VARIATION. 325 



action together. We have heterogeneity among progenitors, 

 which, did it act uniformly and alone in generating, by com- 

 position of forces, new deviations, would impress such new 

 deviations to the same extent on all offspring of the same 

 parents; which it does not. We have functional variation 

 in the parents, which, acting either alone or in combination 

 with the preceding cause, would entail the same structural 

 variations on all young ones simultaneously produced ; which 

 it does not. Consequently there is some third cause of varia- 

 tion, yet to be found, which acts along with the structural 

 and functional variations of ancestors and parents. 



87. Already, in the last section, there has been implied 

 some relation between variation and the action of external 

 conditions. The above-cited contrast between the uniformity 

 of a wild species and the multiformity of the same species 

 when cultivated or domesticated, thrusts this truth upon us. 

 Eespecting the variations of plants, Mr. Darwin remarks that 

 " ' sports ' are extremely rare under nature, but far from rare 

 under cultivation." Others who have studied the matter 

 assert that if a species of plant which, up to a certain time, 

 has maintained great uniformity, once has its constitution 

 thoroughly disturbed, it will go on varying indefinitely. 

 Though, in consequence of the remoteness of the periods at 

 which they were domesticated, there is a lack of positive 

 proof that our extremely variable domestic animals have be- 

 come variable under the changed conditions implied by domes- 

 tication, having been previously constant; yet competent 

 judges do not doubt that this has been the case. 



Now the constitutional disturbance which precedes varia- 

 tion, can be nothing else than an overthrowing of the pre- 

 established equilibrium of functions. Transferring a plant 

 from forest lands to a ploughed field or a manured garden, is 

 altering the balance of forces to which it has been hitherto 

 subject, by supplying it with different proportions of the 

 assimilable matters it requires, and taking away some of the 

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