NATURAL SELECTION 105 



Divergence of Character. 



In species which have a wide range the struggle for exist- 

 ence will often cause some individuals or groups of individuals 

 to adopt new habits in order to seize upon vacant places in 

 nature where the struggle is less severe. Some, living among 

 extensive marshes, may adopt a more aquatic mode of life ; 

 others, living where forests abound, may become more arboreal. 

 In either case we cannot doubt that the changes of structure 

 needed to adapt them to their new habits would soon be 

 brought about, because we know that variations in all the 

 external organs and all their separate parts are very abundant 

 and are also considerable in amount. That such divergence of 

 character has actually occurred we have some direct evidence. 

 Mr. Darwin informs us that in the Catskill Mountains in the 

 United States there are two varieties of wolves, one with a 

 light greyhound-like form which pursues deer, the other more 

 bulky with shorter legs, which more frequently attacks sheep. 1 

 Another good example is that of the insects in the island of 

 Madeira, many of which have either lost their wings or have 

 had them so much reduced as to be useless for flight, while the 

 very same species on the continent of Europe possess fully 

 developed wings. In other cases the wingless Madeira species 

 are distinct from, but closely allied to, winged species of Europe. 

 The explanation of this change is, that Madeira, like many 

 oceanic islands in the temperate zone, is much exposed to 

 sudden gales of wind, and as most of the fertile land is on the 

 coast, insects which flew much would be very liable to be 

 blown out to sea and lost. Year after year, therefore, those 

 individuals which had shorter wings, or which used them least, 

 were preserved ; and thus, in time, terrestrial, wingless, or im- 

 perfectly winged races or species have been produced. That 

 this is the true explanation of this singular fact is proved by 

 much corroborative evidence. There are some few flower- 

 frequenting insects in Madeira to whom wings are essential, 

 and in these the wings are somewhat larger than in the same 

 species on the mainland. We thus see that there is no general 

 tendency to the abortion of wings in Madeira, but that it is 

 simply a case of adaptation to new conditions. Those insects 

 1 Origin of Species, p. 71. 



