182 Proceedings of the Royal Physic(il Society. 



While the younger Palaeozoic systems were being ac- 

 cumulated, excess of upheaval over depression resulted in 

 the gradual increase of the land. The continental plateau 

 came more and more to the surface, in spite of many 

 oscillations of level. It is quite possible, nay, even pro- 

 bable that this persistent growth of land, and consequent 

 modification of oceanic currents, may have rendered the 

 climatic conditions of later Palaeozoic times less uniform; 

 but, if so, such diminished uniformity has left no recognisable 

 impress on either faunas or floras. For fossils characteristic 

 of the Devonian and Carboniferous strata of temperate 

 latitudes occur far within the Arctic Circle. 



Descending to the Mesozoic era, we find that the character 

 and distribution of marine faunas are still indicative of 

 uniformity. There could have been little difference of 

 temperature at that time between Arctic seas and those 

 of our own latitude. Cosmopolitan species abounded in the 

 Jurassic waters, but were relatively less numerous in those 

 of the Cretaceous period. Professor JSTeumayr maintains 

 that already, in the Jurassic period, the climate had become 

 differentiated into zones. This, he thinks, is indicated by 

 the fact that coral reefs abound in the Jurassic strata of 

 Central Europe, while they are wanting in the con-, 

 temporaneous deposits of boreal regions. Dr Heilprin, on 

 the other hand, is of opinion that this and certain other 

 distinctive features of separate Jurassic life-provinces may 

 not have been due to differences of temperature, but rather 

 to varying physical conditions, such as character of the 

 sea-bottom, depth of water, and so forth. Perhaps the safest 

 conclusion we can come to, in the present state of the 

 evidence, is tliat the climatic conditions of tlie IVIesozoic 

 era were, upon the whole, less obviously uniform than 

 those of earlier ages, but that marked zones of climate 

 like the present had not as yet been evolved. At the same 

 time, when we consider how many great geographical revolu- 

 tions took place during the period in question, we must be 

 prepared to admit that these could hardly fail to have 

 influenced the climate, and thus to have induced modifica- 

 tions in the distribution of faunas and floras. And probably 



