FRESH-WATER LIFE. The fresh-water fauna presents certain characteristic features. and is divisible into 'fluviatile' forms, inhabiting streams and rivers, and `laeustrine' forms, inhabiting large lakes, where. as in the sea, the life is di visible into 'littoral,' pelagic," and 'deep-water.' DisPEastox. The slow or rapid spread of a species from its point of origination will depend upon its powers of locomotion and its adaptabil ity to new ciremnstances, In the manunalia this is a matter of walking and swimming•. Some are excellent swimmers, and the great spread of the tiger throughout the Orient is mainly due to his natatorial ability. The faculty of flight has made bats more nearly cosmopolitan than is any other order of mammals. The wings of birds and insects give them a superior means of dispersal: flightless birds are, and always have been, much more circumscribed than the fliers. keptiles and amphibians are poorly provided with means of locomotion, and are often very sluggish, while fresh-water fishes. except anadromons ones, usu ally dwell in confined waters. As for the mol lusks. lower order., of insects, worms, and small sedentary animals of the shores. their principal. and often sole, resource is in the spread of their eggs or free-swimming larvle. Mechanical abb., however, render important assistance in the dis persal of certain species. a-, is the ease with so many plants. Land animals. large or small, fre quently float across spaces of sea on lee cakes or driftwood, and some islands have undoubtedly been colonized in this way: yet it is a remote chance to trust, for unless a pair or a pregnant female were thus transported. no gain would result. Parasitic animals are carried about by their hosts. Small erustaeva and mollusks may be carried great distances by wind. or by adher ing to the feet of birds. Infusoria. the eggs of rotifers. other mieroseopic forms may be transported in the dried condition by the wind. Darwin brought forward nmeh curious informa tion on this subject in the second of hi.
-s/(( ( I London.
It will be seen that the varying :thiliti•s among the different groups of animals would make their distribution equally different one from the other. Salt water is a poison to larval amidtibians, while Illay fl} across more than 1000 miles of sea space. Mammals may run about rapidly, while the reptiles and small invertebrate crea tures must creep slowly or not at all. Finally, marine animals occupy an aria and medium en tirely different from that held by terrestrial ani mals. These variations must be borne in mind in attempting to reduce to scheme and system the vastly diverse phenomena of zoogeography.
Heretofore We have been considering only the dispersion and restriction of a species. Put as a rule several kindred co exist in any given area, and these usually differ in their range, while occasionally a species of the genus is to he found somewhere else, entirely disconnected front its fellows. This shows that the gi-ographical area covered by a genus is greater than that of a species; and, carrying the same inquiry further, it appears true of families, orders. and classes. The larger the group in the scheme of classification the wider its geographical area. To ascertain and record the spread. past and present, of the groups of animals is the business of zoogeography. The earlier students drew up a map in which they con fidently set apart a series of realms, provinces, and subprovinces. A vast anamnt of such in formation hail been tabulated by Selater, Dar win, Schmarda, Mn•ray, and others previously to the publication, in 1876, of Alfred Russell \Val lace's monumental work on the subject. Wallace, following Selater (1S5i), divided the globe into six grand `regiono,' each characterized by groups of animals absent from, or very scantily repre sented in, any other. These were: Paicarctic llegion.—Europe, Africa north of the Sahara. and Asia north of the Himalayas.