NATURAL SELECTION 



fluence of the nutritive processes, and because 

 natural selection would no longer necessitate 

 the slaughter of all clumsy bodied individuals. 

 Thus in course of time the breed of antelopes 

 would become so thoroughly altered as to con- 

 stitute a distinct species from their graceful, 

 swift, and timid ancestors. It is in just these 

 ways that New Zealand birds, freed by insular 

 isolation from the attacks of mammalian ene- 

 mies, have grown large and clumsy, and have 

 lost the power of flight which their partly- 

 aborted wings show that they once possessed. 



By the same kind of illustration we may 

 form a rough notion of the way in which a sin- 

 gle species bifurcates into two well-defined spe- 

 cies. Suppose a race of ruminants to have been 

 living in Africa before the introduction of car- 

 nivora, and suppose that, for sundry reasons, 

 the vitality of the race was but little afi^ected by 

 moderate variations in the sizes of its individ- 

 uals, so that while some were comparatively 

 light and nimble, others were comparatively 

 large and clumsy. Now introducing upon the 

 scene the common ancestor of the lion and the 

 leopard — by immigration either from Asia or 

 from some other adjacent territory now sub- 

 merged — let us note some probable features 

 of the complex result. First, as regards the 

 attacked ruminants, it is likely that in course 

 of time the lightest and swiftest individuals, 

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