500 THE EVOLUTION OF LIFE. 



perate and less intemperate, than when, some one or two 

 millions of years later, the Earth's orbit has reached its ex- 

 treme of eccentricity. 



Thus, besides those daily variations in the quantities of 

 light and heat received by organisms, and responded to by 

 variations 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 responses in the changed functions 

 of organisms. The whole vegetal and animal kingdoms, 

 are subject to quadruply-compounded rhythms in the inci- 

 dence of the forces on which life primarily depends — 

 rhythms so involved in their slow working round that at no 

 time during one of these vast epochs, can the incidence 

 of these various 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; in some cases from salt water 

 to fresh water; and in some cases from fresh water to salt 

 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 which are able to 

 move about with some rapidity; so must the alterations of 

 temperate and intemperate climates produce extensions and 

 restrictions of habitats. These, though slow, must be uni- 

 versal — must affect the habitats of stationary organisms as 

 well as those of locomotive ones. For if, during an astro- 

 nomic 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 



