i88 THE POPULAR SCIENCE MONTHLY. 



their external modification offers endless opportunities for the 

 action of natural selection in producing mimicking forms in this 

 great branch of air-breathing invertebrates. In the other great 

 branch of invertebrate life, the land mollusks, the modifications 

 of structure have been mainly internal. Outwardly they present 

 comparatively few types. This sameness in exterior features has 

 been unfavorable to the development of mimicking forms of 

 mollusks ; but, while true mimicry is rare among them, most 

 interesting cases of protective resemblance and of special protect- 

 ive structures occur not infrequently. 



I well remember hunting the snail Helix thyroides upon the 

 wooded bluffs along the Mississippi. Both shell and soft parts of 

 this mollusk have the brown' tint of the fallen oak and hickory 

 leaves with which the forest floor is thickly carpeted. Indeed, 

 the colors correspond so closely that a person standing can 

 scarcely distinguish snail from leaves, even when knowing where 

 to look. 



The assimilation of this snail to the general color-scheme of its 

 environment must prove very beneficial ; although one occasion- 

 ally finds a heap of empty shells by the side of a fallen log or 

 stump, showing that the jays and crows sometimes find enough 

 of them for a meal. 



It may be stated as a general rule that snails which live quite 

 ufjon the ground have dark or dull-colored shells, while the 

 shells of those living in exposed situations are bright. Turning 

 to the tropics, where all Nature flaunts attire more gaudy than in 

 the sober North, we find many illustrations of this rule. In the 

 Philippine Islands there is a group of arboreal snails (Cochlostyla) , 

 some of which are vivid green in color, like the foliage whereon 

 they live. It should be noted that these snails are so exclusively 

 arboreal that they even deposit their eggs in a bag made by 

 twisting leaves ! 



In tropical America we have a group of tree-climbing snails 

 which subsist chiefly upon fruit. Like the Philippine Island 

 species they are vividly colored ; but in this case the colors are 

 the most brilliant hues of yellow, orange, and red, corresponding 

 admirably to the tints of the ripe fruits upon which they live.* 



It is a noteworthy fact that certain ground-living allies of these 

 brilliantly painted snails are dull colored, as are ground snails 

 generally. 



All the foregoing are instances of what has been called cryptic 

 — that is, concealing — protective coloration. In other words, the 



* Specimens of Helix picla, H. alauda, H. marginclla, and other bright-colored fruit- 

 eating snails of the West Indies, are occasionally imported to the Philadelphia and New- 

 York markets upon banana bunches. 



