A PHYSICAL PHENOMENON 63 



Cells, as living entities, cannot refuse to absorb 

 substances, and it is also a rule that they cannot "pick 

 and choose" what they absorb. For instance, a cell 

 cannot take from a solution which surrounds it a pro- 

 tein and refuse an alkaloid. If it is surrounded by both 

 these substances it has to take both. On theoretical 

 grounds, I believe that a solution could be prepared 

 (although we have not yet been able to assure our- 

 selves that such is the case) from which a given cell 

 would be able to absorb nothing; but such a contin- 

 gency, as far as can be seen, would be impossible in the 

 body. A cell does not appear to " feed " in the ordinary 

 sense of the term that is to say, it cannot seek after 

 food. It has to take what is there according to certain 

 laws, even if it dies in consequence; but it cannot 

 "help itself" in any sense of the term. The life of a 

 cell depends upon substances in its surroundings, even 

 its reproduction depends upon them, and the associa- 

 tion between them and its life depends on the diffusion 

 of these substances into the cell itself, which diffusion 

 is in its turn undoubtedly dependent on physical laws 

 over w r hich the cells themselves can individually exert 

 no control. A cell cannot take a cake and leave a bun, 

 so to speak: it has to take a bit of cake and a bit of 

 bun w r hether it likes them both or not a law which 

 has been amply confirmed by work extending over a 

 period of five years. 



One is open to criticism in this matter; for the 

 objection may be raised that it is well known that 

 some cells of the body are affected by some agents 

 or drugs, while others apparently are not. This 



