3io THE PLACE OF MIMICRY 



and the transformation appears to be going on very 

 gradually. 1 



13. Dimorphism and Polymorphism in Procryptic 

 Defence. — It is common for different individuals of a 

 species to adopt two or more different appearances, each 

 of which resembles some special object that is of no 

 interest to an enemy. Thus the Oriental 'leaf-butter- 

 flies' {Kallima) represent many of the well-known 

 appearances borne by dead leaves. Caterpillars and 

 chrysalises are also frequently dimorphic — green and 

 brown, — the two dominant colours of the normal 

 environment. Such differences extend the area over 

 which an enemy must search in order to obtain its food. 

 It has been shown that green forms predominate in the 

 offspring of green parents, and similarly with the brown. 

 We are, therefore, led to the conclusion that the two 

 forms intermixed are more advantageous than either 

 alone, and that otherwise one would quickly replace the 

 other altogether. 



14. Seasonal Changes in the Individual. — Seasonal 

 changes may occur in the course of the individual life, as 

 in the Alpine Hare, Ptarmigan, &c. In many cases it is 

 known that the change is brought about by moulting, but 

 if an observation by Captain James Ross can be depended 

 on, the whitening may begin in an existing coat in the 

 course of a single night. The animal experimented upon 

 was the Hudson's Bay Lemming, which was suddenly 

 exposed to intense cold. The results were so surprising 

 that it would be most desirable to repeat the experiment. 



1 5. Seasonal Dimorphism in Procryptic Defence. — This 

 mode of protection is seen in many forms with more than 

 one brood in the course of the year. Concealment is 

 effected by different colours and even shapes, correspond- 

 ing to the different environments provided by the two 

 seasons. Thus it has already been pointed out on 

 pp. 206-7 tnat tne hooked apex of the fore wing and 



1 See also Handbuch der paliiarklischen Gross-Schmetterlinge, M. Stand- 

 fuss, Jena, 1896. This author's observations on Melanism are stated in 

 a condensed form by Dr. F. A. Dixey in Science Progress, vol, vii (vol. ii 

 of New Series) ; No. 7, April 1898, pp. 196-202. 



