436 EVOLUTION AND GENETICS 



water. Among other cases amphibia with gills and lungs breathe 

 in either medium. Euglena can carry on either plant or animal 

 metabolism, and the quail can l^oth run rapidly and fly. With 

 the capacity for two or more kinds of activity an organism can 

 carry on either. 



Evidences of this kind are so numerous that latitude of heredi- 

 tary possi]:>ility seems a necessary quality in organisms which 

 are to evolve. A species cannot be expected suddenly to acquire 

 something new to meet a given condition; the abundance of 

 extinct species shows that many an organism has failed to meet 

 the conditions surrounding it. If a species has a sufficient range 

 of possibilities to meet the conditions of a changing environment 

 it may survive, as in the past many species have survived. Grant- 

 ing its ability to survive a change of environment, the more 

 valuable characters of the species may persist and be developed 

 while others are being reduced and eliminated. The horse attained 

 a large third digit while losing all others. 



The Environment. Like the heritage the environment has 

 always been variable. We witness from season to season and 

 from year to year fluctuations of all of the factors of the physical 

 environment. One year may be wet, another dry. In one region 

 the nights may be cold and the days hot, while in another tempera- 

 ture may be almost constant. Paleontology shows that this is 

 not the limit of fluctuation, but that gradual climatic changes 

 covering thousands of years have occurred. Temperate North 

 America has been both tropical and arctic in the past. Moreover 

 at any given time different parts of the earth's surface present 

 different climatic conditions because of their physiographic features 

 and their varying relations to the sun. 



The effects of such fluctuations may be very great. The physical 

 environment is so closely associated with the metabolism of green 

 plants that an exceptionally dry season may mean death to a 

 great many, or if their seeds fall on the wrong kind of soil or in 

 too shady a spot they may perish. The flora of a region is there- 

 fore very largely an indication of its physical conditions, and by 

 the flora the fauna may be definitely influenced. Arboreal animals 

 are not found in extensive prairies, for example, nor grazing 

 species in dense forests. Animals may also respond directly to 

 physical conditions; to this relationship is due the fact that 

 amphibia cannot live in deserts nor in the far north. 



