1 86 INDIVIDUALITY IN ORGANISMS 



specialization results from difference in metabolic rate 

 there are various possibilities. In a physico-chemical 

 complex like living protoplasm a change in tempera- 

 ture of a certain amount alters the rate of chemical 

 reaction to a certain degree, but it also alters many 

 other conditions in protoplasm, e.g., osmotic conditions, 

 surface-tension, aggregate condition of colloids, etc., 

 and it alters some in a greater, others in a less, degree. 

 In such a case the change in each particular process or 

 condition in the living protoplasm may be quantitative, 

 but since different factors are altered in different degree 

 the total change may determine qualitative differences 

 in the reactions or their products. Changes of this 

 sort may result, not merely from differences in tempera- 

 ture, but from other primarily quantitative changes. 

 In fact, it is very doubtful whether we can alter metabolic 

 rate to any great extent without bringing such changes 

 in quality somewhere in the complex. 



Elsewhere I have called attention to various facts 

 which have as yet received but little attention, but 

 which indicate that a relation exists between morpho- 

 logical structure and metabolic rate.^ Structural fea- 

 tures which are stable with a certain metaboHc rate are 

 eliminated when the rate increases, while decrease in 

 rate may determine the addition of new structural sub- 

 stances, and so on. Metabolic rate is apparently a 

 factor, though of course by no means the only one, in 

 determining what substance or substances accumulate 

 in the living cell as structural substratum, and the 

 structural substratum is an important factor in determin- 

 ing the character of the reactions which occur in it. 



* Child, Senescence and Rejuvenescence, iqis, pp. 47-54. 226-27. 



