388 Studies in the Theory of Descent. 



possessing it, nevertheless it has been established 

 that each of the elements of marking occurring in 

 the larvae of the Sphingidcz originally possessed a 

 decided biological significance, which was produced 

 by natural selection. 



In the case of the three chief elements of the 

 markings of the Sphingidcz, it can be further 

 shown that not only the initial stages but also their 

 ultimate perfection the highest stages of their 

 development, are of decided advantage to their 

 possessors, and have a distinct biological value, so 

 that the gradual development and improvement of 

 these characters can be traced to the action of 

 natural selection. 



But although natural selection is the factor 

 which has called into existence and perfected the 

 three chief forms and certain of the subsidiary 

 markings, in the repetition of the local character 

 on the other segments, as well as in the formation 

 of new elements of marking at the points of inter- 

 section of older characters now rudimentary, we 

 can recognize a second factor which must be 

 entirely innate in the organism, and which governs 

 the uniformity of the bodily structure in such a 

 manner that no part can become changed without 

 exerting a certain action on the other parts an 

 innate law of growth (Darwin's " correlation "). 



Only once during the whole course of the in- 

 vestigations was it for an instant doubtful whether 

 a phyletic vital force did not make itself apparent, 



