98 ANALYSIS OF: THE FOUR PRINCIPLES. 
individuals of one form have over those of other forms, but simply to 
the higher ratio of multiplication in the more fertile forms securing the 
intergeneration of the more fertile. /ecundal selection cooperating 
with natural selection insures, in the descendants, the predominance of the 
better adapted of the more fertile, and the more fertile oj the better adapted. 
(4) Partial positive segregation may be greatly strengthened by coope- 
rating negative segregation.—It seems to be a fundamental law that 
vigor and variation in the offspring depend on some degree of diversity 
of constitution in the parents, and diversity of constitution that is not 
entirely fluctuating depends on some degree of positive segregation ; 
therefore vigor and variation depend on the breaking down of incip- 
ient segregations and on the interfusion of the slightly divergent 
forms that had been partially segregated. But in the history of every 
race that is winning success by its vigor and variation there is liable to 
come a time when some variety, inheriting sufficient vigor to sustain 
itself, even if limited to the benefits of crossing with the individuals of 
the same variety, becomes partially segregated. As we have already 
seen, when positive segregation is correlated with segregate fecundity, 
the segregated types tend to become more and more dominant in 
number; but, in the very nature of things not only will the segrega- 
tion be for many generations only partial, but partial segregation— 
unless it is aided by some other principle—although it may greatly 
delay the submerging of different groups in one common group, will 
never prevent that result being finally reached. Though the siphon 
which connects two tanks of water be ever so small, the water 
will in time find a common level in both tanks, unless there are 
additions or subtractions of water that prevent such a result. So, 
in the case under consideration, final fusion will take place, unless 
differentiation progresses more rapidly than the fusion, or some other 
influence comes in to counteract the leveling influence of occasional 
crosses. If, under such conditions, some branch of the partially segre- 
gated variety becomes more fertile when generating with members of 
the same variety, and less fertile when generating with other varieties, 
a principle will be introduced tending to strengthen any form of 
partial segregation that already exists between the varieties. This 
coéperation of segregate fecundity with partially segregative endow- 
ments will produce pure masses of each variety, when, without the 
action of this principle, all distinctions would be absorbed by the 
crossing. We know that a transition from integrate fecundity to 
segregate fecundity usually takes place at a point in the history of 
evolution intermediate between the formation of an incipient variety 
and a strongly marked species; and though the causes that produce 
