142 THE CAUSATION OF DISEASE. 



most negative E compatible with existence ; but all that part 

 of the mental world which results from conscious education 

 certainly belongs to the region of natural variation ; for it has 

 a structural equivalent which has been acquired, and an acquired 

 structural peculiarity is a natural variation. We inherit, be it 

 remembered, no knowledge, only the power of acquiring it. We 

 inherit, in fact, an inconceivably plastic cortex, one ever ready, 

 under specific forms of E, to take on natural variations. It is true 

 that a considerable amount of knowledge could originate under 

 the most negative E consistent with life ; and this shows how 

 largely the distinction between hereditary and acquired charac- 

 ters rests upon theory. Indeed, it is probable that many of 

 our mental powers that are apparently acquired are in reality 

 strictly inherent — the sheer outcome of heredity. For much of 

 what is apparently acquired may be due, as Bastian observes,* 

 to a post-partem evolution of the nervous system which is 

 strictly developmental. The ganglia in the cervical region of 

 the spinal cord are, as we had already occasion to observe, very 

 imperfectly developed, and much of the subsequent development 

 of them, whereby the individual becomes capable of executing 

 complex movements, may be in a large measure purely develop- 

 mental. 



I have already said enough, I think, to make clear what I 

 mean by the statement that the offspring of the same parents 

 tend towards a certain mean, and that the deviations from this 

 mean are due to differences of E. Such deviations, we have 

 seen, are natural variations. Now, the theoretical mean is 

 itself a natural variation, seeing that it differs from each 

 parent, so that the nnlikeness of the parents may be looked 

 upon as one cause of natural variations. But if all individuals 

 were to come into the world exactly alike, differences would 

 be rapidly impressed upon them by the varying E, and it is 

 this varying E which must be regarded as the primary cause 

 of the variations. Having once got these differences, the 

 marriage of unlike individuals must lead to still further 

 variations, owing to the tendency towards the mean structural 

 product of the two. Hence we may regard differences in E 



* " Brain as an Organ of Mind " : International Science Series. 



