446 MIGRATION OF LIFE IN GEOLOGICAL TIME. 



found in the faunas of successive rocks. A slight difference in the 

 life in an overlying deposit was thought to imply a moderate denu- 

 dation, or a short interval in deposition. Great changes were held 

 to imply great denudation, and vast gaps utterly unrepresented by 

 strata. We escape from both of these hypotheses in better geogra- 

 phical knowledge of the distribution of life. For, although we must 

 appeal on the one hand to great upheavals and depressions of land, 

 which are essentially catastrophes in their effects, and have no choice 

 but accept the evidences of evolution concerning the variability of 

 organic types, and must allow that many of these are lost to us 

 through denudation of strata during the elevation and depression of 

 ancient lands, yet nothing but the succession of life provinces, super- 

 imposed vertically upon each other, can account for the succession of 

 life which the fossiliferous strata disclose. 1 



Migration of Life Provinces. When life is traced back in time 

 in any part of the earth, the geographical distribution of species is 

 found to differ from the existing distribution. Thus, in the peat of 

 the fen district of the eastern counties, several mammals occur fossil- 

 ised, such as the Bos primigenius, the Bos frontosus, and the Cervus 

 megaceros, which are now extinct, and these are associated with the 

 brown bear, wolf, beaver, roebuck, and other species which have long 

 since become extinct in these islands, or more limited in range. Simi- 

 larly we may compare the mammals of the peat with those found in 

 the valley gravels, with the result that some species of the peat, like 

 the Bos primigenius and the Cervus megaceros, are still met with, 

 associated with hippopotamus, lion, rhinoceros, elephants, and other 

 types which present a remarkable correspondence to the mammal life 

 of Northern Africa. We can only explain this succession of life in 

 the same district on the hypothesis of migration from some neigh- 

 bouring district, and must seek back for the origin of the fauna of the 

 gravel in the Newer Tertiary strata. We may not be justified in 

 inferring that the gravel fauna reached North Africa by migration 

 from Europe, though it is highly probable that the upheaval of 

 Northern Europe which produced the glacial period necessitated a 

 migration of the crag fauna southward, and that the mammalia re- 

 turned again to the north as the land resumed its ordinary vegetation 

 on bemg again depressed. Another instance which might be adduced 

 as tending to show how the limits of mammalian provinces of life 

 have varied in past geological time may be found in the types by 

 which the Crag deposits of the eastern counties show connections with 

 the Oriental region. Not to mention the mastodons, elephants, rhino- 

 ceroses, which have Eastern affinities, there is a tapir in the Crag com- 

 parable to the Malayan tapir, and the Hyaenarctos, which closely 

 resembles the same genus in the Siwalik rocks of Northern India. 

 This migration of mammal life is in harmony with the migration of 

 plant life. 



At the present day Japan belongs to a different natural history 



1 "Laws which have determined the Distribution of Life and of Rocks," 

 Annals and Mag. Nat. Hist., December 1867. 



