402 ZOOLOGICAL GEOGRAPHY. [part hi. 



not only on account of what we know of the permanence of 

 continents and deep oceans, but because such a connection must 

 have led to much more numerous and important cases of simi- 

 larity of natural productions than we actually find. And if 

 within the life of species such interchange may have taken 

 place across seas of greater or less extent, still more easy is it 

 to understand, how, within the life of genera and families, a num- 

 ber of such interchanges may have occurred ; yet always limited 

 to those groups whose conditions of life render transmission 

 possible. Had an actual land connection existed within the 

 temperate zone, or during a period of warmth in the Antarctic 

 regions, there would have been no such strict limitations to the 

 inter-migration of animals. It may be held to support the view 

 that floating ice has had some share in the transmission of fish 

 and amphibia, when we find that in the case of the narrow 

 tropical sea dividing Borneo from Celebes and the Moluccas, no 

 proportionate amount of transmission has taken place, but 

 numerous species, genera, and whole families, terminate abruptly 

 at what we* have other reasons for believing to be the furthest 

 limits of an ancient continent. We can hardly suppose, how- 

 ever, that this mode of transmission would have sufficed for 

 such groups as tree-frogs, which are inhabitants of the more 

 temperate or even warm portions of the two southern lands. 

 Some of these cases may perhaps be explained by the supposi- 

 tion of a considerable extent of land in the South-Temperate and 

 Antarctic regions now submerged, and by a warm or temperate 

 climate analogous to that which prevailed in the Arctic regions 

 during some part of the Miocene epoch ; while others may be 

 due to cases of survival in the two areas of once wide-spread 

 groups, a view supported in the case of the Amphibia by the 

 erratic manner in which many of the groups are spread over 

 the globe. 



From an examination of the facts presented by the vari- 

 ous classes 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 the 

 various scattered resemblances in their natural productions 



