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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 sur- 

 vival 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 ten- 

 dency of natural selection to enlarge or to reduce the wings would 

 depend on whether a greater number of individuals were saved by suc- 

 cessfully 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 pres- 

 ence of rudknentary organs is frequently to be demonstrated. What 

 explanation, other than that of degradation and decay owing to dis- 

 use, can be offered of the case of the crabs from the Kentucky Cave ? 



Crabs possess compound eyes 

 borne at the extremities of high- 

 ly movable stalks, these stalks 

 in the sentinel crab (Fig. 3) be- 

 ing 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 exer- 

 cise a feeble sense of sight. Here the modification or darkness has 

 simply affected the function of the eye ; in due time the effects of dis- 

 use 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 anom- 

 alous 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 struc- 

 tural 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 



Fig. 3. 



