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 Poulton and Sanders’ 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°/, of 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. 906—909. 
