DEVELOPMENT. 179 



general truth that development is a change from incoherent, 

 indefinite homogeneity, to coherent, definite heterogeneity, 

 becomes a self-evident one when observation has shown us 

 the state in which organisms begin and the state in which 

 they end. 



Just in the same way that the growth of an entire organ- 

 ism is carried on by abstracting from the environment 

 substances like those composing the organism; so the pro- 

 duction of each organ within the organism is carried on by 

 abstracting from the substances contained in the organism, 

 those required by this particular organ. Each organ at the 

 expense of the organism as a whole, integrates with itself 

 certain kinds and proportions of the matters circulating 

 around it; in the same way that the organism as a whole, 

 integrates with itself certain kinds and proportions of 

 matters at the expense of the environment as a whole. So 

 that the organs are qualitatively differentiated from each 

 other, in a way analogous to that by which the entire organ- 

 ism is qualitatively differentiated from things around 

 it. Evidently this selective assimilation illustrates 



the general truth, set forth and illustrated in First Principles, 

 that like units tend to segregate. It illustrates, moreover, 

 the further aspect of this general truth, that the pre-exist- 

 ence of a mass of certain units produces a tendency for 

 diffused units of the same kind to a^regate with this mass 

 rather than elsewhere. It has been shown of particular salts, 

 A and B, co-existing in a solution not sufficiently concen- 

 trated to crystallize, that if a crystal of the salt A be put 

 into the solution, it will increase by uniting with itself the 

 dissolved atoms of the salt A; and that similarly, though 

 there otherwise takes place no deposition of the salt B, yet if 

 a crystal of the salt B is placed in the solution, it will exer- 

 cise a coercive force on the diffused atoms of this salt, and 

 grow at their expense. Probably much organic assimilation 

 occurs in the same way. Particular parts of the organism 

 are composed of special units, or have the function of secret- 



