48 ZOOLOGICAL GEOGRAPHY. [part iii. 



number and variety of the tropical forms has given them the ad- 

 vantage. Tlins we find that among the Lncanidte, Buprestida^, and 

 Longicorns, the northern element is hardly perceptil^le. Most of 

 these are either purely Neotropical, or allied to Neotropical genera, 

 with the admixture, however, of a decided Australian element. 

 As in the case of the Amphibia and fresh-water fishes, the Aus- 

 tralian affinity, as shown by insects, is of two kinds, near and 

 remote. We have a few genera common to the two countries ; 

 but more commonly the genera are very distinct, and the affinity 

 is shown by the genera of both countries belonging to a group 

 peculiar to them, Init whicii may be of very great age. In the 

 former case, we must impute some of the resemblance of the two 

 faunas to an actual interchange of forms within the epoch of 

 existing genera— a period of vast and unknown duration in the 

 class of insects ; while in the latter case, and perhaps also in 

 many of the former, it seems more in accordance with the whole 

 of the phenomena, to look upon most of the instances as 

 survivals, in the two southern temperate areas, of the relics of 

 groups which had once a much wider distribution. That this is 

 the true explanation, is suggested by the numerous cases of dis- 

 continuous and scattered distribution we have had to notice, in 

 which every part of the globe, without exception, is implicated ; 

 and there is a reason why these survivals should be rather more 

 frequent in Australia and temperate South America, inasmuch 

 as these two areas agree in the absence of a considerable number 

 of otherwise cosmopolitan vertebrate types, and are also in many 

 respects very similar in climatic and other physical conditions. 

 The preponderating influence of the organic over the physical 

 environment, as taught by Mr. Darwin, leads us to give most 

 weight to the first of the above-mentioned causes ; to which we 

 may also impute such undoubted cases of survival of ancient 

 types as the Centetidaj of the Antilles and Madagascar— both 

 areas strikingly deficient in the higher vertebrate forms. The 

 probable mode and time of the cross migration between Australia 

 and South America, has been sufficiently discussed in our chapter 

 on the Australian region, when treating of the origin and aflBnities 

 of the New Zealand fauna. 



