262 THE INDUCTIONS OF BIOLOGY. 



their feet, while in others, there was present on each hind-foot, 

 what is called the " dew-claw " — a rudimentary fifth digit. 



Thus, induction points to three causes of variation, all in 

 action together. We have heterogeneity among progenitors, 

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

 tion of forces, new deviations, would impress such new devia- 

 tions to the same extent on all offspring of the same parents ; 

 which it does not. ATe have functional variation in the pa- 

 rents, which, acting either alone or in combination with the pre- 

 ceding cause, would entail like variations on all young ones 

 simultaneously produced ; which it does not. And there is 

 consequently some third cause of variation, 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 wild species and the multiformity of the same species 

 when cultivated or domesticated, thrusts this truth upon us. 

 Respecting 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 do- 

 mestication, 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 s ia 



