which was formerly notoriously sterile, as Buckle observed, is now prolific in new achievement. But to this branch of biology, even more than elsewhere, has Mr. Spencer applied the doctrine that acquired variations are transmissible—that is to say, since mental evolution depends on structural evolution in the nervous system, he supposes that the nervous systems of man and other animals have arisen through the accumulation of acquired variations. It would be vain to discuss his writings in detail. The arguments already set forth against the theory that acquired traits are transmissible, apply in full force in this particular case. À priori, the transmission of acquired changes in nervous tissue seems impossible; it is unbelievable that acquired changes in nervous tissue can so affect the germ cell as to cause it after fertilization to proliferate into an organism with inborn variations similar to the acquired variations in the parent; à posteriori, though we frequently see psychical traits acquired, yet we never see acquired psychical variations transmitted, or so very rarely that such apparent transmissions may be set down as mere fortuitous coincidences.
The study of psychology is extraordinarily difficult, owing to the complexity and obscurity of the subject. We deal here with the intangible, non-material products of the functional activity of an organ, concerning which, in nearly all essential details, we are in ignorance. Though we are sure that every mental phenomenon has its physical side, yet we cannot express mind in terms of matter, and probably never shall be able to do so. Differences of structure in the nervous system, so minute as to be inappreciable to us, have commonly for their concomitants enormous psychical differences. Compare, for instance, the nervous systems on the one hand, and the mental traits on the other, of two allied
