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 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