Park. — The Loiver Mesozoic Bocks of Neiv Zealand. 375 



universal throughout the globe in the Palaeozoic and Second- 

 ary epochs, and that differentiation began with the dawn of 

 present life in the early Eocene. But this contention brings 

 •us no nearer the solution of the problem of dispersion, and 

 throws no light on the perplexing difficulties which surround 

 the processes of migration. 



Furthermore, it is well known to palaeontologists that at 

 certain geological epochs, apparently throughout the world, 

 there was a sudden appearance of animals and plants 

 belonging to orders, and even sub-orders, that were not 

 represented by any probable ancestors in older strata. For 

 example, there was the sudden and almost, if not quite, 

 simultaneous appearance in the Upper Cretaceous of acan- 

 thopterygian fishes and dicotyledonous angiospermous plants, 

 now the common existing types throughout the globe. 



Dispersion to the remoter parts from the centre of origin, 

 or even to the meeting-points from two or more centres of 

 migration, would occupy countless ages measured by our 

 telluric standard of time. A thousand years is but a fraction 

 of a geologic day ; and a migration starting at the dawn of 

 a new day would probably reach the remotest regions before 

 the advent of the coming day. 



Considering the question of distribution over wide areas 

 from a biological standpoint, it seems unlikely that all species 

 originated in one particular hemisphere. On the contrary, it 

 is probable that some species originated in the Northern 

 Hemisphere and other species in the Southern. 



If we entertain the unicentral theory of origin we must 

 suppose that migration progressed in a succession of great 

 waves ; that each wave encountered no insuperable bai'rier to 

 its forward movement ; that each wave maintained its distinc- 

 tive characteristics notwithstanding the diverse life, latitude, 

 and climate encountered in travelling from one hemisphere to 

 the other; and, further, that each wave was endowed with a 

 migratory instinct that impelled it forward through uncon- 

 genial latitudes until it finally arrived at its natural habitat. 

 The problem of the distribution of life is surrounded with 

 many difficulties. Existing conditions are doubtless a reflec- 

 tion of the past, and until we are able to explain the present 

 there seems little hope of unravelling the more tangled pro- 

 blems of geologic time. 



There is considerable evidence in favour of the belief that 

 there existed a synchronous parallelism of life in both hemi- 

 spheres — that is, in corresponding biological provinces and 

 bathymetrical horizons — up till later geological times, when 

 the extreme differentiations of modern types became the 

 dominant inhabitants of the globe. 



It is, at any rate, certain that the continuous succession of 



