4 MH. A. MUllRAr ON THE OlCOGIlArillCAL llELATIONS Ob' 



teresting problem of the occurrence of similar forms in tlie tem- 

 perate and cold regions of both hemispheres. Hitherto the 

 hypotheses by which this has been attempted to be explained 

 have, I believe, either been : Ist, by accidental introduction ; 

 2nd, by the supposition that the glacial epoch had so modified the 

 climate of the globe as to allow an extension or interchange of 

 faunas lying on different sides of the equator ; 3rd, by the suppo- 

 sition that one general fauna had formerly extended over the 

 Avhole world, and that the similarities which we find in antipodal 

 countries are relics of this general fauna ; and 4th, that a former 

 geographical connexion with identity of fauna and flora must have 

 subsisted between the two regions. There is another hypothesis 

 to the effect that the similar species are representative species, 

 meaning by representative something dift'ercnt from derivative and 

 independent of affinity. This latter proposition, I frankly confess, 

 is beyond my conception. I cannot conceive of any other kind 

 of representation in species than that arising from derivation. 

 The other hypotheses are entitled to more consideration, and I 

 shall briefly state my opinion \ipon each. 



As to similarity being due to the introduction of species 

 by accidental dispersal, it is to be noted that this cause must 

 always be in its very nature exceptional and isolated, and cannot 

 be expected to make its impress on a whole fauna. 



The Atlantic islands, for example, which were, and perhaps still 

 are, very generally cited as an instance of colonization by acci- 

 dental introduction, have been shown by Mr. Wollaston to have 

 all one coleopterous subfauna, and that one peculiar to themselves. 

 The general basis is European, but overlying that is something 

 else, a number of species of special type found in all and found no- 

 where else. Now if these islands, as is maintained by some, have 

 been peopled by chance visitors from Europe, how have they all 

 got in addition this special type ? and why should there be, as in 

 Trophonius Cave, " vestigia nulla retrorsum " ? Why have none 

 of these special forms ever wandered to Europe ? Why should 

 things only come from Europe and nothing ever go back in re- 

 turn ? There is no law against reciprocity here, and yet it is 

 rigorously excluded. Moreover the explanation, if ti-ue, sliould 

 apply to every part of the globe, and illustrations of its existence 

 should be in greatest force in the lands which are nearest to each 

 other and which have most intercourse with each other. And 

 this is notoriously not the case. Australia, although so much 



