CHAP. IV EVOLUTION THE KEY TO DISTRIBUTION 



61 



each formed by a combination of certain extreme characters 

 which had before existed in some of its varieties. 



The changes of conditions which lead to such selection 

 of varieties are very diverse in nature, and new species 

 may thus be formed, diverging in many ways from the 

 original stock. The climate may change from moist to 

 dry, or the reverse, or the temperature may increase or 

 diminish for long periods, in either case requiring a 

 corresponding change of constitution, of covering, of vege- 

 table or of insect food, to be met by the selection of 

 variations of colour or of swiftness, of length of bill or of 

 strength of claws. Again, competitors or enemies may 

 arrive from other regions, giving the advantage to such 

 varieties as can change their food, or by swifter flight or 

 greater wariness can escape their new foes. We may thus 

 easily understand how a series of changes may occur at 

 distant intervals, each leading to the selection and pre- 

 servation of a special set of variations, and thus what was 

 a single species may become transformed into a group of 

 allied species differing from each other in a variety of ways, 

 just as we find them in nature. 



Among these species, however, there will be some which 

 will have become adapted to very local or special condi- 

 tions, and will therefore be comparatively few in number 

 and confined to a limited area ; while others, retaining the 

 more general characters of the parent form, but with some 

 important change of structure, will be better adapted to 

 succeed in the struggle for existence with other animals, 

 will spread over a wider area, and increase so as to become 

 common species. Sometimes these will acquire such a 

 perfection of organisation by successive favourable modi- 

 fications that they will be able to spread greatly beyond 

 the range of the parent form. They then become what 

 are termed dominant species, maintaining themselves in 

 vigour and abundance over very wide areas, displacing 

 other species with which they come into competition, and, 

 under still further changes of conditions, becoming the 

 parents of a new set of diverging species. 



Definition and Origin of Genera. — As some of the most 

 important and interesting phenomena of distribution relate 



