182 ORIGIN OF SPECIES 



mation given me by Mr. Watson, Dr. Asa Gray, and Mr. 

 Vv'^ollaston, that generally, when varieties intermediate be- 

 jtween two_other forms occurTTKey^^Tejnuch rarer numeri- 

 'cally 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 en- 

 dure for very long periods :— why, as a general rule, they 

 should be exterminated and disappear, sooner than the forms 

 wEicbthey^'origiirany linked togetherT^ 



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 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 intermedi- 

 ate 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 number of well-marked varieties than do 

 the rarer species. I may illustrate what I mean by supposing 

 three varieties of sheep to be kept, one adapted to an exten- 

 sive mountainous region ; a second to a comparatively narrow, 

 hilly tract; and a third to the wide plains at the base; and 

 that the inhabitants are all trying with equal steadiness and 

 skill to improve their stocks by selection ; the chances in this 

 case will be strongly in favour of the great holders on the 

 mountains or on the plains, improving their breeds more 



