( Ixxxviii ) 



I personally have always felt anxious to obtain confirmation 

 of that oft-quoted instance communicated to Darwin by 

 Moritz Wagner, according to which a Texan species of 

 Saturnia transported to Switzerland became transformed by 

 feeding the larvje on JiKjlnns refiia instead of J. niijra. But 

 if food can modify an organism — and all the evidence on this 

 point cannot be swept aside — consider the difficulties of inter- 

 pretation. In the first place, we must be sure that it is the 

 food material as such which induces the change, and not 

 some other concomitant external change which had not been 

 taken into account. In the next place, the food-stuff itself 

 may act directly as a disturber of physiological processes, and 

 so affect external characters with which the processes are 

 correlated. But an action of this kind could, on Weisman- 

 nian principles, produce only a temporary local race which 

 would revert as soon as the source of disturbance was 

 removed. How many systematists have applied a test of this 

 kind to ascertain whether a named " type " represents a per- 

 manent species ? Then, again, a species might be physio- 

 logically susceptible to the influence of food through having 

 been compelled by stress of circumstances to adapt itself to 

 changes of diet. Such individual adaptability could be con- 

 ferred by natural selection by the picking out of those indi- 

 viduals whose capabilities of assimilation were congenitally 

 the more elastic. It is not unreasonable to suppose, for 



the translation of his paper by Dr. Dixey, in the " Entomologist " for Mai-cli, 

 1895, p. 75). Mr. F. W. Merrifield informs me that his own experience is in 

 accordance with this, i.e., so far as concerns Lepidoptera the nature of 

 khe larval food does not appear to influence the character of the 

 imago, excepting in so far as it affects vigour, and "that vigour affects 

 colouring in the way of rendering it more vivid, and, where there are several 

 colours, in rendering the contrasts between them more striking. Where this 

 is the case it is always or ixsually associated with greater size or robust- 

 ness." This may be translated into the statement that certain kinds of food 

 stimulate all the physiological functions. Mr. Beddard states that the tiger- 

 moth {Cheloiiia caja) is " almost ihe classical instance of the effects of 

 food upon colour" ("Animal Coloration," p. 52). If this be so — and it is 

 certainly not in accordance with my experience — it is remarkable that 

 varieties should be so rare, in view of the fact that the larva feeds on such a 

 number of different plants. Moreover, when well-marked aberrations 

 occur, they appear generally singly out of a whole brood fed on the same 

 plant. 



