174 DIFFICULTIES ON THEORY. Chap. VI. 
present time to meet with numerous transitional vari¬ 
eties in each region, though they must have existed 
there, and may be embedded there in a fossil condition. 
But in the intermediate region, having intermediate 
conditions of life, why do we not now find closely-linking 
intermediate varieties ? This difficulty for a long time 
quite confounded me. But I think it can be in large 
part explained. 
In the first place we should be extremely cautious 
in inferring, because an area is now continuous, that it 
has been continuous during a long period. Geology 
would lead us to believe that almost every continent 
has been broken up into islands even during the later 
tertiary periods; and in such islands distinct species 
might have been separately formed without the possi¬ 
bility of intermediate varieties existing in the interme¬ 
diate zones. By changes in the form of the land and 
of climate, marine areas now continuous must often 
have existed within recent times in a far less continuous 
and uniform condition than at present. But I will pass 
over this way of escaping from the difficulty; for I 
believe that many perfectly defined species have been 
formed on strictly continuous areas; though I do not 
doubt that the formerly broken condition of areas now 
continuous has played an important part in the forma¬ 
tion of new species, more especially with freely-crossing 
and wandering animals. 
In looking at species as they are now distributed 
over a wide area, we generally find them tolerably 
numerous over a large territory, then becoming some¬ 
what abruptly rarer and rarer on the confines, and 
finally disappearing. Hence the neutral territory be¬ 
tween two representative species is generally narrow in 
comparison with the territory proper to each. We see 
the same fact in ascending mountains, and sometimes 
