em for seven 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[45] 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
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%; on bark, 66%; on walls,
54%; and among nettles, 57%. 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 interpretative value of the theory_. Let us consider this
point in greater detail.

In the first place it is necessary to emphasize what is often
overlooked, 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_. It is part of the essence of selection,
that it not only causes a part to _vary_ till it has