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undue accumulation of life on its loftiest pinnacles, 

 for, even allowing a certain number of species (which 

 even formerly were only just able to find a sufficient alti- 

 tude for their development) to have perished, we shall 

 have concentrated at that single elevation the residue of 

 all those which have survived from the ancient elevations 

 above it. But, if, on the other hand, an area, already 

 peopled, be in parts greatly upheaved, there will be 

 either a universal dying-out, from the cold, of a large 

 proportion of its inhabitants, or else an instinctive 

 striving amongst them to desert the higher grounds on 

 which they have been lifted up, and to descend to their 

 normal altitudes: in both cases, however, the present 

 summits will display the same feature, namely, utter 

 desolation. 



Such are a few of the effects which elevation and 

 subsidence, even on a small scale, would seem (when 

 tested by theory and practice) to produce. It yet 

 remains for us to suggest, that the latter, when carried 

 to its maximum, so as to cause the actual separation by 

 the sea of one district from another, is a contingency of 

 immense significance in regulating the distribution of 

 the Annulose tribes. Their outward contour and aspect 

 we have shown in a previous chapter to be very largely 

 beneath the control of isolation, provided a sufficient 

 time can be granted for the change : but their ultimate 

 absence from any particular place, through the impedi- 

 ment which it offers to their migratory progress, we 

 have not yet touched upon. Let us conceive, therefore, 



