268 Heredity. 



relations of the physical and the moral may be conceived as a 

 relation of equivalence, so that in the last analysis there exists 

 only one species of phenomena, neither material nor spiritual, but 

 which, from a purely human point of view, we call physiological 

 when we grasp them from without and through the senses, and 

 psychological when we grasp them from within and through the 

 consciousness. As we have remarked, however, this is but an 

 hypothesis, the value of which will be better and better deter- 

 mined in the progress of the sciences ; but the fate of which is of 

 no importance for the experimental portion of our thesis. 



In the next place, passing from speculation to facts, from meta- 

 physics to biology, we showed, on the ground of experience, that 

 it is extremely probable, if not certain, that every mental state 

 implies a corresponding nervous state, and vice versa; so that, 

 were our science more perfect, we might from the mental state of 

 a being infer the nervous state, and from the nervous state infer 

 the mental state. 



If these premisses be accepted, the problem of the cause may 

 be more clearly stated. In fact, all our science consists in ap- 

 prehending' relations between simple phenomena or groups of 

 phenomena. We have here two groups of phenomena, the one 

 physiological and, above all, nervous, the other psychological; 

 from the standpoint of heredity there can only subsist between 

 these one or other of these three relations : 



1. A simple relation of simultaneity, physiological and psycho- 

 logical heredity being parallel, though entirely independent of one 

 another. 



2. A relation of causality, psychological heredity being con- 

 sidered as the cause, and physiological heredity as the effect. 



3. Another relation of causality, but with physiological heredity 

 as the cause, and psychological heredity as the effect. 



We will not stop to examine the first hypothesis, which appears 

 to us to be an artificial question. It rests on the strange notion of 

 nvo substances, the body and the soul, perfectly distinct, entirely 

 different, and so alien to one another, that it is matter for sur- 

 prise to find them travelling together and in constant relations 

 with one another. The question might have been put in this 

 form in the seventeenth century, but in the present state of science 



