300 



VAKIATION OF PLANTS AND ANIMALS. [Ch. XXXVI. 



species 



blue ; and the same change is always produced by a particular 



sort of yellow loam. 



Whether there are definite limits to the variability of a 



—In former editions of this work (from 1831 to 1853),* 

 I contended that there are limits to that deviation from an 

 original type of which species are susceptible. My argu- 

 ment was founded chiefly on the rapid rate at which we may 

 bring about considerable modifications in a brief period in 

 domesticated animals and cultivated plants, and the slow 

 progress which we can afterwards make in modifying the 

 same races when our experiments are persevered in for a 

 great many generations. In illustration of this principle I 

 observed, that when man uses force or 



stratagem against 

 wild animals, the persecuted race soon becomes more 

 cautious^ watchful^ and cunning ; new instincts seem often to 

 be developed^ and to become hereditary in the first two or 

 three generations : but let the skill and address of man in- 

 crease^ however gradually, no farther variation can take place, 

 no new qualties are elicited by the increasing dangers. The 

 alteration of the habits of the species has reached a point 

 beyond which no ulterior modification is possible, however 

 indefinite the lapse of ages during which the new circum- 

 stances operate. Extirpation then follows, rather than such 

 a transformation as could alone enable the species to perpe- 

 tuate itself under the new state of thincrs. 



But in the first place Mr. Darwin has shown that even 

 in those species such as the pigeon, our common cattle, 

 sheep or pigs, which have been made to vary by selection 

 from the remotest periods, there are no signs of a positive 

 limit having been reached beyond which no farther change 

 can be brought about. All have been altered within quite 

 modern times, and ' the tendency to general variability seems 

 unlimited.'t 



It has also been pertinently remarked by Mr. Wallace that 

 the amount of change in any one direction may at first be 

 comparatively rapid ; as when in the case of the race-horse, 





i 



i 



* 'Principles of Geology,' 1st edi- 

 tion, 1831, vol. ii. chap. iii. p. 37, and 



9th cdjjjl^on, chap. sxxv. p. 592. 

 t Darwin * On Variation/ &c., p. 416. 



