THE CRITERIA OF LIVINGNESS 83 



within its limits. But the image is too general to be ade- 

 quate; we must inquire into the changeful integrity of the 

 organism more carefully. Three points are of outstanding 

 importance: (a) that the changes in the organism are very 

 complex, having essentially to do with protein substances 

 in a colloid state; (6) that they are specific for each kind 

 of creature, and (c) that they are correlated in such a way 

 that they go on, and Ae specific structure likewise persists. 

 Let us take each of these points in turn. 



(a) Metabolism of Proteins. 



According to some physiologists the only absolute differ- 

 ence between living organisms and inorganic bodies is, that 

 proteins are universally present in the former and absent 

 in the latter. Verworn writes : " Since it is known that 

 the nitrogenous proteids, with their allies, which in part 

 are derived from the proteids and in part are necessary to 

 their formation, are the sole organic compounds that are 

 never wanting in living substance, that everywhere they 

 constitute its chief mass and alone are sufficient for its for- 

 mation, it can be said that all living organisms are charac- 

 terised by the metabolism of proteids" (1899, p. 136). 

 These protein compounds, such as white of egg or the gluten 

 of bread, are peculiarly intricate, with a large number of 

 atoms or atom-groups in their molecules; they diffuse very 

 slowly and do not readily pass through membranes; they 

 occur in a colloid state, and although some, e.g., haemoglobin, 

 are crystallisable, they are not known in a crystalloid state 

 in the living organism; they are relatively stable bodies, 

 yet they are continually breaking down and being built 

 up again within the body, partly under the direct influence 

 of ferments or enzymes. The constructive, synthetic, up- 



