^^ x II 



I 



292 



DEFINITE ACTION OF CONDITIONS OF LIFE. Chap. XXIII. 



cases tlie conditions of life play a subordinate part in causing 

 any particular modification; like that which, a spark pl a y s 

 when a mass of combustibles bursts into flame — the nature of 

 the flame depending on the combustible matter, and not on the 

 spark. 



No doubt each slight variation must have its efficient cause • 

 but it is as hopeless an attempt to discover the cause of each 

 as to say why a chill or a poison affects one man differently 

 from another. Even with modifications resulting from the 

 definite action of the conditions of life, when all or nearly all 

 the individuals, which have been similarly exposed, are simi- 

 larly affected, we can rarely see the precise relation between 

 cause and effect. In the next chapter it will be shown that 

 the increased use or disuse of various organs, produces an 

 inherited effect. It will further be seen that certain variations 

 are bound together by correlation and other laws. Beyond this 

 we cannot at present explain either the causes or manner of 

 action of Variation. 



Finally, as indefinite and almost illimitable variability is the 

 usual result of domestication and cultivation, with the same part 

 or organ varying in different individuals in different or even 

 in directly opposite ways ; and as the same variation, if strongly 

 pronounced, usually recurs only after long intervals of time, any 



* 



particular variation would generally be lost by crossing, rever- 

 sion, and the accidental destruction of the varying individuals, 

 unless carefully preserved by man. Hence, although it must 

 be admitted that new conditions of life do sometimes definitely 

 affect organic beings, it may be doubted whether well-marked 

 races have often been produced by the direct action of changed 



conditions without the aid of selection either by man or 

 nature. 



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