Suppose, for instance, that we assert that cold is the actual cause of the winter-sleep of marmots. It is clear that this statement is incorrect, and that not the cold, but the peculiar organization of the marmot causes the reaction of hibernation: cold cannot throw a dog or a bird into a state of slumber for the winter. We are here, therefore, concerned with a special adaptation of the organism to a stimulus—cold—which affects it in such a manner that it escapes from what would otherwise be a destructive influence. We are unable to demonstrate with the microscope the fine 'molecular' or histological variations in the nervous and other systems on which the capacity for hibernation may depend; but some such modifications must exist, and they cannot be regarded as a direct effect of the cold, but must rather be looked upon as arrangements to counteract its influence. Their origin, moreover, can only be assigned to processes of selection.
A thousand other cases are to be explained in a similar manner.
The leaves of a Mimosa, on being touched, bend down and close, the touch merely serving as an exciting stimulus: the actual cause of the movement is due to the peculiar constitution of the plants. The recent observations of Stahl and Haberlandt (see Note I, p. 55) have shown more clearly than ever the advantage which accrues to the plant by this sensitiveness of the delicate leaves; for by its means the plant can to some extent escape from the effects of the falling drops of a tropical rain-shower, by exposing the edges only of the leaflets to its violence.
