o^ 



58 CAUSES OF VARIABILITY. Chap. XXIL 



In accordance with the facts above given, which prove that in 

 many cases a close relation exists between variability and the sterility 

 following from changed conditions, we may conclude that the ex- 

 citing cause often acts at the earliest possible period, namely, on the 

 sexual elements, before impregnation has taken place. That an 

 affection of the female sexual element may induce variability we may 

 likewise infer as probable from the occurrence of bud-variations ; 

 for a bud seems to be the analogue of an ovule. But the male element 

 is apparently much oftener affected by changed conditions, 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 used 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 in this chapter, we may 

 conclude that the variability of organic beings under domes- 

 tication, although so general, is not an inevitable contingent 

 on life, but results from the conditions to which the parents 

 have been exposed. Changes of any kind in the conditions 

 of life, even extremely slight changes, often suffice to cause 

 variability. Excess of nutriment is perhaps the most efficient 

 single exciting cause. Animals and plants continue to be 

 variable for an immense period after their first domestication ; 

 but the conditions to which they are exposed never long 

 remain quite constant. In the course of time they can be 

 habituated to certain changes, so as to become less variable ; 

 and it is possible that when first domesticated they may have 

 been even more variable than at present. There is good 

 evidence that the power of changed conditions accumulates ; 

 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 in- 

 duced by the direct action of the surrounding conditions on 



53 'Dritte Fortsetzung,' &c, s. 123; ' Bastarderzeugucg,' s. 249. 



