RUDIMENTARY ORGANS. 87 



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." 



Amongst 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 ? Crabs possess compound eyes borne at the extremities of 

 highly movable stalks, these stalks in the sentinel crab (Fig. 25) 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, 



had 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 had 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 Kiwi-kiwi (Fig. 26) of New Zealand, a near 

 relative of the ostriches and running-birds in general, represents 

 a still more degraded condition of the organs of flight, for the wing 

 is reduced in size to an extraordinary degree, and exists in a highly 

 abortive condition ; whilst only one complete finger is represented in 

 the hand other birds, as a rule, possessing three modified fingers. 

 The logger-headed duck of South America has wings so reduced that 

 it can but " flap along the surface of the water," a condition of matters 



