184 ZOOLOaiOAIi GEOaEAPIIT OF THE MALAY AECHIPELAGO. 



of these countries must necessarily depend on the same physical 

 changes which the Southern hemisphere has undergone ; and we 

 are therefore led to conclude that insects are much less persistent 

 in their specific forms than flowering plants, while among Mam- 

 malia and land birds (in which no genus even is common to the 

 countries in question) species must die and be replaced much more 

 rapidly than in either. And this is exactly in accordance with the 

 fact (well established by geology) that at a time when the shells 

 of the European seas were "almost all identical with species now 

 living, the European Mammalia were almost all different. The 

 duration of life of species would seem to be in an inverse propor- 

 tion to their complexity of organization and vital activity. 



In the brief sketch I have now given of this interesting subject, 

 such obvious and striking facts alone have been adduced as a tra- 

 veller's note-book can supply. The argument must therefore lose 

 much of its weight from the absence of detail and accumulated 

 examples. There is, however, such a very general accordance in 

 the phenomena of distribution as separately deduced from the 

 various classes or kingdoms of the organic world, that whenever 

 one class of animals or plants exhibits in a clearly marked manner 

 certain relations between two countries, the other classes will cer- 

 tainly show similar ones, though it may be in a greater or a less 

 degree. Birds and insects will teach us the same truths ; and even 

 animals and plants,' though existing under such different conditions, 

 and multiplied and dispersed by such a generally distinct pro- 

 cess, will never give conflicting testimony, however much they 

 may differ as regards the amount of relationship between distant 

 regions indicated by them, and consequently notwithstanding the 

 greater or less weight either may have in the determining of 

 questions of this nature. 



This is my apology for offering to the Linnean Society the pre- 

 sent imperfect outline in anticipation of the more detailed proofs 

 and illustrations which I hope to bring forward on a future 

 occasion. 



