e other species, S. populi and
S. ocellata, we find the beginnings of the same variation, in one more
rarely than in the other, and we can imagine that, in the course of
time, in these two species, coloured lines over the oblique stripes will
arise. In any case these spots are the elements of variation, out
of which coloured lines MAY be evolved, if they are combined in this
direction through the agency of natural selection. In S. populi the
spots are often small, but sometimes it seems as though several had
united to form large spots. Whether a process of selection in this
direction will arise in S. populi and S. ocellata, or whether it is
now going on cannot be determined, since we cannot tell in advance what
biological value the marking might have for these two species. It is
conceivable that the spots may have no selection-value as far as these
species are concerned, and may therefore disappear again in the course
of phylogeny, or, on the other hand, that they may be changed in another
direction, for instance towards imitation of the rust-red fungoid
patches on poplar and willow leaves. In any case we may regard the
smallest spots as the initial stages of variation, the larger as a
cumulative summation of these. Therefore either these initial stages
must already possess selection-value, or, as I said before: THERE MUST
BE SOME OTHER REASON FOR THEIR CUMULATIVE SUMMATION. I should like to
give one more example, in which we can infer, though we cannot directly
observe, the initial stages.

All the Holothurians or sea-cucumbers have in the skin calcareous bodies
of different forms, usually thick and irregular, which make the
skin tough and resistant. In a small group of them--the species of
Synapta--the calcareous bodies occur in the form of delicate anchors of
microscopic size. Up till 1897 these anchors, like many other delicate
microscopic structures, were regarded as curiosities, as natural
marvels. But a Swedish observer, Oestergren, has recently shown that
they have a biological significance: they serve the footless Synapta as
auxiliary organs of locomotion, since, when the body swells up in the
act of creeping, they press firmly with their tips, which are embedded
in the skin, against the substratum on which the animal creeps, and thus
prevent slipping backwards. In other Holothurians this slipping is
made impossible by the fixing of the tube-feet. The anchors act
automatically, sinking their tips towards the gr