50 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 Pqulton 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 the 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 / ; 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 persisting. 



1 Report of the British Association (Bristol, 1898), London, 1899, pp. 900909. 



