No. 589] 



THE EVOLUTION OF THE CELL 



35 



give rise to small-pox, nor vice versa, but each breeds as 

 true to type as do lions and leopards. 



The essential and distinctive characteristic of a living 

 body of any kind whatsoever is that it exhibits while it 

 lives permanence and continuity of individuality or per- 

 sonality, as manifested in specific behavior, combined with 

 incessant change and lability of substance; and further, 

 L hat in reproducing its kind, it transmits its specific char- 

 acteristics, with, however, that tendency to variability 

 which permits of progressive adaptation and gradual evo- 

 lutionary change. It is the distinctively vital property of 

 specific individuality combined with " stuff-change ' ' (if I 

 may be allowed to paraphrase a Teutonic idiom) which 

 marks the dividing line between biochemistry and biology. 

 The former science deals with substances which can be 

 separated from living bodies, and for the chemist specific 

 properties are associated with fixity of substance ; but the 

 material with which the biologist is occupied consists of 

 innumerable living unit-individuals exhibiting specific 

 characteristics without fixity of substance. There is no 

 reason to suppose that the properties of a given chemical 

 substance vary in the slightest degree in space or time; 

 but variability and adaptability are characteristic features 

 of all living beings. The biochemist renders inestimable 

 services in elucidating the physico-chemical mechanisms 

 of living organisms; but the problem of individuality and 

 specific behavior, as manifested by living things, is beyond 

 the scope of his science, at least at present. Such prob- 

 lems are essentially of distinctively vital nature and their 

 treatment can not be brought satisfactorily into relation at 

 the present time with the physico-chemical interactions of 

 the substances composing the living body. It may be 

 that this is but a temporary limitation of human knowledge 

 prevailing in a certain historical epoch, and that in the 

 future the chemist will be able to correlate the individual- 

 ity of living beings with their chemico-physical proper- 

 ties, and so explain to how livinir beinir- first came into 



