408 Studies in the Theory of Descent. 



the property of constancy may belong to a cater- 

 pillar having variable markings. 



There is a soul of truth underlying this objec- 

 tion, since it is true that the various forms of 

 markings which occur in Lepidoptera apparently 

 reach different degrees of constancy. If we speak 

 of the constancy or variability of a species, a 

 different meaning is attached to these expressions 

 according as we are dealing e.g. with a species of 

 Sphinx or a species of Arctia. That which in 

 the latter would be estimated as a high degree of 

 constancy, in the former would be taken as a con- 

 siderable amount of variability. It is of interest, 

 in connection with the question as to the causes of 

 constancy, to note that the power of any form of 

 marking to attain to a high degree of constancy is 

 by no means inversely proportional to the com- 

 plication of the marking, as would have been ex- 

 pected a priori. 



Thus, the species of Sphinx and of allied 

 genera possess on their fore-wings, which are 

 mostly coloured with a mixture of dull grey, white 

 and black, an exceedingly complicated arrange- 

 ment of lines which, in constant species, show a 

 high degree of uniformity : on the other hand, the 

 checquered fore-wings of our Arctiidtz, which are 

 far more coarsely marked, always show, even in 

 the most constant species, well-marked individual 

 differences. The different types of marking must 

 therefore be measured by different standards. 



