CHAP. IV.] EVOLUTION THE KEY TO DISTRIBUTION. 



59 



fore look upon the central group as the type of the species which 

 is best adapted to the conditions in which it has actually to 

 exist. This type will therefore always form the majority, be- 

 cause the struggle for existence will lead to the continual 

 suppression of the less perfectly adapted extremes. But some- 

 times a species has a wide range into countries which differ in 

 physical conditions, and then it often happens that one or other 

 of the extremes will predominate in a portion of its range. 

 These form local varieties, but as they occur mixed with the 

 other forms, they are not considered to be distinct species, 

 although they may differ from the other extreme form quite as 

 much as species often do from each other. 



How neiD Species arise from a variable B'pecies. — It is now 

 very easy to understand how, from such a variable species, one 

 or more new species may arise. The peculiar physical or organic 

 conditions that render one part of the area better adapted to an 

 extreme form may become intensified, and the most extreme 

 variations thus having the advantage, they will multiply at the 

 expense of the rest. If the change of conditions spreads over 

 the whole area occupied by the species, this one extreme form 

 will replace the others ; while if the area should be cut in two 

 by subsidence or elevation, the conditions of the two parts may 

 be modified in opposite directions, so as to be each adapted to 

 one extreme form ; in which case the original type will become 

 extinct, being replaced by two species, each formed by a com- 

 bination 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 vegetable 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 



