430 THE INSTABILITY OF THE HOMOGENEOUS. 



know is produced in inorganic masses by nnlikeness 

 of exposure to incident forces, is strictly paralleled by 

 the first contrast that makes its appearance in all organic 

 masses. 



It remains to point out that in the assemblage of organ- 

 isms constituting a species, the principle enunciated is equal- 

 ly traceable. "We have abundant materials for the induction 

 that each species will not remain uniform, but is ever becom- 

 ing to some extent multiform; and there is ground for the 

 deduction that this lapse from homogeneity to heterogeneity 

 is caused by the subjection of its members to unlike sets of 

 circumstances. The fact that in every species, animal and 

 vegetal, the individuals are never quite alike; joined with 

 the fact that there is in every species a tendency to the pro- 

 duction of differences marked enough to constitute varieties ; 

 form a sufficiently wide basis for the induction. While the 

 deduction is confirmed by the familiar experience that va- 

 rieties are most numerous and decided where, as among cul- 

 tivated plants and domestic animals, the conditions of life 

 depart from the original ones, most widely and in the most 

 numerous ways. Whether we regard " natural selection " 

 as wholly, or only in part, the agency through which varie- 

 ties are established, matters not to the general conclusion. 

 For as the survival of any variety proves its constitution to 

 be in harmony with a certain aggregate of surrounding 

 forces — as the multiplication of a variety and the usurpation 

 by it of an area previously occupied by some other part of 

 the species, implies different effects produced by such aggre- 

 gate of forces on the two, it is clear that this aggregate of 

 forces is the real cause of the differentiation — it is clear that 

 if the variety supplants the original species in some locali- 

 ties but not in others, it does so because the aggregate of 

 forces in the one locality is unlike that in the other — it is 

 clear that the lapse of the species from a state of homogene- 

 ity to a state of heterogeneity arises from the exposure of its 

 different parts to different aggregates of forces. 



