MR. M IV ART AND MR. DARWIN. 285 



at one and the same time, in a sufficient number of in- 

 dividuals, to prevent its being obliterated almost as soon 

 as produced by the admixture of unvaried blood which 

 would so greatly preponderate around it; and indeed 

 the necessity for a nearly simultaneous and similar 

 variation, or readiness so to vary on the part of many 

 individuals, seems almost a postulate for evolution at 

 all. On this subject Mr. Mivart writes : 



" The ' North British Review ' (speaking of the sup- 

 position that species is changed by the survival of a few 

 individuals in a century through a similar and favour- 

 able variation) says 



" ' It is very difficult to see how this can be accom- 

 plished, even when the variation is eminently favourable 

 indeed ; and still more, when the advantage gained is 

 very slight, as must generally be the case. The advan- 

 tage, whatever it may be, is utterly outbalanced by 

 numerical inferiority. A million creatures are bom; 

 ten thousand survive to produce offspring. One of the 

 million has twice as good a chance as any other of sur- 

 viving, but the chances are fifty to one against the 

 gifted individuals being one of the hundred survivors. 

 No doubt the chances are twice as great against any 

 other individual, but this does not prevent their being 

 enormously in favour of some average individual. 

 However slight the advantage may be, if it is 

 shared by half the individuals produced, it will pro- 

 bably be present in at least fifty -one of the sur- 

 vivors, and in a larger proportion of their offspring; 

 but the chances are against the preservation of any 

 one " sport " (i.e., sudden marked variation) in a mime- 



