60 ISLAND LIFE 



developed. If we formed our groups, not by equal 

 numbers, but by equal amount of variation, we should 

 probably find, in accordance with the law of averages, that 

 the central group in which the mean characteristics pre- 

 vailed was much more numerous than the extremes, 

 perhaps twice, or even three times, as great as either of 

 them, and forming such a series as the following — 10 

 maximum, 30 mean, 10 minimum development. In or- 

 dinary cases we have no reason to believe that the mean 

 characters or the amount of variation of a species changes 

 materially from year to year or from century to century, 

 and we may therefore 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, because the struggle for existence 

 will lead to the continual suppression of the less perfectly 

 adapted extremes. But sometimes 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, al- 

 though they may differ from the other extreme form quite 

 as much as species often do from each other.^ 



How 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, 



^ The great variation in wild animals is more fully discussed and 

 illustrated in the author's Darwinism (Chapter III.). 



