THE WAYS OF LIFE 197 



fact that hermit-crabs from the Mediterranean, where there 

 are no tides, did not show the change of tropism as regards 

 light, but were always positively attracted to it. According 

 to Bohn, a night and day rhythm has been to some extent 

 established in the constitution of some sea-anemones, which 

 go on for several days shutting during the day and opening 

 at night, although they are kept in continued darkness. 



There are internal rhythms in the body whose origin is 

 obscure, e.g. in the secretory activity of the kidney, which 

 is at its minimum at about 9 p.m. and at its maximum 

 in the early hours of the day. Now, as Bohn says, we do 

 not need to use psychical terms in referring to this, and 

 why should we in connexion with the rhythms exhibited by 

 shore anima Is in relation to the tides ? 



Establishment of Tropisms. — In many cases it is 

 plain that animals improve by practice ; the nerves and 

 muscles become fitter by exercise ; the creature finds 

 itself. We see this in the individual ; apart from any 

 learning in the strict sense (by association, imitation, and 

 inference), there is an apprenticeship of cells, tissues and 

 organs, and a reward of increased efficiency. This is a 

 matter of observed fact. What remains a subject of 

 debate is whether the reward of individually increased 

 efficiency is in any way entailed, or whether racial progress is 

 wholly due to the selection of the fitter germinal variations. 

 As the data stand at present, the verdict must, we think, 

 be given in favour of the second interpretation. 



Tropisms are hereditary compulsions to certain kinds of 

 movement, and it is a thinkable theory that the particular 

 combinations of them that occur in any particular animal 

 express the result of a long process of selection. It is 

 quite true that the tropisms often lead the animals to 



