DEVELOPMENT. 151 



dovelopraent is a cliange from incolierent, indefinite homo- 

 geneity, to colierent, 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 special kinds and proportions of the matters circulat- 

 ing around it ; in the same way that the organism as a 

 whole, integrates with itself certain special kinds and propor- 

 tions 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 en- 

 tire organism is qualitatively differentiated from things 

 around it. Evidently this selective assimilation 



illustrates the general truth, demonstrable d jmori, that like 

 units tend to segregate. It illustrates, moreover, the further 

 aspect of this general truth, that the pre-existence of a mass 

 of certain units, produces, probably by polar attraction, a 

 tendency for diffused units of the same kind to aggregate 

 with this mass, rather than elsewhere. It has been shown of 

 particular salts, A and B, co- existing in a solution not suf- 

 ficiently concentrated to crystallize, that if a crystal of the salt 

 A be put in ' o 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 exercise 

 a coercive force on the diff'used atoms of this salt, and grow at 

 their expense. No doubt much organic assimilation occurs 

 in the same way. Particular parts of the organism are com- 

 posed of special units, or have the function of secreting 

 special units, which are ever present in them in large quan- 



