230 Studies in the Theory of Descent. 



fluous. The variations which occasionally occur 

 in the larvae, however, to a certain extent furnish 

 a proof of the correctness of the theoretical inter- 

 pretation offered. 



When, in the ontogeny of these species, we 

 actually see before us a series of stages of phyletic 

 development, we must admit that ordinary rever- 

 sion may occur, causing an adult caterpillar to 

 show the characters of the young. Forms 

 reverting to an earlier phyletic stage must, on the 

 whole, occur but seldom, as this stage is removed 

 further back in the ontogeny. Thus, indications 

 of the subdorsal line must occur but rarely in the 

 adult larvae of Euphorbia, and still less frequently 

 in Nic&a, whilst they must be expected to be of 

 more common occurrence in Vespertilio, and also, 

 as has already been seen, in Dahlii. In this last 

 species, as also in Vespertilio, the completely- 

 developed subdorsal line is still present in the 

 third stage, whilst it is possessed by Euphorbia 

 only in the second stage, and then in a rudimentary 

 condition. 



The state of affairs may in fact be thus de- 

 scribed : Among several hundred adult larvse of 

 Dahlii found in Sardinia by Dr. Staudinger, there 

 were some which did not actually possess a dis- 

 tinct subdorsal line, but in place thereof, and as its 

 last indication, a feeble light stripe. One of Dr. 

 Staudinger's caterpillars showed also a distinct 

 line between the closed eye-spots. In the last 



