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