ECOLOGICAL 67 



If we try to group the natural rhythms, we may begin with a 

 great series, inchiding the cosmic periodicities, such as the revolution 

 of a planet on its orbit and the rotation of either planet or sun on 

 its own axis. The revolution of the spinning earth on its orbit gives 

 us the rhythm of spring and summer, autumn and winter; the 

 rotation of the earth on its axis gives us, as familiarly, the rhythmic 

 alternation of day and night. As a consequence of the periodically 

 changing positions of the earth in relation to sun and moon, 

 there is the rhythm of the tides, which has had a far-reaching 

 influence on animal life. 



In a second group may be ranked those sequences in the life of 

 plants and animals that are demonstrably correlated with the cosmic 

 periodicities. This may be illustrated by the rings of growth in the 

 stem of a tree, which are due to the difference in texture between 

 the more porous spring wood and the closer summer or autumn 

 wood. The alternation of two kinds of wood-forming is intelligible 

 in the light of the different vital conditions in spiing and in summer, 

 but there are many cases where the correlation between internal 

 rhythm and external periodicity is certain, though we cannot at 

 present explain how it works. Thus at full moon in October or 

 November the Pacific Palolo worm comes out of its retreats on the 

 shores of Fiji and Samoa and spawns profusely on the surface of the 

 sea. The Atlantic Palolo that frequents the Tortugas and other 

 islands backs out of the crevices of the rocks and breaks off the 

 posterior portion of its body. This rises to the surface and swims 

 rapidly backwards, spawning as it does so. The water is so thick 

 with slender worms that it looks like vermicelli soup; when the 

 myriads of germ-cells are shed it appears almost milky. The headless 

 bodies die, but the head portion, remaining in the burrow, grows a 

 new body for the following year — a strange rhythm. The swarming 

 occurs within three days of the moon's last quarter between June 

 29 and July 28. But what the precise connection between the moon 

 and the worm may be we do not know. In some other sea-worms 

 the egg-laying occurs regularly at the time of the spring tides in 

 June, July, and August, and this may be associated with the abun- 

 dance of food material and consequent increase of vigour at these 

 times. Lunar periodicity in spawning is also well illustrated by one 

 of the common sea-urchins of the Red Sea, Centr echinus setosus, 

 which spawns at each full moon in the summer months. Munro 

 Fox has shown that one and the same sea-urchin may shed its 

 germ-cells at one full moon and have another crop ready a month 

 later. This does not hold for Mediterranean sea-urchins, though the 

 ancients recorded a belief to this effect. 



A very striking rhythm is exhibited by a Californian smelt, called 

 the Grunion. These little fishes come out of the water on the second, 

 third, and fourth nights after the highest tides in spring, usually 



