484 .NATURAL HISTORY OF MAN. 



And first, as to the criteria which best determine the 

 identity or diversity of species recognised as such. Limiting 

 our present view to that part of the scale most approximate 

 to Man, we may state the following conditions as those 

 to which we must apply for solution in each particular case ; 

 viz. the anatomical structure in all its parts ; the average 

 duration of life ; the relation of the sexes and laws of 

 propagation, including the periods of utero-gestation and 

 number of progeny ; the production, or otherwise, of hybrid 

 progeny by mixed breeding; the liability to the same 

 diseases ; and the possession of the same instincts, faculties, 

 and habits of action and feeling. It will readily be admitted 

 that wherever individuals or groups of beings concur as to 

 all these general conditions, there the proof of identity of 

 species is complete. But we have already spoken of that 

 capacity for variation within certain limits in each species, 

 which may as justly be called a law of nature as the division 

 into species itself; and we are in no instance entitled to 

 expect entire conformity to the several conditions stated 

 above. In recurring to them hereafter it will be seen that 

 each condition includes a liability to such variations, more 

 or less, for every species; and it would seem that this 

 increases as we rise upwards in the scale of animal life. In 

 the higher animals, and notably in man himself, this capacity 

 for variation shows itself peculiarly in all that regards the 

 instincts, habits, and mental faculties, as modified by climate, 

 food, culture, and other contingencies. In the phenomena 

 more strictly of physical organisation, a lesser amount of 

 change is likely to occur ; yet here also (and it is an im- 

 portant point in the argument) the familiar experience of 

 every one will indicate innumerable such varieties, more 

 striking as the research is more extended and minute. 



Taking these circumstances into account, our demand for 

 proof of the identity of species need not go beyond a general 



