Chap. XXII. 



CAUSES OF VARIABILITY. 



2G7 



the first generation are generally uniform, but subsequently they display 

 an almost infinite diversity of character. He who wishes, says Kolreuter, 41 

 to obtain an endless number of varieties from hybrids should cross and 

 recross them. There is also much variability when hybrids or mongrels 

 are reduced or absorbed by repeated crosses with either pure parent-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, 42 on whose authority the foregoing statements are made, 

 never succeeded in effecting a union ; but Max Wichura 43 united six dis- 

 tinct 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 the 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 Dianthus barbatus crossed by the hybrid 

 D. chinensi-barbatus were more variable than those raised from this latter 

 hybrid fertilised by the pure D. harbatus. 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 charac- 

 ters, 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 these same characters or to those of more ancient progenitors. 



On the Manner and on the Period of Action of the Games which 

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

 we need here only briefly consider, firstly, whether inherited 

 variations are caused by the organisation being directly acted 

 on, or indirectly through the reproductive system ; and secondly, 

 at what period of life or growth they are primarily caused. 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 organi- 

 sation or certain organs. This direct action of changed con- 

 ditions perhaps comes into play much more frequently than can 

 be proved, and it is at least clear that in all cases of bud- 



41 'Nova Acta, St. Petersburg,' 1794 ibm a 9i 



391 ' ou,J ' *' z± - 



J,^> , j , „„ 4i ' Bastarderzeugung,' s. 452, 507. 



4 - ' Bastarderzeugung, s. 507 51 fi <V79 4^ <-r>- ^, l n n t.x > ?,- 



4 o , _. „ , & ,. 5' "• uu 'W J -o,->/A 45 < Die Bastardbefruchtung, s. 56. 



iA 'Die Bastardbefruchtuua- ' fr f > 46 1T5 , , , ( ,<> 



=» ^S 46 Bastarderzeugung, s. 423. 



