XVIII.] DISTEIBUTION. 221 



mined by those of the other parts, and the law that the 



form and character of the whole organism are determined 



by the mode of its life, are inseparable from each other. 



But, thongh logically in the closest connexion, they have 



bearings on two very distinct parts of biological science. 



The first — the law that the characters of all the parts are Bearingsof 



mutually connected and mutually determined — has special jocYiTue 



bearings on the problems of morpholo<T;y ; the second — the t*" ™or- 



. r cdJ ) phology 



law that they are collectively determined by the mode of and on dis- 

 its life — has special bearings on the problems of distri- ^''ib^ition. 

 bution. If those two laws were not only true, which they 

 are, but all-explaining, which they are not, all the facts 

 of morphology and distribution ought to be explicable by 

 them. The distribution and habitats of every species ought 

 to be explicable by the relation of its organization to its 

 mode of life ; and its morphology ought to be explicable 

 by the relation, both functional and structural, of its organs 

 to each other. Now, is it so ? These questions are, in 

 their nature, capable of being answered by an appeal to 

 facts. I shall consider the facts of distribution first, not as 

 being the most important, but as being the simplest, and 

 the easiest to make intelligible. 



It is to be observed, that even if Cuvier's doctrine were 

 all-explaining, it would not contradict the doctrine of the 

 modifiability of species; it would rather give support to 

 that doctrine ; because we know, from geological evidence, 

 that the physical circumstances of localities as to climate, 

 vegetation, &c. are subject to vast though slow changes ; 

 and if we believe that the characters of species are deter- 

 mined by the external conditions of their existence, it is a 

 reasonable inference, that those characters may be modified 

 by a change in the conditions of existence. But the 

 strongest evidence that we have of such changes in the 

 characters of species is afforded by facts that cannot be in 

 any sense referred to Cuvier's principle, though they are 

 quite consistent with it. 



If Cuvier's doctrine, that the organization of an animal 

 is determined by the conditions and mode of its life, were 



