VARIOUS THEORIES OF THE ORGANISM 23 



but upon particular osmotic or other physical conditions which are 

 present in the experiment. Structures so produced are often 

 evanescent and disappear as the conditions in the medium change, 

 for the chemical processes do not remain localized in the ordinary 

 media of chemical reaction, though where the substance of the 

 structure is insoluble they may persist. 



Within recent years it has been shown that the production of 

 form and structure in connection with chemical reaction is much 

 more readily accomplished when the reaction occurs in the presence 

 of colloids. The colloids in such cases are not necessarily involved 

 in the chemical reaction in any way, but act primarily as a physical 

 substratum in which the reaction occurs. By altering the course 

 and rate of diffusion they serve to establish or maintain differences 

 of concentration; in consequence of the great amount of surface of 

 the colloid particles adsorption may play an important part, and 

 the formation of membranes may also affect the course of the re- 

 action. The effect of the colloid as a localizing factor, as a means 

 of producing form and structure, is greater in the gel than in the 

 sol state of aggregation. 1 



Many have not been slow to call attention to the resemblance 

 between form and structure thus produced and organic form and 

 structure, and more or less adventurous hypotheses of the nature 

 of life have been one result of such researches. On the other hand, 

 many biologists have been inclined to regard experimentation of 

 this sort as of little value for the problem of morphogenesis, but 

 this attitude seems to arise in part from a misconception. The most 

 important point in connection with such experiments is not the 

 resemblance between the forms and structures produced and those 

 of living organisms. Actually of course the resemblances are in 

 many cases very remote and superficial and of minor importance. 

 But the fact that morphological form and structure can be made 

 to arise in such physico-chemical systems is of great importance 

 for biology, for it affords at least a basis for the scientific investiga- 

 tion and interpretation of morphogenesis in the organism. Earlier 

 attempts to formulate theories of morphogenesis have consisted in 



1 Examples of investigation along this line are the work of Leduc, '08, '090, 'ogb, 

 3 10; Liesegang, '09, 'n, '14, and other earlier papers, and Kiister, '13. 



