CHAP. VI.] OF TRANSITIONAL VARIETIES. 127 



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 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 inter- 

 mediate 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 modi- 

 fication, 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 more 

 slowly modified and improved. It is the same principle which, as 

 I believe, accounts for the common species in each country, as 

 shown in the second chapter, presenting on an average a greater 



