rs (Trifolium pratense and incarnatum) do not on a hasty
glance appear to differ in length; yet the hive-bee can easily suck
the nectar out of the incarnate clover, but not out of the common red
clover, which is visited by humble-bees alone; so that whole fields of
the red clover offer in vain an abundant supply of precious nectar to
the hive-bee. Thus it might be a great advantage to the hive-bee to have
a slightly longer or differently constructed proboscis. On the other
hand, I have found by experiment that the fertility of clover greatly
depends on bees visiting and moving parts of the corolla, so as to push
the pollen on to the stigmatic surface. Hence, again, if humble-bees
were to become rare in any country, it might be a great advantage to the
red clover to have a shorter or more deeply divided tube to its corolla,
so that the hive-bee could visit its flowers. Thus I can understand how
a flower and a bee might slowly become, either simultaneously or one
after the other, modified and adapted in the most perfect manner to each
other, by the continued preservation of individuals presenting mutual
and slightly favourable deviations of structure.

I am well aware that this doctrine of natural selection, exemplified in
the above imaginary instances, is open to the same objections which were
at first urged against Sir Charles Lyell's noble views on "the modern
changes of the earth, as illustrative of geology;" but we now very
seldom hear the action, for instance, of the coast-waves, called a
trifling and insignificant cause, when applied to the excavation of
gigantic valleys or to the formation of the longest lines of inland
cliffs. Natural selection can act only by the preservation and
accumulation of infinitesimally small inherited modifications, each
profitable to the preserved being; and as modern geology has almost
banished such views as the excavation of a great valley by a single
diluvial wave, so will natural selection, if it be a true principle,
banish the belief of the continued creation of new organic beings, or of
any great and sudden modification in their structure.

ON THE INTERCROSSING OF INDIVIDUALS.

I must here introduce a short digression. In the case of animals
and plants with separated sexes, it is of course obvious that two
individuals must always unite for each birth; but in the case of
hermaphrodites this is far from obvious. Nevertheless I am strongly
inclined to believe that with all hermaph