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 
