304 MERISTIC VARIATION. [part i. 



Chiton marmoreus, (q.v.), the segments thus undergoing the same 

 variation are not always even adjacent to each other. 



The whole question is a very large one and it is not possible 

 here to do more than refer briefly to a few cases illustrating some 

 of its different aspects. Fuller treatment will be attempted in 

 connexion with the evidence of Substantive Variation. 



463. As examples of a form whose segments in their colour-variations 

 manifest a very close agreement with each other, the Hirudinea may 

 be taken. Figures of numerous varieties of medicinal Leeches are given 

 by Ebrard. Nouvelh monogr. des S'angsues, 1857, and other cases are 

 represented by Moquin-Tandox, Monogr. de la famille des Hirudinees, 

 1827 (see especially PL v. fig. 1). As these figures testify, there is a wide 

 diversity both in the ground-colour and in the size, colour and manner 

 of distribution of the lines and spots with which it is decorated, but 

 so far as may be judged from the figures and descriptions the same 

 decorations are repeated on the various segments. It cannot be doubted 

 that a close scrutiny of the specimens would shew points of difference 

 even between adjacent segments but substantially the patterns are the 

 same for the segments of an individual. The patterns of the varieties 

 may thus, like patterns of ribbon, be each represented by a drawing 

 of a short piece of the body in the way adopted by the writers named. 



As regards the larvse of Lepidoptera a good deal of information 

 bearing on this subject exists, and some of these results, especially 

 those relating to Sphingida?, are of interest 1 . 



*464. In the larvse of many species of Sphingida? there is a more or 

 less regular dimorphism in colour. Notable examples of this are 

 Acherontia atropos, Choerocampa elpenor, C. porcellus and Macroglossa 

 stellatarum, in each of which the larva is known both in a light green 

 and in a dark form 2 . The dark form is the commonest in C. porcellus 

 but in A. atropos it is much rarer than the green form. Judging from 

 the figures, the ground-colour of the segments generally varies as a 



1 The facts which follow are chiefly taken from Wilson, Larvce of Lepidoptera, 

 1880; Weismann, Studies in Theory of Descent, Eng. Trans., 1882; Poulton, Trans. 

 Ent. Soc., 1884, 1885, 1886, 1887; Buckler, Larvce of Brit. Butterf. and Moths, 

 Vol. in. Bay Soc, 1887. 



- That this dimorphism is ' phy tophagic ' is not very likely, but the possibility should 

 be remembered. It seems to be established that in many of the species the colour- 

 varieties are definite and largely discontinuous. Of M. stellatarum Weismann (p. 

 250) bred 140 from one batch of eggs, and of these 49 were of the green form and 63 

 of the brown form, only 28 being transitional. The discontinuous character of the 

 variation was illustrated by one most i - emarkable specimen. In it the body was 

 particoloured, being partly of the green and partly of the brown form. The head, 

 prothorax, all the abdominal segments behind the 2nd, and the right side of the 

 remainder were brown, but tne left side of the meso- and meta-thorax, of the 1st 

 abdominal, and part of the left side of the 2nd abdominal were green [according to 

 the figure 9 on PL m., with which the description in the text, p. 249, differs slightly]. 

 In A. atropos I know no account of any intermediate form. In most of the species 

 the dimorphic or polymorphic character appears in tbe later periods of larval life 

 and especially after the last moult; but in C. porcellus, according to both Weismann 

 (p. 188) and Buckler (p. 117) though the larva? are of both kinds in the penultimate 

 state all or nearlv all after the last moult turn to the dark form. 



