• 



250 



CAUSES OF VARIABILITY. 



Chap. XXII. 



CHAPTEE XXII 



CAUSES OF VAEIABILITY, 



VARIABILITY DOES NOT NECESSARILY ACCOMPANY REPRODUCTION — CAUSES ASSIGNED 



BY VARIOUS AUTHORS — INDIVIDUAL DIFFERENCES 



DUE TO CHANGED CONDITIONS OF LIFE 



VARIABILITY OF EVERY KIND 



ON THE NATURE OF SUCH CHANGES 



CLIMATE, FOOD, EXCESS OF NUTRIMENT — SLIGHT CHANGES SUFFICIENT — EFFECTS 

 OF GRAFTING ON THE VARIABILITY OF SEEDLING-TREES — DOMESTIC PRODUCTIONS 

 BECOME HABITUATED TO CHANGED CONDITIONS — 



CHANGED 



— ON THE ACCUMULATIVE ACTION 

 CLOSE INTERBREEDING AND THE IMAGINATION OF THE 

 MOTHER SUPPOSED TO CAUSE VARIABILITY — CROSSING AS A CAUSE OF THE APPEAR- 

 ANCE OF NEW CHARACTERS —VARIABILITY FROM THE COMMINGLING OF CHARACTERS 

 AND FROM REVERSION — ON THE MANNER AND PERIOD OF ACTION OF THE CAUSES 

 ■WHICH EITHER DIRECTLY, OR INDIRECTLY THROUGH THE REPRODUCTIVE SYSTEM 



w 





consider 



far 



as we can 



almost universal variability of 

 The subject is an 



of the 



obscur 



one 



g 



our domesticated productions. 

 ; but it may be useful to probe 

 Some authors, for instance Dr. Prosper Lucas, 

 look at variability as a necessary contingent on reproduction, 

 and as much an aboriginal law, as growth or inheritance. 

 Others have of late enc 

 view by speaking of inh 

 antagonistic principles, 

 some followers, that va 



ed, perhaps unintentionally 



and 



lability 



qual and 



s. Pallas maintained, and he has had 

 variability depends exclusively on the 

 crossing of primordially distinct forms. Other authors attri- 

 bute the tendency to variability to 



an 



of food, and 



als to 



an excess relatively to the amount of exercise 



That 



taken, or again to the effects of a more genial climate 

 these causes are all effective is highly probable. But we must 

 I think, take a broader view, and conclude that organic beings 



subjected during several generations to any chang 

 their conditions, tend to vary ; the kind of v 



which ensues depending in a far higher deg 



conditions 



of the being, than on the 



of the changed 



» 



