DIFFICULTIES OF THE THEORY 139 



any one species, depending as it does on the range of others, will 

 tend to be sharply defined. Moreover, each species on the confines 

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

 fluctuations 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 

 defined. 



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 we 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 inter- 

 mediate variety, consequently, will exist in lesser numbers from in- 

 habiting 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 num- 

 bers 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 re- 

 marked, run a greater chance of being exterminated than one exist- 

 ing 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, 

 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 



