EVOLUTION 413 



only of selected adults. This is certainly the case for civilised 

 man, in which case 26 per cent of the married population 

 produces 50 per cent of the next generation. Notwith- 

 standing this there might be no progressive change, the 

 selection might be simply periodic. We shall distinguish 

 the two kinds of selection as periodic and secular. Both 

 are really natural selection, but the one only suffices to 

 preserve the constancy or stability of a race, the other 

 produces a secular or progressive change. 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 environment and a secular change takes place, 

 the deviations from the mode previously destroyed giving 

 the requisite material. If the environment be so 

 favourable that no individuals are destroyed selectively,^ 

 then the condition is said to be one of panmixia. Here, 

 unless genetic selection (see Chap. XL, ^ 4) came into play, 

 there would be a sudden change of type, but as far as 

 our knowledge of the laws of heredity goes at present, 

 it would not be progressive ; suspension of selection 

 without genetic selection would mark a sudden change ter- 

 minating in one generation, and not a reversal of natural 

 selection as some writers have asserted. Hence, to deter- 

 mine whether selection is periodic or secular, we must 

 measure our organ or character for the same race in two 

 generations at the same stage of growth. Clearly periods 

 of rapidly changing environment, of great climatological 

 and geological change, are likely to be associated with 

 most marked secular selection. To show that there is 

 little or no change year by year in the types of rabbit 

 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 existence of no secular selection would 



1 The English house-sparrow is asserted to be practically under these 

 conditions since its importation into America ! 



