212 ABSENCE OR RARITY [Chap. VL 



to adapt two varieties to two large areas, and a third 

 variety to a narrow intermediate zone. The inter- 

 mediate variety, consequently, will exist in lesser num- 

 bers 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 varie- 

 ties intermediate between well-marked varieties in the 

 genus Balanus. And it would appear from information 

 given me by Mr. "Watson, Dr. Asa Gray, and Mr. Wol- 

 laston, that generally, when varieties intermediate be- 

 tween two other forms occur, they are much rarer 

 numerically than the forms which they connect. Now, 

 if we may trust these facts and inferences, and conclude 

 that varieties linking two other varieties together gen- 

 erally have existed in lesser numbers than the forms 

 which they connect, then we can understand why in- 

 termediate varieties should not endure for very long 

 periods: — why, as a general rule, they should be ex- 

 terminated 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 exter- 

 minated 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 impor- 

 tant consideration, 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 inter- 

 mediate variety, which exists in smaller numbers in 



