X] MIMICRY AND VARIATION 137 



enemies, and due to some condition or set of conditions 

 of which we are ignorant. It is not inconceivable 

 that the new colour scheme was associated with some 

 physiological pecuHarity which was advantageous to 

 the species in its altered surroundings. If so natural 

 selection may have favoured the new variety, not 

 because of its colour scheme, but owing to the under- 

 lying physiological differences of which the pattern is 

 but an outward sign. And if this could happen in one 

 species there seems to be no reason why it should not 

 happen in others. The weak point of the explanation 

 on the mimicry hypothesis is that it offers no explana- 

 tion of the change in the so-called dominant Ithomiine 

 pattern as we pass from one region to another. What- 

 ever the cause of this change may be there would 

 appear to be nothing against it having also operated 

 to produce similar changes in other unrelated species, 

 in which case the mimicry hypothesis becomes super- 

 fluous. It is not unlikely that the estabhshing of these 

 new forms was due to natural selection. If they were 

 associated with physiological pecuharities better adapted 

 for their environment it is reasonable to suppose that 

 natural selection would favour their persistence as 

 opposed to the older type until the latter was elimi- 

 nated. But such action on the part of natural selection 

 is quite distinct from that postulated on the mimicry 

 hypothesis. On the one view the colour itself is 

 selected because it is of direct advantage to its possessor ; 

 on the other view the colour pattern is associated with 

 a certain physiological constitution which places the 



