184 MIGRATION AND DISTRIBUTION 



globe, but at any rate of the period of its reaching the 

 portion of the earth's surface where we now find it. One 

 great thing to guard against is the presumption that the 

 fauna originated within its present area and has been 

 always contained therein. Thus I take it that the fauna 

 which characterises the New Zealand Region for I follow 

 Professor Huxley in holding that a region it is fully 

 entitled to be called is the comparatively little-changed 

 relic and representative of an early fauna of much wider 

 range ; that the characteristic fauna of the Australian 

 Region exhibits in the same way that of a later period ; 

 and that of the Neotropical Region of one later still. But 

 while the first two regions have each been so long 

 isolated that a large proportion of their fauna remains 

 essentially unaltered, the last has never been so com- 

 pletely severed, and has received, doubtless from the 

 north, an infusion of more recent and therefore stronger 

 forms ; while, perhaps impelled by the rivalry of these 

 stronger forms, the weaker have blossomed, as it were, 

 into the richness and variety which so eminently 

 characterise the animal products of Central and South 

 America. I make no attempt to connect these changes 

 with geological events, but they will doubtless one day 

 be explained geologically. It is not difficult to conceive 

 that North America was once inhabited by the ancestors 

 of a large proportion of the present Neotropical fauna, 

 and that the latter was wholly, or almost wholly, thrust 

 forth perhaps by glacial action, perhaps by the incur- 

 sion of stronger forms from Asia. The small admixture 

 of Neotropical forms that now occur in North America 

 may have been surviviors of this period of stress, or they 

 may be the descendants of the more ancient forms 

 resuming their lost inheritance. Beyond the fact that 

 these few Neotropical forms continue to exist in North 

 America, its fauna seems to be in a broad sense insepar- 

 able from that of the Palaearctic area, and, in my belief, 

 is not to be separated from it. The most difficult 

 problems are those connected with the Ethiopian and 

 Indian (which Mr. Wallace calls the Oriental) areas ; but 



