Page:Popular Science Monthly Volume 15.djvu/30

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20
THE POPULAR SCIENCE MONTHLY.

chance of surviving from not being blown out to sea; and on the other hand those beetles which most readily took to flight would oftenest have been blown to sea, and thus destroyed." An instinct of laziness, so to speak, alone or aided by a shortness of wing, developed stay-at-home habits; and such habits would necessarily tend toward the survival and increase of wingless forms. Other Madeiran insects — such as butterflies, moths, and flower-feeding beetles — have well-developed wings, or possess wings relatively larger than they exhibit elsewhere. This observation, remarks Mr. Darwin, is quite in consistency with the theory of the law of natural selection which favors the survival of the fittest. "For when a new insect first arrived on the island, the tendency of natural selection to enlarge or to reduce the wings would depend on whether a greater number of individuals were saved by successfully battling with the winds, or by giving up the attempt, and rarely or never flying."

Among animals of higher rank in the scale than insects, the presence of rudimentary organs is frequently to be demonstrated. What explanation, other than that of degradation and decay owing to disuse, can be offered of the case of the crabs from the Kentucky Cave?

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Fig. 3.

Crabs possess compound eyes borne at the extremities of highly movable stalks, these stalks in the sentinel crab (Fig. 3) being extremely elongated. In some of the Mammoth Cave crabs the stalk remains, but the eye has completely disappeared. As the eyes in such a case could in no sense disappear from any reason connected with injury to the animal, we are absolutely without any reason for their absence other than that of disuse. Professor Silliman captured a cave rat which, despite its blindness, has large, lustrous eyes. After an exposure for about a month to carefully regulated light, the animal began to exercise a feeble sense of sight. Here the modification or darkness has simply affected the function of the eye; in due time the effects of disuse would certainly alter and render abortive the entire organ of sight.

The possession of flying powers is so notable a characteristic of the class of birds that any exception to this rule, and the want of aerial habits, may be rightly regarded as presenting us with a highly anomalous state of matters. Yet instances of rudimentary wings in birds are far from uncommon; and several groups are, in fact, more notable on account of the absence of powers of flight than for any other structural features. The ostrich, for instance, represents a bird the wings of which are mere apologies for organs of flight, and which are used, as every one knows, simply as aerial paddles. The curious Apteryx or