DISTRIBUTION OF GENERA AND SPECIES 



175 



and, as it -were, scattered geographically, there docs not 

 appear to be any centre where the genus is more predom- 

 inant than elsewhere. In this respect it differs ap- 

 parently from the other genera, which may be said to 

 have each, one or more climatal or topographical centres. 

 In the dispersion of this genus among the islands of the 

 sea, a remarkable contrast is presented to the distribution 

 of the superior classes, and particularly of the mamma- 

 lia, the latter being, in general, entirely wanting in small 

 islands remote from the continents, while, in those adja- 

 cent to them, they consist of the species belonging to 

 the main land. 



