have a great
advantage over the intermediate variety, which exists {177} in smaller
numbers in a narrow and intermediate zone. For forms existing in larger
numbers will always 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 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 favour of
the great holders on the mountains or on the plains improving their breeds
more quickly than the small holders on the intermediate narrow, hilly
tract; and consequently the improved mountain or plain breed will 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 interposition of the supplanted, intermediate
hill-variety.

To sum up, I believe that species come to be tolerably well-defined
objects, and do not at any one period present an inextricable chaos of
varying and intermediate links: firstly, because new varieties are very
slowly formed, for variation is a very slow process, and natural selection
can do nothing until favourable {178} variations chance to 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 new places
will depend on slow changes of climate, or on the occasional immigration of
new inhabitants, and, probably, in a still more important degree, on some
of the old inhabitants becoming slowly modified, with the new forms thus
produced and the old ones acting and reacting on each other. So that, in
any one region and at any one time, we ought only to see a few species
presenting sli