FIELD AND FOREST. 79 



resulting from environing causes, and ever susceptible to the changes 

 of condition that may supervene, while the fishes are a generalized 

 type, and live in a medium where they are much less exposed to the 

 vicissitudes of climate and other conditions, and where change, there- 

 fore, is less likely to supervene; consequently the representatives of the 

 two classes might naturally be expected to indicate differences in the 

 relations of the several faunas to each other, and such is markedly the 

 case. 



It has already been noticed that Mr. Sclater, from an ornithological 

 point of view, segregated the several regions of the globe under two 

 primary groups — Palaeogsean and Neogaean. To a greater extent, per- 

 haps, than would at first be supposed by special students of other 

 classes, he was justified in such a differentiation, for the interchanges 

 of the species of the north and the south with those of the tropics, 

 and vice versa, are so numerous as to give a stamp of comparative ho- 

 mogeneity to the two great areas known as the old and new worlds. 

 The birds, in fact, indicate in the most marked manner the effected 

 accomodation to existing conditions. The fresh-water fishes, on the 

 contrary, point to an entirely different relationship, and if we should 

 take these animals for the determination of the primary regions of the 

 globe, the present combinations of land and water must be entirely 

 ignored, and their faunas correlated de novo on a very different basis. 

 In such case, North America, Europe and Asia would form one 

 great division, in contradistinction to another, which would be con- 

 stituted by Australia, South America and Africa. These great divis- 

 ions, however, are very unequal in one respect : the northern division, 

 or Cenogaea, is comparatively homogenous, and its several regions not 

 very well defined, while the southern district, or Eogsea, is, on the 

 contrary, subdivisible into three very distinct regions, the most gener- 

 alized of which is Australia, and the least so Africa, while South 

 America intervenes between the two, and, on the one hand, shares 

 with Australia some forms, and, on the other hand, some with Africa, 

 the common ones being in each case restricted to the two mentioned 

 together. To some extent our author recognizes these relations (vol. 

 i. pp. 398, 174.) 



These combinations may be explicable by different hypotheses: (i) 

 the forms found in the several regions may be the remnants of a once 

 widely-spread fauna, or (2 ') derivatives of a special fauna, diffused when 



