Plasticity, then, in the wide sense of the word, means the possession of a structure weak enough to yield to an influence, but strong enough not to yield all at once. Each relatively stable phase of equilibrium in such a structure is marked by what we may call a new set of habits. Organic matter, especially nervous tissue, seems endowed with a very extraordinary degree of plasticity of this sort ; so that we may without hesitation lay down as our first proposition the following, that the phenomena of habit in living beings are due to plasticity of the organic materials of which their bodies are composed.

Species do not grow more perfect: the weaker dominate the strong, again and again— the reason being that they are the great majority, and they are also cleverer. Darwin forgot the mind (—that is English!): the weak possess more mind. ... To acquire mind, one must need mind—one loses it when one no longer needs it.
[Criticism of Darwin's Origin of Species.]

Species do not evolve towards perfection: the weak always prevail over the strong—simply because they are the majority, and because they are also the more crafty. Darwin forgot the intellect (that is English!), the weak have more intellect. In order to acquire intellect, one must be in need of it. One loses it when one no longer needs it.
[Criticism of Darwin's Origin of Species.]

The species does not grow in perfection: the weak again and again get the upper hand of the strong,—and their large number and their greater cunning are the cause of it. Darwin forgot the intellect (that was English!); the weak have more intellect. ... One must need intellect in order to acquire it; one loses it when it is no longer necessary.
[Criticism of Darwin's Origin of Species.]