THE EUROPEAN SEAS. 



9 



found similar to it or them, we connect the idea of 

 descent with the definition of a species, and (hypo- 

 thetically) assume the descent of all the individuals 

 of each species from one original stock, monoecious 

 or dioecious, as the case may be. The term specific 

 centre has been used to express that single point 

 upon which each species had its origin, and from 

 which its individuals become diffused. In the course 

 of their diffusion, and during the lapse of time, the 

 species may become extinguished in its original cen- 

 tre, and exist only on some one or several portions 

 of the area over which it became diffused. Groups 

 of the individuals of a single species may thus be- 

 come isolated, and if they be placed far apart, may 

 present the fallacious aspect of two or more centres 

 for the same species. To get at the causes of such 

 phenomena, we must trace the history of the species 

 backwards in time, and inquire into its connection 

 with the history of geological change. We thus 

 trace the genealogy of the species, and unless there 

 has been any endeavour made to develop its pedi- 

 gree, and to connect its history in space with its 

 history in time, no man has a right to cite ano- 

 malous and isolated cases of distribution, as argu- 

 ments against the doctrine of specific centres. In 

 studying the geographical distribution of organized 

 beings philosophically, it is absolutely necessary 

 to call in the aid of geology ; and the time is not 

 far distant when no reasonable man will ven- 

 ture on that most interesting branch of natural 



