192 MR. E. B. POTJLTON ON THE PROTECTIVE [Mar. 1, 



SO as to resemble their surroundings, would be eaten when detected, 

 and this prediction also seemed to receive complete confirmation." 



Tliinking over the whole line of argument and its apparently 

 complete confirmation, I was led to anticipate that a somewliat diflFe- 

 rent method of conducting the experiments would lead to a modification 

 and extension of Wallace's classification of the uses of colour, in the 

 direction of greater elasticity. At the same time it seemed better to 

 withhold the suggestion until 1 had taken the opportunity of sub- 

 mitting it to the experimental test. I was travelling in Italy a few 

 weeks aftt-r delivering the lectures, and took the opportunity of cap- 

 turing many individuals of a few species of South-European Lizards, 

 and of one species of Tree-Fnig {Hyla arhoren, var. meridwnalis). 

 I was thus able to carry out the suggested experiments, which on 

 tlie whole yielded results which confirmed the conclusions I had 

 arrived at a priori, and also produced other results which I had not 

 anticipated. Some of these results were shortly communicated to 

 the Biological Section of the British Association at Birmingham 

 (1886), and an abstract is printed in the volume containing the 

 papers read at that meeting. The suggested extension of Wallace's 

 line of argument, wliich has now beenput to the proof, is as follows: — 

 The acquisition of an unpleasant taste or smell, together with a 

 conspicuous ap[)earance, is so simple a mode of protection, and 

 yet ex hypothesi so absolutely complete, that it seems remarkable 

 that more species have not a^ ailed themselves of this means of 

 defence. What can be the principle which works in antagonism to 

 such a mode of j>rotection ? For in Wallace's theory no suggestion of 

 a true counterbalancing limit appeared — i. e. one which increased 

 with the increasing application of this method of defence, until the 

 latter received a check or, for the time being, was rendered of no 

 avail, or was even turned into an absolute danger. And yet it 

 seemed probable that such an antagonistic principle would appear as 

 the natural outcome of the too complete success of a method of 

 defence which depends on the acquisition of an unpleasant taste or 

 smell together with a conspicuous appearance. If a very common 

 insect, constituting the chief food of one or more Vertebrates, gained 

 protection in this way, the latter animals might be forced to devour 

 the disagreeable objects in order to avoid starvation. And the same 

 result might be readily brought about if a scarce and hard-pressed 

 form adopted the same line, and so became dom.inant, after ousting 

 many species which were much eaten by Vertebrates. If once the 

 Vertebrate enemies were driven to eat any such insect in spite of the 

 unpleasant taste, they would almost certainly soon acquire a relish 

 for what was previously disagreeable, and the insect would be in 

 great danger of extermination, having in the meantime become 

 conspicuous by gaining warning colours. If the reasoning be cor- 

 rect, it is clear that this mode of defence is not necessarily perfect, 

 and that it depends for its apparently complete success upon the 

 existence of relatively abundant palatable forms : in other words, its 

 employment must be strictly limited. It has, indeed, always been 

 recognized that an insect may be distasteful to one Vertebrate 



