A DEFINITION OF EVOLUTION 



thus in perpetual darkness, there is no adverse selection against degen- 

 erative changes of the eyes, and in fact l:)lindness is a general character- 

 istic of such cave dwellers. Their eves show all decrees of deeeneration 

 from just short of the typical functional condition to complete absence of 

 the eyes. Examples include the blind, cave-dwelling salamander of central 

 Europe, Proteus anguineus; the many species of cave-dwelling fishes of 

 the United States as well as other parts of the world; and the blind cray- 

 fishes. The latter have eyestalks which do not, however, bear eyes. While 

 such degenerated eves are easilv understandable on the basis of descent 

 from ancestors with functional eyes, their presence is inexplicable, indeed 

 it is contradictory, on any other theory. 



Typical beetles, as every school boy knows, are strong fliers. The island 

 of Madeira, a wind-swept island about 600 miles off of the coast of Portu- 

 gal and 400 miles to the west of North Africa, has a rich beetle fauna. 

 But a large proportion of these Madeiran species are either wingless or 

 they have much reduced wings which can be of no use for flight. If one 

 makes the assumption that the reduction or loss of these wings has been 

 a result of natural selection, then the selective force may be easily sur- 

 mised. For beetles in flight would be quite likely to be blown out to sea 

 and lost. Thus a strons; selective force should fav^or anv variations toward 

 reduction of wings. Those species which have l)een on the island and 

 subject to this selection longest should be most numerous in the list of 

 flightless species. That this is probably true is indicated by the facts that 

 almost all of the endemic species are flightless, while most of the flying 

 species are also represented in Europe or Africa, or have close relatives 

 on these continents. Only the evolutionary explanation for these facts can 

 be rational. 



When structures undergo a reduction in size together with a loss of 

 their typical function, that is, when they become vestigial, they are quite 

 commonly considered to be degenerate and functionless. But Simpson has 

 recently pointed out that this need not be true at all: the loss of the origi- 

 nal f miction may be accompanied by specialization for a new function. 

 Thus the wing of penguins has become reduced to a point that will not 

 permit flight, but at the same time it has become a highh' efficient paddle 

 for swimming. The wings of rheas, ostriches, and other running birds are 

 also much reduced, and have been described as "at the most still used for 

 display of the decorative wing feathers!" But Siiupson has observed that 

 the rheas, when running, spread the wings and use them as balancers, 

 especially when tmning rapidly. It seems quite probable that this is true 

 of the running birds generally. 



A comparable case is afforded by the pineal gland. This small structure 

 <j;r()ws dorsally from the Forebrain, and its histological structure shows a 

 mixture of glandular and nervous characteristics, in the lampreys and in 

 man\' reptiles, including the \-er\- primitive, lizard-like S})liciiO(lon of New 

 Zealand, this structure bears a third eye, located on the dorsal surface of 

 the skull. Fossil evidence indicates that this is also true of the Crossoptery- 

 gii, the group of fishes from which land vertebrates appear to have been 

 derived. Thus the possibility is open that the possession of a pineal eye 



44 



