will have served to show the vast
extent and complexity of the inquiry, and the extraordinary
contradictions and difficulties which it presents. We have direct proof
of the beneficial results of intercrossing in a great number of cases;
we have an overwhelming mass of facts as to the varied and complex
structure of flowers evidently adapted to secure this intercrossing by
insect agency; yet we see many of the most vigorous plants which spread
widely over the globe, with none of these adaptations, and evidently
depending on self-fertilisation for their continued existence and
success in the battle of life. Yet more extraordinary is it to find
numerous cases in which the special arrangements for cross-fertilisation
appear to have been a failure, since they have either been supplemented
by special means for self-fertilisation, or have reverted back in
various degrees to simpler forms in which self-fertilisation becomes the
rule. There is also a further difficulty in the highly complex modes by
which cross-fertilisation is often brought about; for we have seen that
there are several very effective yet very simple modes of securing
intercrossing, involving a minimum of change in the form and structure
of the flower; and when we consider that the result attained with so
much cost of structural modification is by no means an unmixed good, and
is far less certain in securing the perpetuation of the species than is
self-fertilisation, it is most puzzling to find such complex methods
resorted to, sometimes to the extent of special precautions against the
possibility of self-fertilisation ever taking place. Let us now see
whether any light can be thrown on these various anomalies and
contradictions.

_Intercrossing not necessarily Advantageous._

No one was more fully impressed than Mr. Darwin with the beneficial
effects of intercrossing on the vigour and fertility of the species or
race, yet he clearly saw that it was not always and necessarily
advantageous. He says: "The most important conclusion at which I have
arrived is, that the mere act of intercrossing by itself does no good.
The good depends on the individuals which are crossed differing slightly
in constitution, owing to their progenitors having been subjected during
several generations to slightly different conditions. This conclusion,
as we shall hereafter see, is closely connected with various important
physiological problems, such as the benefit derived from slig