386 ANTIQUITY OF EXISTING EACES OF MANKIND. CHAP. xx. 



'the Negro and Caucasian physiognomies were portrayed 

 as faithfully, and in as strong contrast, as if the likenesses of 

 these races had been taken yesterday.' In relation to the 

 same subject, I dwelt on the slight modification which the 

 Negro has undergone, after having been transported from 

 the tropics, and settled for more than two centuries in the 

 temperate climate of Virginia. I therefore concluded that, 

 ( if the various races were all descended from a single pair, we 

 must allow for a vast series of antecedent ages, in the course 

 of which the long-continued influence of external circum 

 stances gave rise to peculiarities increased in many successive 

 generations, and at length fixed by hereditary transmission.' 



So long as physiologists continued to believe that man had 

 not existed on the earth above six thousand years, they 

 might, with good reason, withhold their assent from the 

 doctrine of a unity of origin of so many distinct races ; but 

 the difficulty becomes less and less, exactly in proportion as 

 we enlarge our ideas of the lapse of time during which dif 

 ferent communities may have spread slowly, and become 

 isolated, each exposed for ages to a peculiar set of conditions, 

 whether of temperature, or food, or danger, or ways of living. 

 The law of the geometrical rate of the increase of population 

 which causes it alwa , s to press hard on the means of subsist 

 ence, would ensure the migration, in various directions, of off 

 shoots from the society first formed abandoning the area where 

 they had multiplied. But when they had gradually penetrated 

 to remote regions by land or water, drifted sometimes by 

 storms and currents in canoes to an unknown shore, barriers 

 of mountains, deserts, or seas, which oppose no obstacle to 

 mutual intercourse between civilised nations, would ensure the 

 complete isolation for tens or thousands of centuries of tribes 

 in a primitive state of barbarism. 



Some modern ethnologists, in accordance with the philoso 

 phers of antiquity, have assumed that men at first fed on the 



