174 Proceedings of the Royal Physical Society. 



apparently restricted in their range. It would be strange, 

 indeed, had it been otherwise ; for, however uniform the 

 climatic conditions may have been, still that uniformity 

 was only comparative. An absolutely uniform world-climate 

 is well-nigh inconceivable. All we can maintain is that 

 the conditions during certain prolonged periods were so 

 equable as to allow of the general diffusion of species over 

 vastly greater areas than now; and that such conditions 

 extended from low latitudes up to polar regions. Now, 

 among the chief factors which in our day determine the 

 limitation of faunas and floras, we must reckon latitude 

 and the geographical position of land and water. What, 

 then, it may be asked, were the causes which allowed of the 

 much broader distribution of species in former ages ? 



It is obvious that before a completely satisfactory answer 

 to that question can be given, our knowledge of past 

 geographical conditions must be considerably increased. If 

 we could prepare approximately correct maps and charts 

 to indicate the position of land and sea during the formation 

 of the several fossiliferous systems, we should be able 

 to reason with some confidence on the subject of climate. 

 But, unfortunately, the preparation of such correct maps 

 and charts is impossible. The data for compilations of 

 the kind required are still inadequate, and it may well 

 be doubted whether, in the case of the older systems, we 

 shall ever be able to arrive at any detailed knowledge of 

 their geographical conditions. [N'evertheless, the geological 

 structure of the earth's crust has been so far unravelled 

 as to allow us to form certain general conceptions of the 

 conditions that must have attended the evolution of our 

 continents. And it is with such general conceptions onl}^ 

 that I have at present to deal. 



I said a little ago that the question of geological climates 

 must now be considered from the point of view of the 

 permanency of the great dominant features of the earth's 

 crust. I need not recapitulate the evidence upon which 

 Dana and his followers have based this doctrine of the 

 primeval antiquity of our continental and oceanic areas. 

 It is enough if I remind you that by continental areas 



