PRECIPITATION STRUCTURES SIMULATING ORGANIC GROWTH. 22Q 



implies corresponding constancy in the reactions taking place 

 under such influence. In other words, if the structural condi- 

 tions are definite, the chemical processes whose rate, nature and 

 position they control must also be definite; and the products of 

 these reactions, if they are of such a nature as to accumulate 

 and add to the structural material of the system, will also be 

 definite and will be deposited in definite situations and at definite 

 rates and times. Apparently deposition of this kind forms an 

 essential part of the organic growth-process. 1 Any structural 

 material thus added will secondarily influence further growth, 

 and by degrees, if metabolic activity continues, a definite struc- 

 ture will be built up whose precise nature, when the final stage 

 is reached, will have been determined by the structure originally 

 present in the growing system- assuming constancy in the 

 original chemical composition of the latter, and in the conditions 

 to which it has been exposed during its growth. 



In any living organism the characteristic physiological activ- 

 ities and external behavior must have as their basis a constancy 

 and specificity in the chemical and physical conditions under- 

 lying the fundamental vital processes. These conditions are in 

 large part structural; if we could account fully for specific 

 structure, we could not doubt account also for what is specific 

 in physiological activity and behavior. Now specific structure 

 is not confined to organisms; it is perhaps most perfectly exem- 

 plified in the specific form of crystals; and crystalline form is to 

 be referred in turn to the chemical character and mode of arrange- 

 of the molecules composing the crystal. In attempting to ac- 

 count for the specific form-characters of organisms we must 

 first inquire, as a matter of general scientific procedure, whether 

 it is not possible to refer these to the more general conditions 

 present in other natural systems which also exhibit definite and 

 specific form-characters. The case of crystals is the clearest; 

 here the visible structure is related to the structure of the mole- 

 cules and varies with changes in the nature and relative positions 

 of the atoms; this relation of molecular configuration to crystal- 

 line form is best illustrated by the differences in the right and 

 left-handed crystals of stereoisomeric compounds like the tar- 



1 Compare the interesting discussion of Child, loc. oil., Chapter 2, pp. 38 seq. 



