31G ILLOGICAL GEOLOGY. 



Iii the last edition of liis Outlines of Astronomy, Sii 

 John Ilerschcl recognizes this as an element in geological 

 processes: regarding it as possibly a part-cause of those 

 climatic changes indicated ly the records of the Karth s 

 past. That it has had much to do with the larger changes 

 &quot;of climate of which we have evidence, seems unlikely, since 

 there is reason to think that these have been fur slower and 

 more lasting; but that it must have entailed a rhythmical 

 exaggeration and mitigation of the climates otherwise pro 

 duced, seems beyond quest inn. And it seems also beyond 

 question that there must have been a consequent rhythmi 

 cal change in the distribution of organisms a rhythmical 

 change to which we here wish to draw attention, as one 

 cause of minor breaks in the succession of fossil remains. 

 Kach species of plant and animal, has certain limits of heat 

 and cold within which only it can exist ; and these limits 

 in a great degree determine its geographical position. It 

 will not spread north of a certain latitude, because it can 

 not bear a more northern Avinter, nor south of a certain 

 latitude, because the summer heat is too great ; or else it 

 is indirectly restrained from spreading further by the effect 

 of temperature on the humidity of the air, or on the distri 

 bution of the organisms it lives upon. 



IJut now, what will result from a slow alteration of cli 

 mate, produced as above described? Supposing the pe 

 riod we set out from is that in which the contrast of seasons 

 is least marked, it is manifest that during the progress to 

 wards the period of the mo.^t violent contrast, each species 

 of plant and animal will gradually Wian^e its limits of dis 

 tribution will be driven back, here; by the winter s increas 

 ing cold, and there by the summer s increasing heat will 

 retire into those localities that are still lit for it. Thus dur 

 ing 10,000 years, each species will ebb away from certain 

 regions it was inhabiting; and during the succeeding 

 10,000 years will ilow back into those regions. IVom tin- 



