264 



THE AMERICAN NATURALIST [Vol. LI 



teeted forms might be maintained through the interaction 

 of natural factors. 



In an attempt to avoid this preliminary difficulty Poul- 

 ton declares that it has always been recognized that an 

 insect may be distasteful to one vertebrate enemy, but 

 palatable to another. He suggests a different counter- 

 balancing limit, which he admits would certainly in time 

 become identical with the other. He argues that a verte- 

 brate enemy may be forced by stress of hunger to eat an 

 unpalatable insect, by implication asserts that this adapt- 

 ability of potential enemies of forms tending to assume 

 warning colors would limit the number of species de- 

 veloping them, and considers the truth of his suggestion 

 confirmed by experimental tests. 



It seems, however, that as long as warning coloration 

 confers any advantage, other species should move in the 

 same direction, for the same reasons that those did in 

 which it first appeared. The number of warningly col- 

 ored species should therefore increase until the situation 

 conceived by Poulton materialized. But in that event the 

 experimental proof that hungry animals are not so fas- 

 tidious as those that are well fed is insufficient to meet 

 the exigencies of the situation ; for one has no reason to 

 believe that animals which might become adjusted to the 

 new condition would confine themselves to a diet of in- 

 sects whose warning coloration was recently acquired, 

 and would leave intact the vested interests of those that 

 first attained it. Indeed, if one is permitted to speculate, 

 it seems not wholly unreasonable to anticipate a swing 

 of the pendulum in the other direction, so that conspicuous 

 forms might face the possibility of almost complete ex- 

 tinction. This is suggested by the well-known fact that 

 interspecific adjustments are not rigid, and that a state 

 of approximate equilibrium is frequently modified* by 

 climatic or other factors, and restored only after a series 

 of more or less definite oscillations. Examples in point 

 are furnished by Howard 's^^ observations upon iclmeu- 



is"New Nature Library," Vol. 7, Pt. 1 (The Insect Book), p. 68. New 

 York, Doubleday, Page and Co. 



