THE ACTIVITIES OF THE ORGANISM 113 



stimulation of the gland cells by " secretory nerve 

 fires." Now the investigations carried out — and no 

 part of the physiology of the mammal has been so 

 patiently studied as the salivary gland — fail, so far, 

 completely to describe the function in terms of these 

 elements. In the end we have to refer the secretion 

 to intra-cellular processes, and then we begin to invoke 

 again processes of osmotic pressure, diffusibility, and 

 so on with reference to the formation of the drops of 

 secretion which we can see formed in the gland cells. 

 We are forced to the formulation of a logical hypo- 

 thesis as to the nature of these intra-cellular processes, 

 and since much that goes on in the cell substance 

 is, so far, beyond physico-chemical investigation, our 

 hypothesis will be as difficult to disprove as to verify. 



Let us return now to Huxley's comparison of the 

 activity of the green plant with the chemical reaction 

 which occurs when an electric spark is passed through 

 a mixture of oxygen and hydrogen. The lecture on 

 the " Physical Basis of Life " was published in 1869 ; 

 in 1852 William Thomson published his paper " On 

 a Universal Tendency of Nature to Dissipation of 

 Energy," and a year or two before that Clausius had 

 applied Carnot's law to the kinetic theory of heat : 

 the second principle of energetics had therefore even 

 then been exactly formulated, but its significance for 

 biological speculation had not been recognised by 

 Huxley, any more than it has generally been recog- 

 nised by most biologists since 1869. What, then, does 

 the comparison of Huxley show ? Clearly that the 

 physical changes which occur in the explosion of a 

 mixture of oxygen and hydrogen trend in a different 

 direction from those which occur in the photo-synthesis 



