CuAi-. VI. OF TRANSITIOXAL VARIETIES. 171 



one exist inn" in laro-c numbers; and in this particular case the 

 interniedi:it(! form would be cminentl}"^ liable to the inroads 

 of closely-allied forms existing on both sides of it. But a far 

 more important consideration, as I believe, is that, during the 

 process of further modification, by Avhich two varieties are 

 supposed to be converted and perfected into two distinct spe- 

 cies, the two which exist in larger numbers, from inhabiting 

 larger areas, will have a great advantage over the intermediate 

 variet}', which exists in smaller numbers in a narrow and inter- 

 mediate zone. For forms existing in larger numbers will al- 

 ways have a better chance, within any given period, of present- 

 ing further favorable variations for natural selection to seize 

 on, than will the rarer fcjrms 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 extensive 

 mountainous region ; a second to a comparatively narrow, hilly 

 tract ; and a third to 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 favor of the great holders on the mountains or 

 on the plains improving th.eir breeds more quicldy than the 

 small holders on the intermediate narrow, hilly tract; and con- 

 sequently the improved mountain or j^lain breed Avill soon 

 take the place of the less improved hill-breed ; and thus the 

 two breeds, which originally existed in greater numbers, will 

 come into close contact with each other, without the interpo- 

 sition of the supplanted, intermediate hill-variety. 



To sum up, I believe that species come to be tolerabh' well- 

 dcfini^d objects, and do not at any one period present an inex- 

 tricable chaos of varying and intermediate links : first, because 

 new varieties are very slowly formed, for variation is a slow 

 ])roccss, and natural selection can do nothing until favorable 

 individual diflerences or variations occur, and until a place in 

 the natural polity of the country can be better filled by some 

 modification of some one or more of its inhabitants. And such 

 «ncw places will depend on slow changer> of climate, or on the 

 occasional innnigratinn of new inhabitants, and, ]m)bably, in a 



