144 DARWINISM chap. 



■would be ample beneficial variations ; if a darker colour or 

 a longer beak or Aving were required, there are always a con- 

 siderable number of individuals darker and lighter in colour 

 than the average, with longer or with shorter beaks and wings, 

 and thus the beneficial variation must always be present. And 

 so with every other part, organ, function, or habit ; because, as 

 variation, so far as we know, is and always must be in the two 

 directions of excess and defect in relation to the mean amount, 

 whichever kind of variation is Avanted is always j^resent in some 

 degree, and thus the difficulty as to " beneficial " variations 

 occurring, as if they were a special and rare class, falls to the 

 ground. No doubt some organs may vary in three or perhaps 

 more directions, as in the length, breadth, thickness, or curva- 

 ture of the bill. But these may be taken as separate varia- 

 tions, each of which again occurs as " more " or " less "; and thus 

 the " right " or " beneficial " or " useful " variation must always 

 be present so long as any variation at all occurs ; and it has not 

 yet been proved that in any large or dominant species, or in 

 any part, organ, or faculty of such species, there is no variation. 

 And even were such a case found it would prove nothing, so 

 long as in numerous other species variation was shoAvn to exist ; 

 because we know that great numbers of species and groups 

 throughout all geological time have died out, leaving no 

 descendants ; and the obvious and sufficient explanation of this 

 fact is, that they did not vary enough at the time Avhen varia- 

 tion was required to bring them into harmony with changed 

 conditions. The objection as to the "right" or "beneficial" 

 variation occurring when required, seems therefore to have no 

 weight in view of the actual facts of variation. 



Isolation to prevent Intercrossing. 



Most writers on the suliject consider the isolation oi a 

 portion of a species a very important factor in the formation 

 of new species, while others maintain it to be absolutely 

 essential. This latter view has arisen from an exaggerated 

 opinion as to the power of intercrossing to keep doA\ai any 

 A^ariety or incipient species, and merge it in the parent stock. 

 But it is evident that this can only occur Avith varieties Avhich 

 are not useful, or Avhich, if useful, occur in very small 

 numbers; and from this kind of variations it is clear that 



