264 EEV, J. T GULTCK ON DIVERGENT EVOLUTION 



Tlie Influence of Segregate Vigour. 



I think we may say we have here come in sight of one form of 

 the still wider fourfold law already mentioned ; for on the same 

 priueiple that Segregate Eecundity increases when once allied 

 with partial Segregation in vigorous forms, Segregate Vigour 

 must also tend to increase when brought into the same alliance ; 

 and I believe it will be found that there is a similar principle 

 tending to the self-accumulation of Segregate Adaptation. 



At the point where they both arise, that is during the 

 period that immediately follows the act of impregnation, it is 

 difficult to distinguish between the two principles, and the mor- 

 tality of the hybrid embryo before birth, or before it leaves the 

 egg, may be conveniently classed as Segregate Fecundity. * 



Though the two principles are so closely related, it would be a 

 great mistake not to distinguish them ; for there is no close 

 correspondence between the degrees in which the two qualities 

 occur in the relations of individuals or varieties ; and in some cases 

 we find Segregate Fecundity associated with Integrate Vigour. 

 The mule, though absolutely sterile, possesses vigour equal, if 

 not superior, to that of either parent. In the record of experi- 

 ments given by Darwin in ' Cross- and Self -Fertilization in the 

 Vegetable Kingdom ' mention is made of certain species in which 

 self-fertilized flowers are more fertile than the cross-fertilized, 

 while the plants produced from the crossed seed are the more 

 vigorous ; and of other species in which cross-fertilized flowers are 

 by far the most productive, while the plants produced from the 

 crossed seed are neither taller nor heavier than the self-fertilized.f 

 In the same work the common pea {Pisum sativurti), the common 

 tobacco {Nicotiana tahacum), and Canna Warscewiczi are shown 

 to be more vigorous when raised from self-fertilized seed than 

 when raised from seed crossed with other individuals of the same 

 strain ; but in the case of the tobacco and the pea, great increase of 

 vigour is produced by a cross with a slightly diff'erent variety 

 while the fertility is increased but little if any. 



But the most interesting of all his experiments as bearing on 

 the subject of Segregate Vigour, is given in the history of " The 

 Descendants of the self-fertilized Flant, named Hero, which ap- 

 pearedin theSixth Self-fertilized Generation of Ipomcea purpurea.' 

 "A cross between the children of Hero did not give to the 

 * See ' Origin of Species,' 6th edition, p. 249. f See pages 322-329. 



