270 



CAUSES OF VARIABILITY. 



Chap. XXII. 



I 



I 



I 



ditions, at least in a visible manner, than the female element or ovule ; and 

 we know from Gartner's and Wichura's statements that a hybrid nsed as the 

 father and crossed with a pure species gives a greater degree of variability 

 to the offspring, than does the same hybrid when used as the mother. 

 Lastly, it is certain that variability may be transmitted through either 

 sexual element, whether or not originally excited in them, for Kolreuter 

 and Gartner 53 found that when two species were crossed, if either one was 

 variable, the offspring were rendered variable. 



/Summary. — From the facts given 

 may conclude that the variability of 



in this chapt 



we 



organic 



beings under 



domestication, although 



6 



al. is not an 



table 



con- 



g 



on growth and reproductio 



but 



esults from the 



Changes 



conditions to which the parents have been exposed. 



of any kind in the conditions of life, even extremely slight 



changes, often suffice to cause variability. Excess of nutriment 



perhaps the most effi 



single exciting cause. 



Animals 

 and plants continue to be variable for an immense period after 



their first domestication ; but the conditions to which they 



posed never long remain quite 



In the course of 



time they can be habituated to certain changes, so as to become 



iable ; and it is possible that when first domesticated 



} variable than at present. 



less va 



they may have been even mor 



There 



accumulates 



5 



ood evidence that the power of changed conditions 

 ; so that two, three, or more generations must be 



exposed to new conditions before any effect is visible. The 

 crossing of distinct forms, which have already become variable, 

 increases in the offspring the tendency to further variability, by 

 the unequal commingling of the characters of the two parents, 

 by the reappearance of long-lost characters, and by the appear- 

 ance of absolutely new characters. Some variations are induced 

 by the direct action of the surrounding conditions on the whole 

 organisation, or on certain parts alone, and other variations are 



induced indirectly through the reproductive system bein 



affected in the same manner as is so common with organic 

 beings when removed from their natural conditions. The causes 



gamsm, on 



which induce variability act on the mature o 

 embryo, and, as we have good reason to believe, on both 

 elements before impregnation has been effected. 



the 

 ;ual 



68 ' Dritte Fortsetzung,' &c, s. 123; ' Bastarderzeugung,' s. 249. 



