PREFACE XI 



According to that hypothesis, two factors are at 

 work, variation and selection. Next to nothing is 

 known of the causes of the former process ; nothing 

 whatever of the time required for the production 

 of a certain amount of deviation from the existing 

 type. And, as respects selection, which operates 

 by extinguishing all but a small minority of 

 variations, we have not the slightest means of 

 estimating the rapidity with which it does its 

 work. All that we are justified in saying is that 

 the rate at which it takes place may vary almost 

 indefinitely. If the famous paint-root of Florida, 

 which kills white pigs but not black ones, were 

 abundant and certain in its action, black pigs 

 might be substituted for white in the course of 

 two or three years. If, on the other hand, it was 

 rare and uncertain in action, the white pigs might 

 linger on for centuries. 



T. H. HUXLEY. 



HODESLEA, EASTBOURNE, 

 April, 1894. 



