STRUCTURE AND GEOGRAPHICAL DISTRIBUTION. 57 



physical agents ; and I may add that the vegetable king- 

 dom presents a series of facts identical with these. This 

 proves that all the higher relations among animals and 

 plants are determined by other causes than by mere phy- 

 sical influences. 



While all the representatives of the same genus are iden- 

 tical in structure, 1 the different species of one genus differ 

 only in their size, in the proportions of their parts, in 

 their ornamentation, in their relations to the surrounding 

 elements, etc. The geographical range of these species 

 varies so greatly that it cannot afford in itself a criterion 

 for the distinction of species. It appears further, that 

 while some species which are scattered over very exten- 

 sive areas, occupy disconnected parts of that area, other 

 species, closely allied to one another, and which are gene- 

 rally designated under the name of representative species, 

 occupy respectively such disconnected sections of these 

 areas. The question then arises, how these natural boun- 

 daries assigned to every species are established. It is now 

 generally believed that each species had, in the beginning, 

 some starting point, from which it has spread over the 

 whole range of the area it now occupies ; and, that this 

 starting point is still indicated by the prevalence or con- 

 centration of such species in some particular part of its 

 natural area, which, on that account, is called its centre of 

 distribution or centre of creation, while at the external 

 limits of the area its representatives thin out, as it were, 

 occurring more sparsely, and sometimes in a reduced 

 condition. 



It was a great progress in our science, when the more 

 extensive and precise knowledge of the geographical dis- 

 tribution of organized beings forced upon its cultivators 



1 See hereafter, Chap. II, Sect. 5. 



