ection. 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 calcereous
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 ground when the corresponding part of the body thickens, and
returning to the original position at an angle of 45 degrees to the
upper surface when the part becomes thin again. The arms of the anchor
do not lie in the same plane as the shaft, and thus the curve of the
arms forms the outermost part of the anchor, and offers no further
resistance to the gliding of the animal. Every detail of the anchor,
the curve