212 ABSENCE OE EAEITY [Chap. VI. 



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 

 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 infor- 

 mation given me by Mr. Watson, Dr. Asa Gray, and 

 Mr. "Wollaston, that generally, when varieties inter- 

 mediate between 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 con- 

 clude that varieties linking two other varieties together 

 generally have existed in lesser numbers than the 

 forms which they connect, then we can understand why 

 intermediate varieties should not endure for very long 

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

 minated and disappear, sooner than the forms which 

 they originally linked together. 



For anv 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 

 important 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, 

 fiom inhabiting larger areas, will have a great advantage 

 over the intermediate variety, which exists in smaller 



