,30 The Selection Theory 



of the initial stages, and the stages in the increase of variations, 

 as has been already shown. But the selection-value of a finished 

 adaptation can in many cases be statistically determined. Cesnola 

 and Poulton have made valuable experiments in this direction. The 

 former attached forty-five individuals of the green, and sixty-five of 

 the brown variety of the praying mantis {Mantis religiosa), by a silk 

 thread to plants, and watched them for seventeen days. The insects 

 which were on a surface of a colour similar to their own remained 

 uneaten, while twenty-five green insects on brown parts of plants had 

 all disappeared in eleven days. 



The experiments of Poulton and Sanders 1 were made with 600 

 pupae of Vanessa urticae, the " tortoise-shell butterfly." The pupae 

 were artificially attached to nettles, tree-trunks, fences, walls, and to 

 the ground, some at Oxford, some at St Helens in the Isle of Wight. 

 In the course of a month 93 % °f * ne pupae at Oxford were killed, 

 chiefly by small birds, while at St Helens 68 % perished. The experi- 

 ments showed very clearly that the colour and character of the 

 surface on which the pupa rests — and thus its own conspicuousness — 

 are of the greatest importance. At Oxford only the four pupae which 

 were fastened to nettles emerged ; all the rest — on bark, stones and 

 the like — perished. At St Helens the elimination was as follows : on 

 fences where the pupae were conspicuous, 92 % ; on bark, 66 °/ ; on 

 walls, 54% I and among nettles, 57%. These interesting experi- 

 ments confirm our views as to protective coloration, and show further, 

 that the ratio of elimination in the species is a very high one, and 

 that therefore selection must be very keen. 



We may say that the process of selection follows as a logical 

 necessity from the fulfilment of the three preliminary postulates of 

 the theory : variability, heredity, and the struggle for existence, with 

 its enormous ratio of elimination in all species. To this we must 

 add a fourth factor, the intensification of variations which Darwin 

 established as a fact, and which we are now able to account for 

 theoretically on the basis of germinal selection. It may be objected 

 that there is considerable uncertainty about this logical proof, be- 

 cause of our inability to demonstrate the selection- value of the initial 

 stages and the individual stages of increase. We have therefore to 

 fall back on presumptive evidence. This is to be found in the inter- 

 pretative value of the theory. Let us consider this point in greater 

 detail. 



In the first place, it is necessary to emphasise what is often over- 

 looked, namely, that the theory not only explains the transformations 

 of species, it also explains their remaining the same ; in addition to 

 the principle of varying, it contains within itself that of persisthig. 



1 Report of the British Association (Bristol, 1898), London, 1899, pp. 906—909. 



