GENESIS, HEREDITY, AND VARIATION. 345 



so long as continuance of evolution permits further segrega- 

 tion; and only when further segregation cannot go on, will 

 the like forces tend to assimilate the units. Hence, where 

 there is no prolonged maintenance of an approximate organic 

 balance, self-fertilization may be possible for some gener- 

 ations; but it will be impossible in organisms distinguished 

 by a sustained moving equilibrium. 



95. The interpretation which it affords of sundry pheno- 

 mena familiar to breeders of animals, adds probability to the 

 hypothesis. Mr. Darwin has collected a large " body of facts, 

 showing, in accordance with the almost universal belief of 

 breeders, that with animals and plants a cross between dif- 

 ferent varieties, or between individuals of the same variety 

 but of another strain, gives vigour and fertility to the off- 

 spring; and on the other hand, that close interbreeding di- 

 minishes vigour and fertility," a conclusion harmonizing 

 with the current belief respecting family-intermarriages in 

 the human race. Have we not here a solution of these facts ? 

 Relations must, on the average of cases, be individuals whose 

 physiological units are more nearly alike than usual. Ani- 

 mals of different varieties must be those whose physiological 

 units are more unlike than usual. In the one case, the un- 

 likeness of the units may frequently be insufficient to pro- 

 duce fertilization; or, if sufficient to produce fertilization, not 

 sufficient to produce that active molecular change required 

 for vigorous development. In the other case, both fertiliza- 

 tion and vigorous development will be made probable. 



Nor are we without a cause for the irregular manifestations 

 of these general tendencies. The mixed physiological units 

 composing any organism being, as we have seen, more or less 

 segregated in the reproductive centres it throws off; there 

 may arise various results according to the degrees of difference 

 among the units, and the degrees in which the units are segre- 

 gated. Of two cousins who have married, the common grand- 

 parents may have had either similar or dissimilar constitu- 



