34:6 ILLOGICAL GEOLOGY. 



In the last edition of his Outlines of Astro7io'my, Sii 

 John Herschel recognizes this as an element in geological 

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

 climatic changes indicated by the records of the Earth's 

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

 of climate of which we have evidence, seems unlikely, since 

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

 more lasting ; but that it must have entailed a rhythmical 

 exaggeration and mitigation of the climates otherwise pro- 

 duced, seems beyond question. 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. 

 Each 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 winter, 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. 



But 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 most violent contrast, each species 

 of plant and animal will gradually ^abange 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 fit 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 flow back into those regions. From the 



