CHAP. VIII. TRANSMITTED EFFECTS OF A CBOSS. 305 



the advantage derived from a cross between two plants 

 was not confined to the offspring of the first generation. 

 That constitutional vigour due to cross-parentage is 

 transmitted for many generations may also be inferred 

 as highly probable, from some of Andrew Knight's 

 varieties of the common pea, which were raised by 

 crossing distinct varieties, after which time they no 

 doubt fertilised themselves in each succeeding gene- 

 ration. These varieties lasted for upwards of sixty 

 years, " but their glory is now departed." * On the 

 other hand, most of the varieties of the common pea, 

 which there is no reason to suppose owe their origin 

 to a cross, have had a much shorter existence. Some 

 also of Mr. Laxton's varieties produced by artificial 

 crosses have retained their astonishing vigour and 

 luxuriance for a considerable number of generations ; 

 but as Mr. Laxton informs me, his experience does 

 not extend beyond twelve generations, within which 

 period he has never perceived any diminution of vigour 

 in his plants. 



An allied point may be here noticed. A& the force 

 of inheritance is strong with plants (of which abundant 

 evidence could be given), it is almost certain that seed- 

 lings from the same capsule or from the same plant 

 would tend to inherit nearly the same constitution ; and 

 as the advantage from a cross depends on the plants 

 which are crossed differing somewhat in constitution, 

 it may be inferred as probable that under similar con- 

 ditions a cross between the nearest relations would 

 not benefit the offspring so much as one between non- 

 related plants. In support of this conclusion we have 

 some evidence, as Fritz Miiller has shown by his 



* See the evidence on this mestication,' chip. ix. vol. i. 2nd 

 head in my Variation -under Do- edit. p. R97 



X 



