ce, 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 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 slight modifications of
structure in some degree permanent; and this assuredly we do see.

Secondly, areas now continuous must often have existed within the
recent period in isolated portions, in which many forms, more especially
amongst the classes which unite for each birth and wander much, may have
separately been rendered sufficiently distinct to rank as representa