238 THE ORIGIN OF SPECIES 



inhabiting a very large area, we shall have to adapt two 

 varieties to two large areas, and a third variety to a nar- 

 row intermediate zone. The intermediate variety, conse- 

 quently, will exist in lesser numbers from inhabiting a 

 narrow and lesser area; and practically, as far as I can 

 make out, this rule holds good with varieties in a state 

 of nature. I have met with striking instances of the 

 rule in the case of varieties intermediate between well- 

 marked varieties in the genus Balanus. And it would 

 appear from information given me by Mr. Watson, Br. 

 Asa Gray, and Mr. Wollaston, that generally, when vari- 

 eties intermediate between two other forms occur, they 

 are much rarer numerically than the forms which they 

 connect. Now, if we may trust these facts and infer- 

 ences, and conclude that varieties linking two other vari- 

 eties together generally have existed in lesser numbers 

 than the forms which they connect, then we can under- 

 stand why intermediate varieties should not endure for 

 very long periods — why, as a general rule, they should 

 be exterminated and disappear, sooner than the forms 

 which they originally linked together. 



For any form existing in lesser numbers would, as 

 already remarked, run a greater chance of being extermi- 

 nated than one existing in large numbers; and in this 

 particular case the intermediate form would be eminently 

 liable to the inroads of closely-allied forms existing on 

 both sides of it. But it is a far more important consider- 

 ation that during the process of further modification, by 

 which two varieties are supposed to be converted and 

 perfected into two distinct species, the two which exist 

 in larger numbers, from inhabiting larger areas, will have 

 a great advantage over the intermediate variety, which 



