Initial Stages of Variation 29 



farther, and leads us to the same alternative on a still more secure 



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 (Smcrhdhus tiliae) 

 similar blood-red spots unite to form a line-like coloured seam in 

 the last stage of larval life, while in 8. ocellata rust-red spots appear 

 in individual caterpillars, but more rarely than in S. populi, 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 8. tiliae, in which species the coloured 

 stripes are a normal specific character. In the other species, S. populi 

 and 8. 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 8. 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 



