Presidential Address. 2^1 



with the distribution of life, the great advantage of not being 

 tied up to definite astronomical cycles of glaciation, which 

 may not always suit the geological facts, and of correlating 

 elevation and subsidence 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, published in his ' Principles of Geo- 

 logy' two imaginary maps which illustrate 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 the 

 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 distri- 

 bution as that represented in his maps had actually occur- 

 red, though this supposition has been sometimes attributed 

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

 case to illustrate 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 con- 

 tinents, though with great modifications in the matter of 

 submergence and of land conditions. The union, indeed, of 

 these two great principles of fixity and diversity of the 

 continents lies at the foundation of theoretical geology. 



We can now more precisely indicate this than was pos- 

 sible when Lyell produced his ' Principles,' and can repro- 

 duce the conditions of our continents in even the more an- 

 cient 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 magnificent flora of that age — the 

 earliest certainly known to us — made its appearance. Ima- 



