ADDRESS. 25 



tied tip to definite astronomical cycles of glaciation, which may not 

 always suit the geological facts, and of correlating elevation and subsid- 

 ence of the land with changes of climate affecting living beings. It will, 

 however, be necessary, as Wallace well insists, -that we shall hold to that 

 degree of fixity of the continents in their position, notwithstanding the 

 submergences and emergences they have experienced, to which I have 

 already adverted. Sir Charles Lyell, more than forty years ago, pub- 

 lished in his * Principles of Geology ' two imaginary maps which illus- 

 trate the extreme effects of various distribution of land and water. In 

 one all the continental masses are grouped around the equator. In the 

 other they are all placed around the poles, leaving an open equatorial 

 ocean. In the one case the whole of tho land and its inhabitants would 

 enjoy a perpetual summer, and scarcely any ice could exist in the sea. 

 In the other the whole of the land would be subjected to an arctic climate, 

 and it would give off immense quantities of ice to cool the ocean. But 

 Lyell did not suppose that any such distribution as that represented in 

 his maps had actually occurred, though this supposition has been some- 

 times attributed to him. He merely put what he regarded as an extreme 

 case to illudtrate what might occur under conditions less exaggerated. 

 Sir Charles, like other thoughtful geologists, was well aware of the 

 general fixity of the areas of the continents, though with great modifica- 

 tions in the matter of submergence and of land conditions. The union, 

 indeed, of these two great principles of fixity and diversity of the con- 

 tinents lies a the foundation of theoretical geology. 



We can now more precisely indicate this than was possible when Lyell 

 produced his * Principles,' and can reproduce the conditions of our con- 

 tinents in even the more ancient periods of their history. Some examples 

 may be taken from the history of the American continent, which is more 

 simple in its arrangements than the double continent of Europ-asia. 

 We may select the early Devonian or Erian period, in which the magni- 

 ficent flora of that age — the earliest certainly known to us — made its 

 appearance. Imagine the whole interior plain of North America sub- 

 merged, so that the continent is reduced to two strips on the east and 

 west, connected by a belt of Laurentian land on the north. In the great 

 mediterranean sea thus produced the tepid water of the equatorial current 

 circulated, and it swarmed with corals, of which we know no less than 

 one hundred and fifty sp'-'iies, and with other forms of life appropriate 

 to warm seas. On tho islands and coasts of this sea was introduced 

 the Erian flora, appearing first in the north, and with that vitality and 

 colonising power of which, as Hooker has well shown, the Scandinavian 

 flora is the btst modern type, spreading itself to the south.' A very simi- 



> As I have elsewhere endeavoured to show {Report on Silurian and Devonian 

 Plants of Canada), a warm climate in the Arctic region seems to have afforded the 

 necessary conditions for the great colonising floras of all geological periods. Gray 

 had previously illustrated the : tme fact in the case of the more modern floras. 



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