412 THE EVOLUTION OF LIFE. 



severally less temperate and less intemperate, than when, 

 some one or two millions of years later, the Earth's orbit haa 

 reached its extreme of eccentricity. 



Thus, besides those daily variations in the quantities of light 

 and heat received by organisms, and responded to by varia- 

 tions in their functions ; and besides the annual variations in 

 the quantities of light and heat which organisms receive, 

 and similarly respond to by variations in their functions ; 

 there are variations that severally complete themselves in 

 21,000 years and in some millions of years variations to 

 which there must also be a response in the changed functions 

 of organisms. The whole vegetal and animal kingdoms, 

 are subject to a quadruply-compounded rhythm in the in- 

 cidence of the forces on which life primarily depends a 

 rhythm so involved in its slow working round, that at 

 no time during one of these vast epochs, can the in- 

 cidence of these forces be exactly the same as at any other 

 time. To the direct effects so produced on organ- 



isms, have to be added much more important indirect effects. 

 Changes of distribution must result. Certain redistributions 

 are occasioned even by the annual variations in the quantities 

 of the solar rays received by each part of the Earth's surface. 

 The migrations of birds thus caused, are familiar. So too 

 are the migrations of certain fishes : in some cases from one 

 part of the sea to another ; and in some cases from salt water 

 to fresh water. Now just as the yearly changes in the amounts 

 of light and heat falling on each locality, yearly extend 

 and restrict the habitats of many organisms that are able to 

 move about with some rapidity ; so must these alternations 

 of temperate and intemperate climates produce extensions 

 and restrictions of habitats. These extensions and restric- 

 tions, though slow, will be universal will affect the habitats 

 of stationary organisms as well as those of locomotive ones. 

 For if during an astronomic era, there is going on at any 

 limit to a plant's habitat, a diminution of the winter's cold 

 or summer's heat, which had before stopped its spread ai 



