EXTERNAL FACTORS. 501 



had before stopped its spread at 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 

 which 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 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 decreasing rises and falls. There 

 will be an alteration of spring tides and neap tides, answer- 

 ing 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, 

 new soil and surface. 



§ 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 in course of formation, 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 modi- 

 fled; 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 



