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. Fecundal selection cooperating 

 with natural selection insures, in the descendants, the predominance o] the 

 better adapted of the more fertile, and the more fertile of 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 

 cooperation 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 



