ARTICLES 421 



that, to reach the settlement, adult mosquitoes would have 

 to fly from half a mile to a mile across, or at right angles to, 

 the stiff breezes which prevail at Gatun. It was eventually 

 discovered " that a regular purposive flight of mosquitoes to- 

 wards the town took place in the evenings before nightfall from 

 6.30 to 7 p.m. After dark the flight was reduced to prac- 

 tically nothing. During the period of flight, the observers 

 were bitten continuously. After the flight ceased, the observers 

 were bitten only once or twice in an hour's time. A return 

 flight began at 6 a.m. and took place with extraordinary 

 rapidity. As daylight became stronger, the speed of the 

 returning Anopheles increased. The termination of both 

 forward and return flights was remarkably abrupt. One 

 observer said the flight stopped with almost mechanical pre- 

 cision when there was too much daylight or too much dark- 

 ness." 



Willey ' records an interesting example of periodic habit 

 occurring among crows and so-called " flying foxes " (really 

 fruit-eating bats) on the coast of Ceylon. At one place, a 

 small lighthouse islet off the coast of Ceylon, they congregate 

 in the palm-trees alternately by night and by day. " At 

 sundown," says Willey, " the passage of immense flocks of 

 crows and flying foxes in opposite directions across the strait 

 which divides the island from the mainland can be witnessed, 

 the former bound for the island to rest for the night, the latter 

 speeding their way to the mainland intent upon their nocturnal 

 forage. . . . The reverse passage — namely, the matutinal 

 flight — takes place towards sunrise, the bats returning from 

 the mainland to rest for the day suspended in rows from the 

 midribs of the palm-leaves, the crows crossing over on their 

 daily quest for garbage." As a result of these markedly 

 periodic habits, the two classes of animals are able to make 

 their homes in the same trees, without in the slightest degree 

 interfering with each other. 



The effect of external periodicities on the organism and its 

 behaviour is nowhere better seen than on the seashore. The 

 case of Convoluta roscoffensis is perhaps too familiar to need 

 much description. Convoluta roscoffensis is a minute, elongated 

 flat-worm covered with cilia, and containing green algal 

 cells living with it symbiotically. Its habitat is a narrow 

 strip of sandy beach on the coasts of Normandy and Brittany 

 situated at the level reached by high-water at the slackest 

 of neap-tides. Though of very small size, the worms occur 

 in such enormous numbers as to form at low tide great 

 patches of green scum. As the tide laps the edges of the 

 colony the green patches disappear, the worms remaining 

 1 Willey, A., Convergence in Evolution. (London: Murray, 191 1.) 



