Seite - 182 - in The Origin of Species

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