176 DIFFICULTIES ON THEOEY. Chap. VI. 



the same rule will probably apply to both ; and if we in 

 imagination adapt a varying species to a very large area, 

 we shall have to adapt two varieties to two large areas, 

 and a third variety to a narrow intermediate zone. The 

 intermediate 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 information given 

 me by Mr. Watson, Dr. Asa Gray, and Mr. Wollaston, 

 that generally when varieties 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 inferences, and therefore conclude 

 that varieties linking two other varieties together have 

 generally existed in lesser numbers than the forms which 

 they connect, then, I think, we can understand why in- 

 termediate 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 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 a far more important 

 consideration, as I believe, is that, during the process 

 of further modification, by which two varieties are sup- 

 posed on my theory 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 exists 



