206 Darwin, and after Darwin, 
admit of brief and yet adequate consideration. But 
of course it will be understood that the less isolated 
the region, and the shorter the time that it has been 
isolated, the smaller amount of peculiarity should we 
expect to meet with on the part of its present in- 
habitants. Or, conversely stated the longer and the 
greater the isolation, the more peculiarity of species 
would our theory expect to find. The object of the 
present chapter will be to show that these, and other 
cognate expectations, are fully realized by facts ; but, 
before proceeding to do this, I must say a few words 
on the antecedent standing of the argument. 
Where the question is, as at present, between the 
rival theories of special creation and gradual trans- 
mutation, it may at first sieht well appear that no test 
can be at once so crucial and so easily applied as this 
of comparing the species of one geographical area 
with those of another, in order to see whether 
there is any constant correlation between differences 
of type and degrees of separation. But a little further 
thought is enough to show that the test is not quite so 
simple or so absolute—that it is a test to be applied 
in a large and general way over the surface of the 
whole earth, rather than one to be relied upon as 
exclusively rigid in every special case. 
In the first place, there is the obvious consideration 
that lands or seas which are discontinuous now may 
not always have been so, or not for long enough to 
admit of the effects of separation having been exerted 
to any considerable extent upon their inhabitants. 
Next, there is the scarcely less important consideration, 
that although land areas may long have been sepa- 
rated from one another by extensive tracts of ocean, 
