12 FACTORS IX VARIATION. 



Bastian will probably demonstrate this by imdcmiable 

 laboratory experiments) . 



The dominance in a va.riety of habitats of such a plant 

 as Oxalis corniculata and such an animal as Epiviys rattus 

 might 1)6 mentioned as an instance of the energy of an 

 organism. But surely we must recognise that at the 

 foundation of this energ}' or "■ initiative "" there are 

 physico-chemical causes. 



The character of the environment is obviously of vital 

 importance. It is a mere truism to saj' that without a 

 suitable environment, containing water, carbon dioxide 

 and salts, life would be impossible. There are reciprocal 

 relations between adaptations in organisms and special 

 conditions in the environment. Changes in environment 

 necessaril}^ precede special adaptations in organisms. 

 Thus, on primary grounds, environment is the dominant 

 factor in evolution. Temperature and humidity are of 

 paramount importance, and, as ftointed out by Gadow, 

 a '■ change into a colder environment is a more powerful 

 factor than change into a warmer climate."* A prolonged 

 drought will change the character of the biota of a country 

 and will exterminate whole species. Parallel effects must 

 obviously have been associated A\-ith the great glacial 

 periods of the past, and, in certain areas, with the great 

 periods of volcanic activity. Subsequent changes doubtless 

 provided special opportunities for organic extension. We 

 may surely quote here from R. S. Lull a record of 

 palaeontologj' : — " The stream of life flows so slowly that 

 the imagination fails to grasp the immensit}- of time 

 required for its passage, but like many another stream, 

 it pulses as it flows. There are times of quickening, the 

 expression points of evolution, and the.se are found to be 

 coincident with geologic change." f Dar\\in"s inference 

 ■■ that variability mainly depends on changed conditions 

 of life "% is now surely 2:»roven. It will probably be found, 



*H. Gadow : The Wanderings of Animak, 1913, p. r»3. 



fR. S. \a\\\: The Evohition of tin- Eaith and it.s Inhaliitant.s, Yale, 

 1918. 



IDarwin : Variation of Animals and Plants, II., 2nd edit.. 1890. 

 p 413. 



