368 Studies in the Theory of Descent. 



correlated habit of hiding themselves by day 

 when in the adult condition. This is the case 

 with Sphinx Convolvuli, Deilephila Vespertilio, 

 and Acherontia Atropos. 



Thus far all has been easily explicable by natural 

 selection ; but when we also see a u tendency" to 

 acquire a dark colour in the course of develop- 

 ment, in those species which neither conceal 

 themselves nor are adaptively coloured, but are 

 very conspicuously marked and if, further, it can 

 be shown that these species, such for instance as 

 Deilephila Galii, actually possess immunity from 

 the attacks of foes, how can this tendency to the 

 formation of a dark colour be otherwise explained 

 than by the admission of a phyletic vital force 

 urging the variations in this direction ? 



Nevertheless I believe that also on this point 

 an appeal to unknown forces can be dispensed 

 with. In the first place, dark ground-colours can 

 be of use to a species otherwise than as means of 

 adaptation. In D. Galii, as well as in D. Euphor- 

 bia, the light ring-spots appear rather at their 

 brightest on the pitchy-black ground ; and if this 

 caterpillar must ( sit venia verbo /) become con- 

 spicuous, this purpose would be best attained by 

 acquiring a dark ground-colour, such as that of 

 D. Euphorbia. 



The tendency, apparently common to all these 

 Sphingidce, to acquire a dark colour with increasing 

 age, depends therefore on two quite distinct adap- 



