404 Studies in the Theory of Descent. 



When, in the following pages, I speak of 

 variability, I do not refer to the occurrence of local 

 varieties, or to variations which occur in the course 

 of time, but I mean a high degree of individual 

 variability a considerable fluctuation of characters 

 in the individuals of one and the same district or of 

 the same brood. I consider a species to be con- 

 stant, on the other hand, when the individuals from 

 a small or large district differ from one another 

 only to a very slight extent. Constant forms are 

 likewise generally, but not invariably, such as are 

 poor in local varieties, whilst variable forms are 

 those which are rich in such variations. Since the 

 terms " variable " and " constant " are but relative, 

 I will confine myself to the most extreme cases, 

 those in which the individual peculiarities fluctuate 

 within very wide or very narrow limits. 



As no observations upon the degree of varia- 

 bility shown by a species in the different stages of 

 its development were available, I was obliged to 

 fall back upon my own, at least so far as relates 

 to the larval and pupal stages, whilst for the 

 imaginal stage the wide experience of my esteemed 

 friend Dr. Staudinger has been of essential service 

 to me. 



Let us in the first place confine our attention to 

 the three chief forms \which every Lepidopteron 

 presents, viz. larva, pupa, and imago. With 

 respect to the constancy or variability of these 

 three forms, we actually find in nature" all the 



