208 Hints on the 



wise, the question of tlieir specific value may ever remain in 

 doubt. 



5. But if we admit the doctrine of species above referred to, 

 a few examples of identical types in distant areas are not 

 exceptions to the general plan of nature, but an integral part of 

 it. The general fact may be thus stated : a number of pairs of 

 analogues from two distinct provinces may be so arranged, that 

 the amount of difference between each two shall successively 

 diminish from very distinct species to species less distinct, and 

 then to species scarcely distinguishable, until at length the 

 series shall terminate in two forms quite indistinguishable from 

 each other, that is, in one species. In other words, if the pairs 



be arranged on both sides of an angle, as in the accompanying 

 figure, with the amount of difference expressed by the distance 

 across the angle, as from a to a', h to h', Sic, the vanishing 

 point of the differences will be in an identical type, as g. 



6. The areas of species vary from a few miles to several 

 thousand miles in diameter. There are few which exceed three 

 or four thousand miles. But a great number exceed two 

 thousand miles ; and, with the exception of insular terrestrial 

 faunfe, a large majority of the areas exceed one thousand miles 

 in diameter. 



7. The areas of insular terrestrial species, excepting those 

 which have the power of flight, do not usually exceed the 

 islands which they inhabit, if the islands are thirty to fifty 

 miles distant from each other. As to islands which are sepa- 

 rated by one hundred miles or more of water, the examples of 

 species common to two or more are rare. 



