Chap. XXII. CAUSES OF VARIABILITY. 255 



form ; and a still higher degree of variability when three distinct 

 species, and most of all when four species, are blended together by 

 successive crosses. Beyond this point Gartner, 4 * on whose authority 

 the foregoing statements are made, never succeeded in effecting a 

 union ; but Max Wichura 43 united six distinct species of willows 

 into a single hybrid. The sex of the parent species affects in an 

 inexplicable manner the degree of variability of hybrids; for 

 Gartner 44 repeatedly found that when a hybrid was used as a father 

 and either one of the pure parent-species, or a third species, was 

 used as the mother, the offspring were more variable than when the 

 same hybrid was used as the mother, and either pure parent or the 

 same third species as the father: thus seedlings from Diardhus 

 barbatus crossed by the hybrid D. chintimi-barbatus were more 

 variable than those raised from this latter hybrid fertilised by the 

 pure D. barbatus. Max Wichura 45 insists strongly on an analogous 

 result with his hybrid willows. Again Gartner 46 asserts that the 

 degree of variability sometimes differs in hybrids raised from re- 

 ciprocal crosses between the same two species ; and here the sole 

 difference is, that the one species is first used as the father and then 

 as the mother. On the whole we see that, independently of the 

 appearance of new characters, the variability of successive crossed 

 generations is extremely complex, partly from the offspring partaking 

 unequally of the characters of the two parent-forms, and more 

 especially from their unequal tendency to revert to such characters 

 or to those of more ancient progenitors. 



On the Manner and on the Period of Action of the Causes ichich 

 induce Variability. — This is an extremely obscure subject, and 

 we need here only consider, whether inherited variations are 

 due to certain parts being acted on after they have been 

 formed, or through the reproductive system being affected 

 before their formation ; and in the former case at what period 

 of growth or development the effect is produced. We shall 

 see in the two following chapters that various agencies, such 

 as an abundant supply of food, exposure to a different climate, 

 increased use or disuse of parts, &c, prolonged during several 

 generations, certainly modify either the whole organisation or 

 certain organs ; and it is clear at least in the case of bud- 

 variation that the action cannot have been through the repro- 

 ductive system. 



42 ' BastarderzeuguDg,' s. 507, 516, 44 ' Bastarderzeugung.'s. 452, 507 

 572. 45 ' Die Bastardbefruchtung,' s. 56 



43 ' Die Bastardbefruchtung,' &c, 4C ' B.istarder.zeuguDg,' s. 423. 

 1865, s. 24. 



