494 THE WONDER OF LIFE 



value. In the darkness of the great abysses some animals 

 may possibly use their luminous organs as lanterns. Where 

 the luminous organs are arranged on a definite pattern 

 which is sometimes different in the two sexes it is quite 

 likely that they serve as recognition marks. But for our 

 present purpose it is enough to indicate that this peculiar 

 transformation of energy not in itself necessarily useful 

 may be seized upon, utilized, and specialized towards diverse 

 ends by different types. 



Sleep. It appears to us to be characteristic of the life 

 or activity of organisms that it can be slowed down and 

 quickened again. We may hunt out analogies, such as 

 that of a fire which, after vigorous burning, smoulders, 

 and then breaks out again ; but no mere mechanisms have 

 the organism's power of taking a rest, and one of the many 

 forms of rest is ordinary sleep that familiar but puzzling 

 state in which we spend about a third part of our existence. 

 Analogous but quite different vital adjustments are to be 

 seen in the hibernation of such animals as snail and dor- 

 mouse, and in the so-called ' sleep ' of plants which often 

 makes itself plain in the altered position of the leaves and 

 the closing of the flowers. 



Normally, there can be no doubt, the sleeping habit was 

 established in relation to sunset, but every one knows that 

 we can adjust our capacity for going to sleep so as to suit 

 our particular circumstances. The important point is, 

 that once the habit is established, say of going to sleep at 

 11 p.m., it asserts itself with some insistence unless the 

 attention is strongly diverted. In the course of a short 

 time, varying with the individual, the rhythm can be 

 changed and the man sleeps as soundly (other things equal) 

 by day as he formerly did by night. 



