146 



ISLAND LIFE. 



[part I. 



: It would be easy to suggest other probable changes which would 

 produce a marked effect on climate; but we will only refer to 

 the subsidence of the Isthmus of Panama, which has certainly 

 happened more than once in Tertiary times. If this subsidence 

 were considerable it would have allowed much of the accumulated 

 warm water which initiates the Gulf Stream to pass into the 

 Pacific ; and if this occurred while astronomical causes were 

 tending to bring about a cold period in the northern hemisphere, 

 the resulting glaciation might be exceptionally severe. The 

 effect of this change would however be neutralised if at the 

 same epoch the Lesser and Greater Antilles formed a connected 

 land. 



Now, as such possible and even probable geographical 

 changes are very numerous, they must have produced important 

 effects ; and though we may admit that the astronomical causes 

 already explained were the most important in determining 

 the last glacial epoch, we must also allow that geographical 

 changes must often have had an equally important and perhaps 

 even a preponderating influence on climate. We must also 

 remember that changes of land and sea are almost always 

 accompanied by elevation or depression of the pre-existing 

 land: and whereas the former produces its chief effect by 

 diverting the course of warm or cold oceanic currents, the 

 latter is of not less importance in adding to or diminishing 

 those areas of condensation and ice-accumulation which, as 

 we have seen, are the most efficient agents in producing 

 glaciation. 



If then Sir Charles Lyell may have somewhat erred in attach- 

 ing too exclusive an importance to geographical changes as 

 bringing about mutations of climate, his critics have, I 

 think, attached far too little importance to these changes. We 

 know that they have always been in progress to a sufficient 

 extent to produce important climatal effects; and we shall 

 probably be nearest the truth if we consider, that great extremes 

 of cold have only occurred when astronomical and geographical 

 causes were acting in the same direction and thus produced a 

 cumulative result, while, through the agency of warm oceanic 

 currents, the latter alone have been the chief cause of mild 



