The Origin of the Markings of Caterpillars. 231 



stage of Vespertilio this line appears still more 

 frequently, whilst in Euphorbia it is extremely 

 rare, and when present it only appears as a faint 

 indication. This is the case with one of the 

 specimens figured in Hiibner's work as an " aber- 

 ration," and also with one in Dr. Staudinger's 

 collection. Of Niccea I have at most seen only 

 eight specimens, none of which showed any trace 

 of the long-vanished subdorsal line. 



It must be expected that any ontogenetic stage 

 would most readily revert to the preceding phyletic 

 stage, so that characters present in the preceding 

 stage are consequently those which would most 

 commonly arise by reversion. This postulate of 

 the theory also finds confirmation in the facts. 

 Caterpillars which, when full grown, belong to the 

 seventh phyletic stage, e.g. D. Euphorbia ', not 

 unfrequently show variations corresponding to the 

 sixth stage, i. e. only one instead of two rows of 

 ring-spots the upper and first-appearing series. 

 On the other hand, forms reverting to the fifth 

 phyletic stage (ring-spots with connecting sub- 

 dorsal line) occur but very rarely. I have never 

 met with such cases in adult living caterpillars of 

 D. Euphorbia, although in one instance such a 

 larva was found in the fourth ontogenetic stage ; 

 but the strikingly dark, brownish subdorsal line 

 which connected the otherwise perfectly developed 

 ring-spots, completely disappeared in the fifth stage 

 of the ontogeny. Those larvae which, in the adult 



