APHORISMS AND REFLECTIONS 59 



ever, which justify the biologisi in assigning any, 

 even approximately definite, period of tinae, either 

 long or short, to the evolution of one species from 

 another by the process of variation and selection. 

 In the elsay on Geological Contemporaneity and 

 Persistent Types of Life I have taken pains to 

 prove that the change of animals has gone on 

 at very different rates in different groups of living 

 beings ; that some types have persisted with little 

 change from the palaeozoic epoch till now, while 

 others have changed rapidly within the limits 

 of an epoch. In 1862 (see Coll. Ess. viii. pp. 303, 304) m 

 1863 (vol. ii., p. 461) and again in 1864 {ibhl, pp. 89-91) 

 I argued, not as a matter of speculation, but from 

 palseontological facts, the bearing of which I believe, 

 up to that time, had not been shov/n, that any 

 adequate hypothesis of the causes of evolution must 

 be consistent with progression, stationariness and 

 retrogression, of the same type at different epochs ; 

 of different types in the same epoch ; and that 

 Darwin's hypothesis fulfilled these conditions. 



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, v/hich 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. 



