Variation and Heredity 267 



maintain a given race under given conditions. " Each new 

 adolescent generation is not the product of the entire preced- 

 ing generation, but only of selected individuals. This is cer- 

 tainly the case for civilized man, in which case twenty-six per 

 cent of the married population produce fifty per cent of the 

 next generation." 



Pearson believes that " if a race has been long under the 

 same environment it is probable that only periodic selection 

 is at work, maintaining its stability. Change the environ- 

 ment and a secular change takes place, the deviations from 

 the mode previously destroyed giving the requisite material." 

 " Clearly periods of rapidly changing environment, of great 

 climatological and geological change, are likely to be asso- 

 ciated with most marked secular selection. To show that 

 there is little or no change year by year in the types of rab- 

 bit and wild poppy in our English fields, or of daphnia in our 

 English ponds, is to put forward no great argument for the 

 inefficiency of natural selection. Take the rabbit to Australia, 

 the wild poppy to the Cape, the daphnia into the laboratory, 

 and change their temperature, their food supply, and the 

 chemical constituents of water and air, and then the exist- 

 ence of no secular selection would indeed be a valid argument 

 against the Darwinian theory of evolution." In regard to 

 the last point, it should be noted that, even if under the 

 changed conditions a change in the mode took place, as 

 Pearson assumes, it does not follow necessarily that selection 

 has had anything to do with it, but the environment may have 

 directly changed the forms. Furthermore, and this is the 

 essential point, even if selection does act to the extent of 

 changing the mode, we should not be justified in concluding 

 that this sort of change could go on increasing as long as the 

 selection lasts. All that might happen would be to keep the 

 species up to the highest point to which fluctuating variation 

 can be held. This need not lead to the formation of new 

 species, or direct the course of evolution. 



