212 Hints on the 



Relations of the subject to theories of the origin of species. 



20. It should not be assumed, before it has been proved, that 

 each species has originated in a single central point in its area, 

 nor that physical agents have been the principal causes of the 

 existing phenomena of distribution.* 



21. Natural agents may somewhat enlarge the area of a 

 species, but are not likely to transplant a species into a distinct 

 remote area. Thus if currents or locomotion transport a species 

 into a new habitat, the same species will be likely to occur at 

 places intermediate between this new habitat and the original 

 area. Consequently the effect of physical causes must usually 

 be limited to the enlargement of specific areas. 



22. The small areas of insular terrestrial species and large 

 areas of continental species suggest the theory of centres of 

 creation ; for it ma}'' be said that the insular species would have 

 spread over equally large areas, if they had not been restrained 

 by water. But although such a theory accounts for these facts, 

 the theory is not proved, because the facts are as satisfactorilj^ 

 accounted for by another theory, viz. that the original plan of 

 creation was different in the two cases. 



23. The fact, that the number of species in an insular 

 province is generally much greater, proportionally to the area 

 of the provinces, than in a contiuental province, proves that the 

 original plan of creation was different. Thus Jamaica contains 

 more known species of terrestrial Mollusca than the whole 

 of North America, from the Isthmus to Melville Island. It is 

 indeed probable that there are more unknown species in 

 ]\[exico and in Central America than in Jamaica ; but it is not 

 probable that enough remain undiscovered materially to affect 



* Some writers account for the facts of distribution, and for the introduction of 

 varieties of species by physical agents, while they designate such a method of 

 accounting for species and genera as atheistical. But it should ever be rcnietii- 

 bcred that physical agents are only the agents of the Divine will ; consequently 

 such opinions are not atheistical. 



