14 INTRODUCTION. 



external and internal differences may in certain tribes present 

 themselves as constant which can scarcely be denied even in 

 people originally of the same stock if it cannot be shown that 

 there is a difference in the form and mode of development of in- 

 tellectual life, if it cannot be shown that some, under equally 

 or still more favourable external conditions of development, 

 are detained in a lower scale than others by original weakness, 

 the proof of specific difference is not complete. We do not 

 mean to assert that whatever great and constant external diver- 

 sities may prevail among mankind, it still would, from a 

 similar mental endowment, follow that they belong to the same 

 species : we acknowledge in this respect the equal rights of 

 physical and psychological proofs, but we cannot, as is often 

 done, deem the latter of less importance than the former, as a 

 mere secondary consideration of not much account. The ques- 

 tion whether we have to decide for or against unity of species, 

 where there is a considerable constant physical difference com- 

 bined with equal mental endowments, or physical equality 

 with psychical dissimilarity, may be left in abeyance, as it has 

 no practical signification. Nature has seemingly relieved us 

 from this embarrassment, in combining almost everywhere the 

 same pyschical endowment with the same physical characters, 

 without, however (in individuals as little as in whole nations), 

 adopting a strict parallelism of external and internal develop- 

 ment as a fixed law. If such a parallelism, as some modern 

 authors have indicated,* cannot be shown to obtain generally in 

 the animal kingdom, since the development of the organization 

 does not always correspond to that of the intellect, and though 

 even in the human race it is still doubtful whether the degrees 

 of intellectual development correspond to those of the body, 

 and specially of the brain, there has, as yet, neither in animals 

 nor in man been found an instance of a combination of specific 

 physical equality, with a specifically different psychical endow- 

 ment. 



Though we may be justified in classing animals, of whose 

 psychical life we know so little, according to their external 



* Compare Volkmann, Art. "Gehirn," in "Wagner's Handworterb. der 

 Physiol." 



