290 



DEFINITE ACTION OF THE 



Chap. XXIII. 



Oil 



tlie conditions of life sometimes act in a definite manner 

 onr already variable domesticated productions ; and, as the action 

 of changed conditions in causing general or indefinite varia- 

 bility is accumulative, so it may be with their definite action. 

 Hence it is possible that great and definite modifications of struc- 

 ture may result from altered conditions acting during a Ion? 

 series of generations. In some few instances a marked effect 

 has been produced quickly on all, or nearly all, the individuals 

 which have been exposed to some considerable change of climate 

 food, or other circumstance. This has occurred, and is now 

 occurring, with European men in the United States, with Euro- 

 pean dogs in India, with horses in the Falkland Islands, appa- 

 rently with various animals at Angora, with foreign oysters 

 in the Mediterranean, and with maize grown in Europe from 

 tropical seed. We have seen that the chemical compound 

 secreted by plants and the state of their tissues are readily 

 affected by changed conditions. In some cases a relation appa- 

 rently exists between certain characters and certain conditions, 

 so that if the latter be changed the character is lost — as with 

 cultivated flowers, with some few culinary plants, with the fruit 

 of the melon, with fat-tailed sheep, and other sheep having 

 peculiar fleeces. 



The production of galls, and the change of plumage in parrots 

 when fed on peculiar food or when inoculated by the poison of a 

 toad, prove to us what great and mysterious changes in structure 

 and colour may be the definite result of chemical changes in tlie 

 nutrient fluids or tissues. 



s 



We have also reason to believe that organic bein 



o 



of nature may be modified in various definite ways by 



ditions to which they have been 



of American 



comparison 



exposed, s 

 their repr 



r> 



• . • 



Europe. But in all such cases it is most difficult to disti 

 between the definite results of changed conditions, and the 

 cumulation through natural selection of serviceable variat 

 which have arisen independently of the nature of the con( 

 If, for instance, a plant had to be modified so as to become fitted 

 to inhabit a humid instead of an arid station, we have no reason 

 to believe that variations of the right kind would occur more 

 frequently if the parent-plant inhabited a station a little more 







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