Initial Stages of Variation 29 
farther, and leads us to the same alternative on a still more secure 
basis. 
Many years ago I observed in caterpillars of Smerinthus populi 
(the poplar hawk-moth), which also possess white oblique stripes, 
that certain individuals showed red spots above these stripes ; these 
spots occurred only on certain segments, and never flowed together 
to form continuous stripes. In another species (Smerinthus tiliae) 
similar blood-red spots unite to form a line-like coloured seam in 
the last stage of larval life, while in S. ocellata rust-red spots appear 
in individual caterpillars, but more rarely than i in S. popult, and they 
show no tendency to flow together. 
Thus we have here the origin of a new character, arising from 
small beginnings, at least in S. tiliae, in which species the coloured 
stripes are a normal specific character. In the other species, S. popult 
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 sum- 
mation. 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 (Figs. A, B). 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 
