XXXV111 INTRODUCTION. 



then, we cannot, with many physiologists, maintain that 

 species are immutable, and exempt from the laws of change, 

 to which all organic matter seems subject, we can say that 

 species may remain unchanged for periods of time beyond 

 any to which our inquiries, for the purposes of useful infer- 

 ences, need extend. It is matter of merely speculative in- 

 quiry, whether now, as in all the period of the past, the 

 earth, the air, and the relations which connect external 

 nature with the living kingdom, are not undergoing pro- 

 gressive though insensible changes, which may in the course 

 of unmeasured periods of time, react upon all the existing 

 species, not excepting man himself. It suffices for us to know 

 that species are to us realities, and remain constant in their 

 essential characters for a time which we cannot compute. 



But there is a class of changes in organic forms which fall 

 more within our cognizance, and which merit our attention 

 in an especial degree ; this is the class of changes, which 

 produce what we term Varieties or Races, in which the spe- 

 cific type is generally so far preserved that the animals may, 

 with more or less certainty, be referred to it, although very 

 often the divergence is so great that nothing can be traced 

 beyond the affinities which we terrn generic. The human 

 races, as well as the lower tribes, are subject to this class of 

 changes, under the influence of temperature, food, habitudes, 

 and other agencies. 



Man, it has been seen, of all the Mammalia, constitutes a 

 Genus, into the circle of which none of the tribes, even the 

 nearest to him in conformation, enters. Many divisions have 

 been made of the different groups of 'men according to the 

 external characters, habits, traditions, and affinities of speech, 

 which have been supposed to connect them. 



One great division has been supposed to comprehend, gene- 

 rally, the inhabitants of Western Asia and Europe, from the 

 first historical records to the present time. This group of 

 nations has been termed Caucasian, from the mountainous 

 regions of the Caucasus, where the inhabitants have been 



