374 Transactions. — Geology. 



between the known Carboniferous floras of these respective 

 regions ; or, to quote a more strilcing illustration, the Miocene 

 floras of New Zealand and Tasmania, consisting principally of 

 oaks, elms, beeches, alders, laurels, &c., show a closer rela- 

 tionship to the existing vegetation of Europe than to the 

 present floras of these islands. 



As would naturally be expected, the marine faunas show a 

 closer parallelism than the floras, a fact probably due to the 

 greater facilities for rapid difl'usion possessed by marine in- 

 habitants in a continuous sea compared with the slower difl'u- 

 sion of terrestrial organisms checked by physical obstructions, 

 such as wide stretches of sea and mountain-chains. 



The true explanation of this seeming lack of parallelism of 

 life in geologic time is at present a subject for much conjec- 

 ture and speculation, and doubtless possesses a significance 

 which is not now very obvious to us. 



The theory which is thought to satisfy our present know- 

 ledge of the facts is that which supposes that the Southern 

 Hemisphere was the cradle of organic life, from which life 

 gradually difl'used itself northward by a process of slow migra- 

 tion, difi'erentiated in its progress by accidents of climate and 

 changes in the distribution of land and water. 



Attractive as such a theory must be to southern geologists, 

 I am inclined for my own part, after a perusal of recent dis- 

 coveries in arctic Europe and America, to favour the existence 

 of two independent and widely separated centres of distribu- 

 tion, from which migration slowly radiated into the far-distant 

 regions where types of two distinct ages appear to commingle. 



The notorious likeness existing between the marine faunas 

 of the diflerent geological periods in the Southern Hemisphere 

 with the faunas of the corresponding periods in the Northern 

 Hemisphere, from the Cambrian, when the first assem- 

 blage of life suddenly burst upon the geological horizon — life 

 that in many respects was complex and comparatively highly 

 organized, whose appearance was unexpected and perplexing 

 because there were no known probable ancestors — from then 

 upward to the close of the Tertiary, this likeness may be held 

 to support the bipolar theory of distribution. 



The rate of dispersion, as pointed out by Huxley, Wallace, 

 and other writers, is governed by two dominant factors, of 

 which the position of the centre of origin of each organic unit 

 is one, and the distribution of land and water at the time of 

 dispersion the other. Moreover, in a mixed fauna the powers 

 of migration would not be equal, nor the ability to withstand 

 varying climatic and other adverse conditions. Hence the 

 rate of advance would be unequal ; but of this we have no 

 certain evidence. On the contrary, it is probable that the 

 faunas of the difi"erent geological periods were similar and 



