Page:Popular Science Monthly Volume 42.djvu/202

 188 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 protective 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 occasionally 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 upon 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