ADAPTIVE COLORATION l8l 



selection, one inquires into the structural basis of the resemblance in 

 each instance, it is found that some cases can be explained, without the 

 aid of natural selection, as being direct effects of food, light or other 

 primary factors. Such cases, then, are in a sense accidental. For ex- 

 ample, many inconspicuous green insects are green merely because 

 chlorophyll from the food-plant tinges the blood and shows through the 

 skin. If it be argued that natural selection has brought about a thin 

 and transparent skin, it may be replied that the skin of a green cater- 

 pillar is by no means exceptional in thinness or transparency. More- 

 over, many leaf-mining caterpillars are green, simply because their food 

 is green; for, living as they do within the tissues of leaves, and surrounded 

 by chlorophyll, their own green color is of no advantage, but is merely 

 incidental. 



Again, in the "protectively" colored chrysalides experimented upon 

 by Poulton, the color was directly influenced by the prevailing color 

 of the light that surrounded the larva during the last few days before 

 pupation. Of course, it is conceivable that natural selection may have 

 preserved such individuals as were most responsive to the stimulus of 

 the surrounding light; nevertheless the fact remains that these resem- 

 blances do not demand such an explanation, which is, in other words, 

 superfluous. 



Indeed, a great many of the assumed examples of "protective re- 

 semblance" are very far-fetched. On the other hand, when the re- 

 semblance is as specific and minutely detailed as it is in the Kallima 

 butterflies where, moreover, special instincts are involved the phe- 

 nomenon can scarcely be due to chance; the direct and uncombined 

 action of such factors as food or light is no longer sufficient to explain 

 the facts although these and other factors are undoubtedly important 

 in a primary, or fundamental; way. Here natural selection becomes 

 useful, as enabling us to understand how original variations of structure 

 and instinct in favorable directions may have been preserved and ac- 

 cumulated until an extraordinary degree of adaptation has been attained. 



Value of Protective Resemblance. The popular opinion as to 

 the efficiency of protective resemblances in undoubtedly an exaggerated 

 one, owing mainly to the false assumption that the senses of the lower ani- 

 mals are co-extensive in range with our own. As a matter of fact, birds 

 detect insects with a facility far superior to that of man, and destroy them 

 by the wholesale, in spite of protective coloration. Thus, as Judd has 

 ascertained, no less than three hundred species of birds feed upon pro- 

 tectively colored grasshoppers, which they destioy in immense numbers. 



