18 PHYSICAL INVESTIGATION. 



of the same complex of causes by which that typical similarity is 

 maintained, the term expresses, to the exclusion of every hypo- 

 thesis, merely the fact as observed, and presents no difficulty 

 whatever. But if it be requisite, since differential characters be- 

 tween individuals are nowhere wanting in nature, that in the 

 notion of species there should be included a criterion accord- 

 ing to which we might decide as to the range of variation for 

 each type that is to say, to determine the limits of each, or what 

 magnitude or qualities of the differences between individuals 

 might justify us in including them in the same type or not 

 then the definition of the term species becomes difficult, or rather 

 not the definition itself, but the laying down of a rule as to the 

 extent of the variation for each type to which the term is to be 

 applied. The only positive and valid proof that a certain num- 

 ber of individuals belong to the same species, proceeds from the 

 demonstration that they have descended from the same original 

 stock ; and in all doubtful cases, the question of unity of species 

 can only be decided by analogy with those cases in which unity 

 of stock has been amply demonstrated. But as the extent of 

 this range of variation, and consequently unity or diversity of 

 descent can, in comparatively few cases, be decided by direct 

 evidence, there remains a wide room for doubt as regards 

 unity or difference of species. This is the more the case, as on 

 the one hand within the very same stock later generations ex- 

 hibit considerable deviations from the preceding, and, on the 

 other hand, these deviations, arising in the course of time, may 

 be so constantly transmitted that it cannot with any certainty 

 be maintained, that they do not possess the same absolute con- 

 stancy as that attributed to specific characters. These actual 

 phenomena are designated by the term varieties, which are 

 partly individual, or merely transitory, or more or less perma- 

 nent, in which case the term race is used. The difficulty 

 consists in determining the difference between species and per- 

 manent variety, or race. 



We purpose reviewing the principal attempts made in this 

 direction, in order to pave the way for our investigation of the 

 specific unity of mankind, and learn what weight is to be at- 

 tached to arguments derived from natural history. 



