Geograiihical Distribution of Animals. 213 



the proportion. The species of Cuba, St, Domingo, and Porto 

 Eico arc but partially known ; yet they confirm the general 

 proposition. The same is true of the land shells of the Philip- 

 pine Islands, and of the Sandwich Islands. 



24. The distribution of fresh water continental species also 

 proves that the original creation of the insular species was on a 

 different plan from that of the continental species (§ 15). 



25. If large groups of such islands, as the West Indies, 

 should be united in a common area of dry land, then, according 

 to the theory which accounts for the facts of distribution by 

 actual dispersion from centres, there would be zoological 

 provinces containing five to tenfold as many species as any 

 which now exist. 



26. The geological fact, that continents by submergence 

 become islands, and that islands by emergence become conti- 

 nents, does not affect the foregoing reasoning, because such 

 changes require an amount of time exceeding one geological 

 period, during which time there is a change of faunae. 



27. The original creation of many individuals of a species in 

 different parts of its area has been the principal cause of the 

 present facts of the distribution of the individuals of the species. 

 Physical agents have exerted only a modifying agency. 



28. In organic nature, principles are not observed through- 

 out any department, with mathematical uniformity ; on the 

 contrary each idea appears with various degrees of development 

 from its maximum to its .minimum, and often to a vanishing- 

 point. Hence it is probable that the introduction of species has 

 been accomplished by the creation of original individuals vary- 

 ing in number from a great multitude to a few. In the same 

 manner the actual numbers of the individuals of species now 

 range from inconceivable profusion to extreme raiity. 



Relations of the subject to geological reasoning. 



29. Such insular faunie, as have been described in § 7 and 

 § 13, prove that the islands which they inhabit, have been 



