36 Darwin, and after Darwin. 



there be a sufficient number which are both similar 

 and simultaneous, the desirability of any co-operating 

 form of isolation is correspondingly removed, because 

 natural selection may then have sufficient material 

 wherewith to overcome the adverse influence of free 

 intercrossing, and so of itself to produce monotypic 

 evolution. Now, variations may be numerous, similar, 

 and simultaneous, either on account of some common 

 cause acting on many individuals at the same time, 

 or on account of the structures in question being 

 more or less variable round a specific mean. In 

 the latter case which is the only case that Mr. 

 Allen's measurements have to do with the law of 

 averages will of course determine that half the whole 

 number of variations in any given structure, in any 

 given generation, will be above the mean line. But, 

 equally of course, no one has ever denied that where, 

 for either of these reasons, natural selection is pro- 

 vided with sufficient material, it is correspondingly 

 capable of improving the specific type without 

 the assistance of any other form of homogamy ; 

 so to speak, they protect themselves by their very 

 numbers, and their superiority over others leads to 

 their survival and accumulation. But what is the 

 result ? The result can only be monotypic evolution. 

 No matter how great the number, or how great the 

 range, of variations round an average specific mean, 

 out of such material natural selection can never 

 produce polytypic evolution : it may change the type 

 to any extent during successive generations, and 

 in a single line of change ; but it cannot branch 

 the type, unless some other form of homogamy 

 intervenes. Therefore, when Mr. Wallace adduces 



