328 DIVERGENT EVOLUTION THROUGH SEGREGATION. 
In this latter case, where the Vigor of Hybrids is 7/5 that of Pure-breeds, while 
their Fecundity is equal to that of Pure-breeds, we find a sx, Which is the same 
result as that given in the eighth line of the last column of Table V, where the 
Fecundity of cross unions and of Hybrids is =; that of Pure-breeds, while their 
Vigor is equal. 
THE INFLUENCE OF SEGREGATE VIGOR. 
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 segre- 
gation in vigorous forms, segregate vigor 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 mortality 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 some cases we find segregate fecundity 
associated with integrate vigor. The mule, though absolutely sterile, 
possesses vigor equal, if not superior, to that of either parent. In the 
record of experiments given by Darwin in “ Crossand 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 cross seed are the more vigorous; and of 
other species in which cross-fertilized flowers are by far the most pro- 
ductive, while the plants produced from the crossed seed are neither taller 
nor heavier than the self-fertilized.t In the same work the common 
pea (Pisum sativum), the common tobacco (Nicotiana tabacum), and 
Canna Warscewiczi are shown to be more vigorous when raised from 
self-fertilized seed than when raised from seed crossed with other indi- 
viduals of the same strain; but in the case of the tobacco and the pea 
great increase of vigor is produced by a cross with a slightly different 
variety, while the fertility is increased but little if any. 
3ut the most interesting of all his experiments as bearing on the sub- 
ject of segregate vigor, is given in the history of ‘‘ The Descendants of 
the self-fertilized Plant, named Hero, which appeared in the Sixth Self-fer- 
tilized Generationof Ipomeea purpurea.” “A cross between the children 
of Hero did not give to the grandchildren any advantage over the self- 
fertilized grandchildren raised from the self-fertilized children.” ‘‘ And, 
“See “Origin of Species,” 6th edition, p. 249. 
tSee ‘Cross and Self-Fertilization,” pp. 322-329. 
