482 PROGRESS OF SCIENCE IN THE CENTURY. 



of an assemblage of forms with characters constant 

 to this extent that the species was permanent and 

 discontinuous from other species. But the evolu- 

 tion-idea has changed this, and we regard species as 

 stages in a progressive development whose flux is so 

 slow that the shortness of any man's observational 

 period is almost inadequate to detect it. But the 

 flow of glaciers is not negatived by the fact that they 

 cannot be used as means of transport. 



It seems fairly certain that had not the enquirer 

 been man himself (with obvious vested interests) 

 there would never have been any discussion as to the 

 unity of the human species. The numerous races 

 are quite comparable to the races of pigeons (all 

 descendants of the wild rock-dove, Columba Ztvta), 

 or to the races of cabbages (all derived from the wild 

 kale) ; they are all, so far as we know, fertile inter 

 Be, but precise data on this subject are within 

 a comparatively narrow range; they shade off into 

 one another most perplexingly when identification 

 or definition is the object; in a word, they are 

 varieties. There are no certain cases comparable 

 to mules among mankind. 



It is possible, of course, that some of th re- 

 mains doubtfully identified as human may be those 

 of a precursor species ; it is possible, also, that some 

 form of " isolation," e.g., psychical antipathy, might 

 even now lead to the evolution of a distinct human 

 species non-fertile with the rest of mankind; but, 

 at present, the conclusion seems secure that zoologi- 

 cally considered mankind represents one species. 



We have, however, no enthusiasm on the subject, 

 remembering Darwin's verdict : " It is almost a 

 matter of indifference whether the so-called races of 

 man are thus designated, or ranked as ' species ' or 



