998 LIFE : OUTLINES OF GENERAL BIOLOGY 



spring ; and this is more than we can confidently say in regard to 

 modifications. 



Now let us return to the cave. It may be that the blindness 

 illustrates the hereditary accumulation of the results of disuse and 

 darkness; but there is little experimental evidence in support of 

 this possibility. It requires fewer assumptions to suppose that 

 variants changing in the direction of poor sight, as many variants 

 do, found their way into the grateful shade of the cave, and con- 

 tinued to vary in the same direction. The passage towards complete 

 blindness would be assisted by Nature's sifting out of all increases 

 in useless organs. 



Madeira Beetles. — Darwin was much interested in the beetles 

 of Madeira because they illustrated so well his theory of Nature's 

 Sifting or Natural Selection. In its simplest terms, his theory was 

 that new departures or variations, which are always cropping up, 

 are sifted in the struggle for existence. Any variation that gives its 

 possessors some appreciable advantage will tend to secure their 

 survival, while a variation that handicaps its possessors will tend 

 to their elimination. Thus the fitter, which means the better suited 

 to particular conditions of life, will automatically tend to survive. 

 The struggle for existence, which does the sifting, includes all the 

 answers-back that living creatures make to environing difficulties 

 and limitations; it means much more than internecine competition 

 among fellows, it may be a struggle against a changeful environment, 

 against foes of an entirely different type; it may even be other- 

 regarding as well as self-regarding. 



As to the beetles in Madeira, the peculiarity is that so many 

 different kinds are wingless. Out of the 580 species of beetles that 

 were known in Madeira when Wollaston made his study, over 200 

 are so defective in wing-development that they cannot fly. Moreover, 

 of the twenty-nine genera peculiar to the island, no fewer than 

 twenty- three have all their species flightless. What is the meaning 

 of this ? It has no doubt something to do with the fact that Madeira 

 is wind-swept, so that there is great risk of flying beetles being blown 

 out to sea. But it is necessary to be more precise. The non-flying 

 beetles did not intelligently shed their wings so as not to be blown 

 away, for they are hatched out from the pupa-state without wings, or 

 with no more than vestiges of wings. It is an old-established consti- 

 tutional peculiarity, we may be sure of that ; and it is seen in many 

 beetles which live in haunts where they are not exposed to wind. 



Another possibility is that the native beetles of Madeira learned 

 to lie low, and in the course of generations lost their wings by disuse. 

 But, as we have said, there is little evidence that the results of disuse 

 can be hereditarily entailed. On the whole, the probability is that 

 Nature's sifting favoured germinal variations in the direction of 



