298 STUDIES, SCIENTIFIC AND SOCIAL 



to understand how new species may arise. For example, 

 the peculiar physical or organic conditions that render 

 one part of the area occupied by a species better adapted 

 to an extreme variety may become intensified. The most 

 extreme variations in this direction will then have the 

 advantage, and will multiply at the expense of the rest. 

 If this change of conditions should extend over the whole 

 area occupied by the species, this one extreme form will 

 replace all the others ; while, if the area should be cut in 

 two by subsidence or elevation, the conditions of the two 

 portions may be modified in opposite directions, each 

 becoming adapted to one extreme form. The original 

 type of the species will then have become extinct, being 

 replaced by two species, each distinguished by a com- 

 bination of certain extreme characters which had before 

 existed in some of its varieties. 



Effect of Changed Environment. 



The changes of conditions which lead to such selection 

 of varieties are very diverse in their nature ; and new 

 species may thus be formed diverging in many ways from 

 the parent stock. The climate may change from moist to 

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

 diminish during long periods, in either case requiring 

 some 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 adjacent countries, giving the advantage 

 to such varieties as can change their food, or by swifter 

 fiight or greater wariness can escape their new foes. In 

 this way several series of changes may occur, each brought 

 about by the pressure of changed conditions ; and thus 

 what was before a single species may become transformed 

 into a group of allied species, differing from each other 

 in a number of slight characters, just as we find them in 

 nature. 



