INSULAR FAUNAS. The faunas of most islands near continents date back to the time when these islands were a part of the adjacent mainland, or were separated front it only by narrow straits. The explanation of the broad and conspicuous disparity that exists between. the faunas of such large islands as Australia and Madagascar, and those of Asia and Africa. respectively, is ac counted for by the evidently very ancient (late at which they became dissevered. "It is evident." remark Flower and Lydekker. "that Australia has been isolated from the Asiatic Continent from some very remote geological epoch. at which period it is probable that monotretnes and marsupials were the dominant if not the solo representatives of the mammalia then existing. Consequently Australia has never been able to receive an influx of the Entherian orders, which have probably swept away all the marsupials except the small American opossums from the rest of the globe. Again. the large island of Madagascar. which has a fauna of an African type, but still very markedly different from that of the mainland. may be considered to have been connected with the latter at a time when the Entheria had become the dominant forms, but has been separated for a suffieiently long period to have enabled a large number of its species and genera to have become distinct from those of the adjacent continent. Similarly there is evidence to show that South America was prob ably cut off for a considerable period from the northern half of the American Continent, in consequence of which its lowly organized fauna of Edentat.s were enabled to attain such a re markable development in the later geological 14.1'110-, C011-111t Island I,ifc (Ness' York. 1.-4titt.. See I SOL.V1111N.
• :11)01c instances [and their converse, ex emplitied by the identity of insular and conti fauna. where a land union is known have existed lately] are suni,ient to show . . h.\ largely the present distribution of 1112111111:11ia 11 life is 1)01111d 113) with the past his tory of our globe. IVe must, however, not omit to na !Ilion another very important agency of past time. which has likewise had great influ ence on the present distribution of the various faunas of the Northern Hemisphere. is the
Epoch, which took place im mediately before the establishment of the present state of thing-, and appears to have been the cause of the extinetion of many of the larger mammalian types which formerly inhabited Put while Many species have had a far IA iller distribution in past ages than now (and in a general way a widely. distrilmled form may be regarded as an old form), many species seem never to have expanded much, some being limited to a short stretch of coast, or a single river valley. or to a certain mountain or island.
DEsrit-rs, Morx.rxiss, ETC., AS BARRIERS. Other I11:111 the sea, therefore. have been ef fective in impeding the spread of crescent varieties. species, and groups. :Most prominent among these is a desert region. The Sahara cuts off almost completely the fauna of Europe from that of Africa ; and the eont inuat ion of the arid treeless area northeastward across Asia divides the northern .\siatie animals front the southern in a most emphatic way. Similarly, the fauna of North Ameriva stops and that of the southern continent begins where the plains and tablelands of Arizona and northern Alexi...) a seantily planted space of semi-desert and sterile hills. uninviting to either set of animals as a xvhole, yet invaded by both.
Long and lofty mountain ranges stand next in importance, probably, as physical harriers, but this is more marked in the warmer than in the cooler parts of the world, since animals accus• touted to the torrid conditions of tropical plains ea 111101 01' will 1101 endure the cold and lack of customary food eneountl•red even on the passes, and so do not cross over the highlands. The mountains of North America seem to make less difference with the distribution of our ani mals. as between the .\ tlant iv and Pacific slopes, than does the central plains area : and that of Enrope has been little affected by the presence of mountains. The Atlas range is more marked as a boundary, but that is because of the neigh boring desert: and similarly the great Central .\ -inn ranges are part of the lofty, cold. and arid region which as a whole forms probably the most effective inland harrier in the world.