It is difficult to disbelieve in the potency of external influences when one sees how invariably all the vital manifestations of animals and plants are ultimately reactions to such influences, and how both animal and plant are comparable to machines so constructed that stimuli from the outer world cause them to act in the most purposeful manner for their own maintenance. How would they have become adapted in so wonderful a degree to these stimuli if they had not themselves helped in any way to bring about the result? And as every vital manifestation is a reaction to stimulus, there is hardly anything left for a developmental force to do.
Although, in my opinion, we are now quite justified in denying that evolution has taken place owing to purely internal causes, it can by no means be said that we are yet quite clear as to the way in which external influences have formed and transformed organisms. There is still a conflict between rival theories, and important points, though often apparently clear, are in reality not so.
To one such point I wish to direct your attention to-day.
It is often assumed, without much proof that a certain variation of a living being is the direct consequence of an external influence simply because the variation in question is, in fact, in some causal connexion with a definite external influence: such an assumption is, however, founded on a totally false idea as to the interconnexion of the phenomena. In many cases this will readily be granted.
