RHYTHM IN ORGANISMS 143 



this shows a curious two-days rhythm which is at present quite 

 unaccountable. Every other day it swells up and appears in 

 greater numbers and on intermediate days it diminishes. Why 

 it does this is quite unknown. 



Another daily periodicity is shown by man both in liis 

 weight and height. The weight is lowest in the morning before 

 breakfast, and highest after the evening meal. This is easily 

 explicable. But the variation in height is rather peculiar. 

 During the day the stature decreases from 1 to 3 centimetres. 

 This is attributed to the compression of the intervertebral 

 discs whilst man is in an upright position, to the curving of 

 the spine, and to the depression in the arch of the foot. It is 

 therefore necessary in taking measurements for comparative 

 purposes to take them at the same time of the day, preferably 

 before breakfast. 



Many aquatic organisms show a periodical ascent and 

 descent in the water in which they live. This is attributed 

 by some to variations in the temperature, in the strength of 

 the light, and in the food supply. One of the chief factors is 

 that many floating animals move towards the bottom when 

 light falls on the water, and upwards when darkness sets in. 

 But another factor seems to enter into this periodical rise 

 and fall. It has been known for some time that the little 

 water-flea DapJmia increases in specific gravity after a meal. 

 There is a marked difference in its specific gravity twelve 

 hours after feeding and after twelve hours' starving. Recent 

 observations have shown that its maximum density is nor- 

 mally acquired between the hours of 6 and 8 a.m. By noon 

 there is a less well-defined but stifl marked minimum specific 

 gravity. Daphnia takes an early and hearty breakfast. The 

 minute algae upon which it lives are most abundant near the 

 surface in the early morning. When the Daphnia rises from 

 the deeper water it finds itself surrounded by an abundant 

 food supply. After a meal it seems to sink slowly. There is 

 a periodical rise and fall of the animal towards and from the 

 surface layers. DapJmia is most scarce in shallow water from 

 midday tiU about 4 a.m., and direct sunlight also seems to 

 drive it downwards. There are, of course, other factors. The 

 number of young in the brood-pouch is one of these. When 



