218 THE PHILOSOPHY OF BIOLOGY 



of an organism varies according to the conditions of 

 the environment. In general they are cases of van't 

 HofFs law, that is, the rapidity at which a chemical 

 reaction proceeds varies according to the temperature. 



They are changes of functioning passively experi- 

 enced by the organism as the result of environmental 

 changes, and we must clearly distinguish between them 

 and such changes as are the result of some activity 

 or effort on the part of the organism. A flounder 

 which lives in a river migrates out to sea when the 

 first of the winter snows melt and flood the estuary 

 with ice-cold water. Brown or striped prawns living 

 on brown or striped weeds become green when they 

 are placed on green weed, changing their pigmentation 

 to match that of the alga. A kitten brought up in a 

 cold-storage warehouse develops a sleeker and more 

 luxuriant coat than does its sister reared in a well- 

 warmed house. An animal which recovers from 

 diphtheria forms an antitoxin which enables it to 

 resist, for a time at least, repeated infection. A man 

 who goes exploring in polar seas puts on warmer 

 clothing than he wears in the tropics. 



It is not necessary that an environmental change 

 should occur in order that an adaptation should be 

 evoked, for the organism may react actively and 

 purposefully to a change in itself. The athlete acquires 

 by running or rowing a more powerful heart ; the 

 blacksmith develops more muscular shoulders and 

 arms ; and the professional pianist more supple wrists 

 and fingers. If one kidney is removed by operation, 

 or if one lung becomes diseased, the organ on the other 

 side of the body becomes hypertrophied. Aphasia, 

 which is due to a lesion in the unilateral speech-centre, 

 may pass away if the previously unused centre on the 

 other side of the brain should become functionally 



