398 ZOO-GEOGRAPHICAL NOTES.

allied varieties, and to this end the regions of moist, warm, dense forests should chiefly be searched. Heuglin, though furnished with comparatively insufficient material, has already pointed out that, as we advance from the north in the direction of the equator, the affinities of the north-eastern fauna with that of the west, and even of the south, grow more numerous, that is, a number of varieties formerly believed to belong exclusively to the west spread themselves much farther to the east than has generally been supposed. My own collections confirm this opinion most thoroughly, and I will attempt to show that the reason of it is to be found in the physical formation of the country and the vegetation which clothes it.

Soon after the Bahr-el-Jebel has passed the rocky defile of Jinja, after leaving the Victoria Lake, and has spread itself out in the wide alluvial plains of eastern Unyoro into a series of shallow, lake-like basins, it is again confined by mountain ranges, and flows between high rocky banks over a succession of steps, which form rapids, to the Mwutan Nzige or Albert Lake. Except the extensive swampy flats to the east of Mrili, which are exposed to the inundations of the river, and the equally flat tracts of the southern Lango district, which in some places extend to the river, and are covered with a vegetation of grass and papyrus peculiar to such bogey districts, both banks of the river are overgown with thick forest. On the northern bank, in consequence of less abundant irrigation, greater declivity, and a very insignificant thickness of surface soil, the leathery-leaved and stiff forms of northern vegetation prevail, which are not atlected by temporary droughts. The southern bank, on the contrary, presents a succulent verdure, and a rich display of foliage belonging to tropical species and dependent on constant moisture. Though the region between the lakes, a decidedly mountainous country, cannot produce the exuberant vegetation of the western tropics, owing to its elevation, yet the dews, continuing all through the year without intermission, and the exceedingly rich and well-watered soil, supply a basis for a considerable development of vegetable growth. Besides the imposing representatives of northern latitudes, the Ficus, Bal-