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 si^ht 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, 



