178 
DIFFICULTIES ON THEOKY. 
Chap. VI. 
tions cliance 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 inha¬ 
bitants, and, probably, in a still more important degree, 
on some of the old inhabitants becoming slowly modi¬ 
fied, 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 struc¬ 
ture 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 
wliich 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 
representative species. In this case, intermediate vari¬ 
eties between the several representative species and 
their common parent, must formerly have existed in 
each broken portion of the land, but these links will 
have been supplanted and exterminated during the pro¬ 
cess of natural selection, so that they will no longer 
exist in a living state. 
Thirdly, when two or more varieties have been formed 
in different portions of a strictly continuous area, inter¬ 
mediate varieties will, it is probable, at first have been 
formed in the intermediate zones, but they will gene¬ 
rally have had a short duration. For these inter¬ 
mediate varieties will, from reasons already assigned 
(namely from what we know of the actual distribution 
of closely allied or representative species, and likewise 
of acknowledged varieties), exist in the intermediate 
zones in lesser numbers than the varieties which they 
