Chap. XXII. CAUSES OF VARIABILITY. 237 



CHAPTER XXII. 



CAUSES OF VARIABILITY. 



VARIABILITY DOES NOT NECESSARILY ACCOMPANY REPRODUCTION — CAVSES 

 ASSIGNED BY VARIOUS AUTHORS — INDIVIDUAL DIFFERENCES — VARIABILITY 

 OF EVERY KIND DUE TO CHANGED CONDITIONS OF LIFE — 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— ON THE ACCUMULATIVE ACTION OF CHANGED 

 CONDITIONS — CLOSE INTERBREEDING AND THE IMAGINATION OF THE 

 MOTHER SUPPOSED TO CAUSE VARIABILITY — CROSSING AS A CAUSE OF 

 THE APPEARANCE OF NEW CHARACTERS — VARIABILITY FROM THE COM- 

 MINGLING OF CHARACTERS AND FROM REVERSION — ON THE MANNER AND 

 PERIOD OF ACTION OF THE CAUSES WHICH EITHER DIRECTLY, OR IN- 

 DIRECTLY THROUGH THE REPRODUCTIVE SYSTEM, INDUCE VARIABILITY. 



We will now consider, as far as we can, the causes of the 

 almost universal variability of our domesticated productions. 

 The subject is an obscure one ; but it may be useful to probe 

 our ignorance. 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 encouraged, perhaps unintentionally, this 

 view by speaking of inheritance and variability as equal and 

 antagonistic principles. Pallas maintained, and he has had 

 some followers, that variability depends exclusively on the 

 crossing of primordially distinct forms. Other authors attri- 

 bute variability to an excess of food, and with animals to an 

 excess relatively to the amount of exercise taken, or again to 

 the effects of a more genial climate. That these causes are 

 all effective is highly probable. But we must, I think, take 

 a broader view, and conclude that organic beings, when sub- 

 jected during several generations to any change whatever in 

 their conditions, tend to vary ; the kind of variation which 

 ensues depending in most cases in a far higher degree on the 

 nature or constitution of the being, than on the nature of the 

 changed conditions. 



