( Ixxxix ) 



example, that the success of polyphagic species, such as the 

 species of Hi/bcrnia, etc., is attributable to this kind of 

 adaptabihty to various distinct kinds of foliage. If, more- 

 over, this adaptability corresponded with a visible effect upon 

 the colour or form of any subsequent stage, we should have a 

 character which might be considered of diagnostic value, and 

 which might be erroneously quoted as another example of 

 the inutility of specific characters. There is also the possi- 

 bility that the action of the food may be only apparent and 

 not real, resulting in fact from the indirect action of the 

 colour of the surroundings upon the physiology of the indi- 

 vidual (Poulton's factor). 



Finally, the action of other external agencies may, as 

 already indicated, be referred to physiological adaptation of 

 the species as a whole, or to individual physiological adapta- 

 bility. These two modes of action must be disentangled, and 

 systematically conducted experimental observations alone will 

 enable us to arrive at a satisfactory conclusion. If a species 

 requiring concealment under variable external conditions 

 acquires a power of individual adaptability by any physio- 

 logical mechanism whatever, the stimulus under which that 

 adjustability was first produced by natural selection will 

 always tend to produce the necessary colour change, even 

 when the individual is removed from its surroundings, by 

 virtue of the conservative tendency of heredity. It inherits a 

 power of responding to light, heat, etc. Natural selection does 

 not concern itself with physiological mechanisms as such ; it 

 utilises their effects whether obvious or latent. The problem 

 before us is to determine whether any, and if so, which of the 

 " specific " characters are physiological correlates. If, as the 

 experiments of Vulpian and Monnier, quoted by De Varigny, 

 seem to show,'-' the two allied frogs, Bana esculenta and 

 R. teviporaria, differ to such an extent in their physiological 

 characters as to react in quite different ways to the same 

 poison, who can say that some of the diagnostic characters of 

 these species are not the correlates of the physiological differ- 

 ences ? These and whole groups of similar cases are await - 



* 0-p. cit, p. 130, etseq. 



