16!8 ABSENCE OR RARITY 



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 con- 

 nect. Now, if we may trust these facts and inferences, 

 and conclude that varieties linking two other varieties to- 

 gether 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 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 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 considera- 

 tion, 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 pre- 

 senting further favorable 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 number of well- 

 marked varieties than do the rarer species. I may illus- 

 trate what I mean by supposing three varieties of sheep to 

 be kept, one adapted to an extensive mountainous region; 

 a second to a comparatively narrow, hilly tract; and a 

 third to the wide plains at the base; and that the inhabi- 

 tants are all trying with equal steadiness and skill to im- 

 prove their stocks by selection; the chances in this case 



