xx Preface to the English Edition. 



The action of Natural Selection, on account of 

 the nature of this agency, can only be exerted on 

 those characters which are of biological im- 

 portance. As it was to be tested whether, besides 

 Natural Selection and the direct action of external 

 conditions, together with the correlative results of 

 these two factors, there might not lie concealed 

 in the organism some other unknown transforming 

 power, it was desirable to select for the investi- 

 gation a group of forms which, if not absolutely 

 excluding, nevertheless appeared possibly to re- 

 strict, the action of one of the two known factors 

 of transformation, that of Natural Selection ; a 

 group of forms consisting essentially of so-called 

 " purely morphological " characters, and not of 

 those the utility of which was obvious, and of 

 which the origin by means of Natural Selection 

 was both possible and probable ab initio. Now, 

 although the colouring can readily be sen to be 

 of value to the life of its possessors, this is not 

 the case with the quite independent markings of 

 caterpillars ; excepting perhaps those occasional 

 forms of marking which have been regarded as 

 special cases of protective resemblance. The 

 markings of caterpillars must in general be con- 

 sidered as " purely morphological " characters, 

 i. e. as characters which we do not know to be of 

 any importance to the life of the species, and 

 which cannot therefore be referred to Natural 

 Selection. The most plausible explanation of 



