VI. 



OF ZOOLOGICAL FOCI, OR POINTS OF ORIGIN WHENCE 

 GENERA AND SPECIES ARE DERIVED. 



The distribution of species, as treated of in the pre- 

 ceding chapter, has thus far been considered only as 

 influenced by existing causes, whose operation is known. 

 These, however, will hardly account for the present con- 

 dition of all the species, or rather, some of them as it 

 would seem, occupy their present positions, notwithstand- 

 ing the restraining influences, which, if the preceding 

 views are correct, should have limited them within more 

 narrow bounds. The species alluded to, are those which 

 occupy dififerent and widely distant countries and conti- 

 nents, and which have already been discovered upon so 

 many points of the earth's surface, as to indicate an 

 almost universal difi"usion. The researches which are 

 now continually undertaken, to elucidate the zoology of 

 almost every part of the world, frequently bring to our 

 knowledge instances of the wide range of species, which 

 had been noticed previously only in a single coimtry ; 

 and the number of these is already large. The most 



