruit of many such species is secured, the powder
which they have collected from the stamens being unconsciously left by
them in visiting the pistils.

How often, during the heat of a summer's day, do we see the males of
dioecious plants, such as the yew-tree, standing separate from the
females, and sending off into the air, upon the slightest breath of
wind, clouds of buoyant pollen! That the zephyr should so rarely
intervene to fecundate the plants of one species with the anther-dust of
others, seems almost to realize the converse of the miracle believed by
the credulous herdsmen of the Lusitanian mares--

Ore omnes versae in Zephyrum, stant rupibus altis
Exceptantque leves auras: et saepe sine ullis
Conjugiis, vento gravidae, mirabile dictu.[831]

But, in the first place, it appears that there is a natural aversion in
plants, as well as in animals, to irregular sexual unions; and in most
of the successful experiments in the animal and vegetable world, some
violence has been used in order to procure impregnation. The stigma
imbibes, slowly and reluctantly, the granules of the pollen of another
species, even when it is abundantly covered with it; and if it happen
that, during this period, ever so slight a quantity of the anther-dust
of its own species alight upon it, this is instantly absorbed, and the
effect of the foreign pollen destroyed. Besides, it does not often
happen that the male and female organs of fructification, in different
species, arrive at a state of maturit