136 ORGANIC EVOLUTION— MENTAI. 



■which was formerly notoriously sterile, as Buckle ob- 

 served, is now prolific in new achievement. But to 

 this branch of biology, even more than elsewhere, has 

 Mr. Spencer applied the doctrine that acquired varia- 

 tions are transmissible — that is to say, since mental 

 evolution depends on structural evolution in the ner- 

 vous 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. A priori, the transmission of ac- 

 quired 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 ; d posterio^'i, 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 



