264 
REV. J. T. GTJLICK ON DIVERGENT EVOLUTION 
The 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 
principle that Segregate Fecundity 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 ( Visum sativum ), 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 different 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 Plant, named Hero, which ap- 
pearedin the Sixth Self-fertilized Generation o/Tpomoea 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. 
