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. 
