SELECTION: ORGANIC AND SOCIAL 199 



It must be admitted that Darwin left the theory 

 in this form : Variations occur abundantly ; there 

 is a complex, subtle struggle for existence ; there is 

 a constant process of sifting and winnowing ; if fit 

 variations occur among the rest, and if there is 

 discriminate elimination so intense that survival 

 depends on the presence or absence of the variation 

 in question, then new adaptations must result. 

 Those who have something of a naturalist's ex- 

 perience and have some appreciation of the enor- 

 mous scale upon which Nature works as to time, 

 as to numbers, as to chances have usually been 

 content to accept this theory of natural selection 

 as a good working hypothesis. 



But what we wish is actual proof of discriminate 

 elimination, that survivors do survive in virtue of 

 particular qualities. A few illustrations in the 

 present may legitimise our belief that similar 

 processes occurred in the past. Let us summarise 

 the best of these illustrations. 



With silk threads Cesnola * tethered forty-five 

 green praying mantises to green herbage, and sixty- 

 five of the brown variety to withered plants. He 

 watched them for seventeen days, and all survived 

 unnoticed by birds. But when he put twenty-five 

 green ones among brown herbage all were killed 

 by birds in eleven days, while of forty-five brown 

 ones on green grass, only ten survived at the end 

 of seventeen days. Here we have definite proof of 

 a selective death-rate, definite proof of the selective 

 value of the protective coloration. 



Poulton and Saunders 2 fastened 600 pupae of the 



1 See " Biometrika," vol. iii. p. 58. 



2 "Report of the British Association, Bristol Meeting" (1899), 

 pp. 906-909. 



