chap, iii.] CONDITIONS AFFECTING DISTRIBUTION. 
47 
families ,) would have a limited range ; but, as is to some extent 
the case in tropical America east of the Andes, every such 
group would be represented over the whole area, by countless 
minute modifications of form adapted to local conditions. 
2. One great physical barrier would, however, even then 
exist ; the hot equatorial zone would divide the faunas and floras 
of the colder regions of the northern and southern hemispheres 
from any chance of intermixture. This one barrier would be 
more effectual than it is now, since there would be no lofty 
mountain ranges to serve as a bridge for the partial interchange 
of northern and southern forms. 
3. If such a condition of the earth as here supposed con- 
tinued for very long periods, we may conceive that the action 
and reaction of the various organisms on each other, combined 
with the influence of very slowly changing physical conditions, 
would result in an almost perfect organic balance, which would 
be manifested by a great stability in the average numbers, the 
local range, and the peculiar characteristics of every species. 
4. Under such a condition of things it is not improbable that 
the total number of clearly differentiated specific forms might be 
much greater than it is now, though the number of generic and 
family types might perhaps be less ; for dominant species would 
have had ample time to spread into every locality where they 
could exist, and would then become everywhere modified into 
forms best suited to the permanent local conditions. 
5. Now let us consider what would be the probable effect of 
the introduction of a barrier, cutting oft’ a portion of this homo- 
geneous and well-balanced world. Suppose, for instance, that a 
subsidence took place, cutting off by a wide arm of the sea a 
large and tolerably varied island. The first and most obvious 
result would be that the individuals of a number of species 
would be divided into two portions, while others, the limits of 
whose range agreed approximately with the line of subsidence, 
would exist in unimpaired numbers on the new island or on the 
main land. But the species whose numbers were diminished 
and whose original area was also absolutely diminished by the 
portion now under the sea, would not be able to hold their 
