266 PSYCHOLOGICAL INVESTIGATION. [PART II. 



tion, and again relapse from its high state, and its capacities 

 decline ; but as the cranial shape (as is usually assumed), 

 remains the same, the assertion that the intellectual faculties 

 are dependent on it, is not even consonant with the doctrine 

 of the immutability of race-crania. We are thus compelled to 

 renounce the doctrine that the capacity of the cranium indi- 

 cates the amount of mental endowment. 



Having disposed of this preliminary question, and shown 

 that the size of the skull presents us with no criterion for 

 the peculiarities of intellectual life, we must endeavour to in- 

 dicate the path we ought to pursue in these investigations. 



As, in physical respects, all men may be considered as be- 

 longing to the same species, if it can be proved that the 

 greatest physical differences occurring among them, are not 

 more considerable than such as may have arisen in the 

 same people in the course of time ; so may we, in psychical 

 respects, count all as belonging to the same species, if it can 

 be shown that the greatest differences of their mental develop- 

 ment and their intellectual and moral culture, are not greater 

 than the differences of the degrees of civilization which the 

 same people passes through in its history. Here the question 

 is not either to prove or to refute that at present, e. g. an 

 individual Negro, or the Negro race generally, is capable of 

 the same intellectual performances as an European civilized 

 people; for nations are as much dependent on the historical 

 basis of their vital development as individuals, and it is impos- 

 sible that peoples passing through different stages of develop- 

 ment should be capable of the same intellectual performances. 

 But if by the term capacity be designated what alone should 

 be designated by it not the possible performances at any 

 given time, but such as are possible to the living generation 

 under the most favourable circumstances; then it becomes 

 clear, not only that the capacities of a people may change in 

 the course of time, but that the judgment, as regards the 

 unity of mankind, depends on the solution of the question, 

 whether, under favourable circumstances, in the course of 

 time, all nations and tribes are capable, or not, of reaching the 

 same degree of mental development. 



