GEOGRAPHICAL DISTRIBUTION. I 23 



of organisms, but in the case of insects the root and leaf of 

 the same tree present greater contrasts in conditions of en- 

 vironment than two trees of the same species a thousand miles 

 apart. Geographical distribution, and other terms associated 

 with it, have reference fundamentally to conditions of environ- 

 ment, whether the distribution is on geographical or other 

 lines. 



Representative Species, Common Descent, and Migration of 

 Species. — Similar species of the genus in other provinces were 

 called representative species by Forbes. Another idea, in- 

 cluded in this hypothesis, was that all the individuals of a 

 species had a conivion deseent. The idea of common descent 

 was associated with the definition of species, and when the 

 same species was recognized in two distinct provinces, the fact 

 was explained by the theory of diffusion, or migration of species; 

 and in defence of the theory of the specific centres Forbes held 

 that provinces, to be understood, must be traced back, like 

 species, to their history and origin in past time; and again, 

 that " species have a definite existence, and a centralization in 

 geological time as well as in geographical space, and that no 

 species is repeated in time." 



Darwin did not deny the Facts, but explained them differently 

 from Forbes. — Darwin, who gave a different interpretation of 

 the facts, recognized the truth of the proposition set forth by 

 Forbes. In his famous " Origin of Species" he says (in reply 

 to the question "whether species have been created at one or 

 more points of the earth's surface," and after some discussion 

 of the topic): " Hence, it seems to me, as it has to many of 

 the naturalists, that the view of each species having been pro- 

 duced in one area alone, and having subsequently migrated 

 from that area as far as its power of migration and subsistence 

 under past and present conditions permitted, is the most 

 probable." 



Forbes' Explanation of the Origin of Species. — In Forbes' no- 

 tion of "specific centres" is included the idea that ancestry is 

 responsible for the "specific characters" of the individual. 

 "Every true species presents in its individuals," he says, 

 "certain features, specific characters, which distinguish it 

 from every other species ; as if the Creator had set an ex- 



