168 ORGANIC EVOLUTION — MENTAL 



several nervous systems are much alike. In love of 

 life, in desire for food, in social feeling, &c., the different 

 species resemble one another, but in this or that 

 particular, in the nest-building, the fighting, the slave- 

 getting, or other instincts, they differ. In them, a 

 difference in nervous tissue, so minute as to be 

 inappreciable to us, results in mental products that 

 differ greatly. Now the huge brain of man, in which, 

 as we cannot doubt, various tracts subserve definite 

 functions, is subject to great variations in size (as 

 shown by differences in the cranial capacity) and in the 

 complexity and amplitude of the convolutions. Surely, 

 if minute differences in the nervous systems of other 

 animals result in mental differences so vast, it is 

 possible to account for much less fundamental and 

 important differences in human faculties, as manifested 

 in different individuals, by supposing that they likewise 

 are due to more or less slight differences in the systems, 

 especially when we remember how small a mass of 

 nerve tissue — e. g. the thermal and breathing centres — 

 may subserve important functions. 



Much, very much, has been written concerning man's 

 moral nature. Numerous authors, especially theolo- 

 gians, have assured us that there is inborn in him a 

 knowledge of good and evil, that, in fact, this know- 

 ledge is instinctive, not acquired. But all. the evi- 

 dence points the other way. On both d priori and 

 a posieriori grounds we are forced to the conclusion 

 that man's moral nature is acquired, not inborn. In 

 the first place, it is impossible to conceive how the 

 possession of a high moral tone can have so affected 

 the survival rate as to secure to the possessors of it 

 such advantages in the struggle for existence, that 

 there was thereby brought about an evolution in moral 

 tone by the survival of the morally high. In the 

 second place, young infants certainly give no indication 



