EXTERNAL FACTORS. 413 



that limit ; then, though the individual plants are fixed, 

 yet the species will move : the seeds of plants living at the 

 limit, will produce individuals that survive beyond the limit. 

 The gradual spread so effected, having gone on for some ten 

 thousand years, the opposite change of climate will begin to 

 cause retreat : the tide of each species will during the one 

 half of a long epoch, slowly flow into new regions, and 

 then will slowly ebb away from them. Further, this rise 

 and fall in the tide of each species, will, during far longer 

 intervals, undergo increasing rises and falls and then de- 

 creasing rises and falls. There will be an alternation of 

 spring tides and neap tides, answering in its period to the 

 changing eccentricity of the Earth's orbit. 



These astronomical rhythms, therefore, entail on organisms 

 unceasing changes in the incidence of forces in two ways. 

 They directly subject them to variations of solar influences, 

 in such a manner that each generation is somewhat differently 

 affected in its functions; and they indirectly bring about 

 complicated alterations in the environing agencies, by carry- 

 ing each species into the presence of new physical conditions. 



149. The power of geological actions to modify every- 

 where the circumstances in which plants and animals are 

 placed, is conspicuous. In each locality, denudation slowly 

 uncovers different deposits ; and slowly changes the exposed 

 areas of deposits already uncovered. Simultaneously, the 

 alluvial beds that are being formed, are qualitatively affected 

 by these progressive changes in the natures and proportions of 

 the strata denuded. The inclinations of surfaces and their 

 directions with respect to the Sun, are at the same time 

 altered ; and the organisms existing on them are thus having 

 their thermal conditions continually altered, as well as their 

 drainage. Igneous action, too, complicates these gradual 

 modifications. A flat region cannot be step by step thrust 

 up into a protuberance, without unlike climatic changes 

 being produced in its several parts by their exposures to dif- 



