possible to bring forward any actual proof of the selection-value 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 ("Report of the British
Association" (Bristol, 1898), London, 1899, pages 906-909.) 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 per cent of the pupae at Oxford were
killed, chiefly by small birds, while at St Helens 68 per cent perished.
The experiments 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 per cent; on bark, 66 per cent; on
walls, 54 per cent; and among nettles, 57 per cent. These interesting
experiments 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, because of our
inability to demonstrate the