IV METHODS AND RESULTS OF ETHNOLOGY 251 



these slow and gradual, but immense alterations, 

 were wholly or in part inhabited before the 

 changes I have indicated began and it is more 

 probable that they were than that they were not 

 what a wonderfully efficient " Emigration Board " 

 must have been at work all over the world long 

 before canoes, or even rafts, were invented ; and 

 before men were impelled to wander by any desire 

 nobler or stronger than hunger. And as these 

 rude and primitive families were thrnst, in the 

 course of long series of generations, from land 

 to land, impelled by encroachments of sea or of 

 marsh, or by severity of summer heat or winter 

 cold, to change their positions, what opportunities 

 must have been offered for the play of natural 

 selection, in preserving one family variation and 

 destroying another ! 



Suppose, for example, that some families of a 

 horde which had reached a land charged with the 

 seeds of yellow fever, varied in the direction of 

 woolliness of hair and darkness of skin. Then, if 

 it be true that these physical characters are 

 accompanied by "comparative or absolute exemp- 

 tions from that scourge, the inevitable tendency 

 would be to the preservation and multiplication of 

 the darker and woollier families, and the elimi- 

 nation of the whiter and smoother haired. In 

 fact, by the operation of causes precisely similar to 

 those which, in the famous instance cited by Mr. 

 Darwin, have given rise to a race of black pigs in 



