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. 
IIow new Species arise from a variable Species . — 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 
