DISTRIBUTION OF GENERA AND SPECIES 
175 
and, as it were, scattered geographically, there does 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. 
