THE GENESIS OF SPECIES. 179 



the illustration loses much or all of its point, when we 

 consider how little the circumstances of the experiment 

 would correspond with what ordinarily happens in 

 nature, how little we know whether the white man's 

 colour would be really an advantage or the reverse, and 

 how complicated are the differences between a white 

 man and a negro. If the blackness of the negro be 

 due to Natural Selection in any considerable degree, we 

 should expect it to suit the conditions which surround 

 him in his native habitation better than a white skin 

 would do. In this case the pallor introduced into the 

 breed by a solitary stranger would gradually disappear 

 in obedience to the principles of Natural Selection, not 

 in opposition to them. To take once more the instance 

 of the giraffe ; the useful variation is here by hypothesis 

 an elongated neck ; it is conceivable that out of large 

 herds the few survivors of a drought might be exclu- 

 sively such as possessed this advantage to some extent. 

 These would probably transmit to a large majority of 

 their descendants the tendency to vary in a given 

 direction which they had themselves all more or less 

 exhibited. Their progeny, moreover, would be placed 

 in exceptionally favourable circumstances by the very 

 fact that in the previous drought so many of the same 

 species had been starved to death, who would otherwise 

 have furnished their chief competitors in the struggle 

 for existence. It is still objected that upon this sup- 

 position many other animals ought to have acquired 

 giraffe-like necks. But such an expectation is far from 

 being warranted by the principles of Natural Selection. 



