But the amount of the change which is beneficial has its
limits, and then a greater amount is injurious. A change to a climate a
few degrees warmer or colder may be good, while a change to the tropics
or to the arctic regions might be injurious.
Thus we see that, both slight changes of conditions and a slight amount
of crossing, are beneficial; while extreme changes, and crosses between
individuals too far removed in structure or constitution, are injurious.
And there is not only a parallelism but an actual connection between the
two classes of facts, for, as we have already shown, many species of
animals and plants are rendered infertile, or altogether sterile, by the
change from their natural conditions which occurs in confinement or in
cultivation; while, on the other hand, the increased vigour or fertility
which is invariably produced by a judicious cross may be also effected
by a judicious change of climate and surroundings. We shall see in a
subsequent chapter, that this interchangeability of the beneficial
effects of crossing and of new conditions, serves to explain some very
puzzling phenomena in the forms and economy of flowers.
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