The geographical distribution of animals, with a study of the relations ...

402 ZOOLOGICAL GEOGRAPHY. [part hi.not only on account of what we know of the permanence ofcontinents and deep oceans, hut because such a connection musthave led to much more numerous and important casesof similarityof natural productions than we actually find. And ifwithin the life of species such interchange may have takenplace across seas of greater or less extent, still more easy is itto understand, how, within the life of genera and families, a numberof such interchanges may have occurred ;yet always limitedto those groups whose conditions of life render transmissionpossible. Had an actual land connection existed within thetemperate zone, or during a period of warmth in the Antarcticregions, there would have been no such strict limitations to theinter-migration of animals.that floating ice has had some share in theIt may be held to support the viewtransmission of fishand amphibia, when -we find that in the case of the narrowtropical sea dividing Borneo from Celebes and the Moluccas, noproportionate amount of transmission has taken place, butnumerous species, genera, and whole families, terminate abruptlyat what we have other reasons for believing to be the furthestlimits of an ancient continent. We can hardly suppose, however,that this mode of transmission would have sufficed forsuch groups as tree-frogs, which are inhabitants of the moretemperate or even warm portions of the two southern lands.Some of these cases may perhaps be explained by the suppositionof a considerable extent of land in the South-Temperate andAntarctic regions now submerged, and by a warm or temperateclimate analogous to that which prevailed in the Arctic regionsduring some part of the Miocene epoch ; while others may bedue to cases of survival in the two areas of once wide-spreadgroups, a view supported in the case of the Amphibia by theerratic manner in which many of the groups are spread overthe globe.From an examination of the facts presented by the variousclasses of vertebrates, we are, then, led to the conclusion,that there is no evidence of a former land-connection be,tween the Australian and Neotropical regions ;but that thevarious scattered resemblances in their natural productions