282 THE l.NDUCTIONS OF BIOLOGY. 



based wholly on observed facts, is just the conclusion to ^'hich 

 the foregoing argument points. That necessary action and 

 the re-action between the parts of an organism and the 

 organism as a whole — that power of the aggregate to re-mould 

 tiie units, which is the correlative of the power of the units to 

 build up into such an aggregate ; implies that any differences 

 existing between the units inherited by an organism, must 

 gradually diminish. Being subject in common to the total 

 forces of the organism, they will in common be modified to- 

 wards congruity with these forces; and therefore towards 

 likeness with each other. If, then, in a self- fertilizing organism 

 and its self-fertilizing descendants, such contrasts as origin- 

 ally existed among the physiological units, are progressive- 

 ly obliterated — if, consequently, there can no longer be a 

 segregation of different physiological units in different sperm - 

 cells and germ-cells ; self-fertilization will become impossible : 

 step by step the fertility will diminish, and the series will 

 finally die out. 



And now observe, in confirmation of this view, that self- 

 fertilization is limited to organisms in which an approximate 

 equilibrium among the organic forces, is not long maintained. 

 While growth is actively going on, and the physiological units 

 are subject to a continually-changing distribution of forces, 

 no decided assimilation of the units can be expected : like 

 forces acting on the unlike units, will tend to segregate them, 

 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 

 fciypo thesis. Mr Darwin has collected a large *^ body of fact^, 



