120 ABSENCE OR RARITY [Chap. VI. 



of its range, where it exists in lessened numbers, will, during fluctu- 

 ations in the number of its enemies or of its prey, or in the nature 

 of the seasons, be extremely liable to utter extermination ; and thus 

 its geographical range will come to be still more sharply denned. 



As allied or representative species, when inhabiting a continuous 

 area, are generally distributed in such a manner that each has a 

 wide range, with a comparatively narrow neutral territory between 

 them, in which they become rather suddenly rarer and rarer ; then, 

 as varieties do not essentially differ from species, the same rule will 

 probably apply to both ; and if Ave take a varying species inhabiting 

 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 conclude 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 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 exterminated than one 

 existing in large numbers ; and in this particular case the inter- 

 mediate 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 modifi- 

 cation, 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 exists in smaller numbers 

 in a narrow and intermediate zone. For forms existing in larger 

 numbers will have a better chance, within any given period, of 

 presenting further favourable variations for natural selection to 

 seize on, than will the rarer forms which exist in lesser numbers. 

 Hence, the more common forms, in the race for life, will tend to 

 beat and supplant the less common forms, for these will be inoro 



