INTRODUCTORY 13 



nature of the surroundings. Among the vast majority of organ- 

 isms, however, periodicity in reproductive activity seems to 

 relate entirely to recurrent environmental changes, such as the 

 seasons, the phases of the moon, or the tides. That among 

 large numbers of species spring and summer are the times for 

 breeding is a matter of common knowledge, and it is equally well 

 known that unusual warmth or cold may hasten or check the 

 periodic development of the sexual instinct and the accompanying 

 internal and external changes in the body. It is recognised 

 also that where conditions are approximately uniform throughout 

 the year, periodicity in the breeding habits of animals may often 

 be obliterated. For instance. Semper states that few things 

 impressed him more in the Philippine Islands than the absence 

 of sexual periodicity among the molluscs, insects, and other land 

 animals, and Westermarck has remarked on a similar fact in 

 connection with the birds of the Galapagos Islands which are 

 situated very near the equator, where conditions are the same 

 throughout the year. The whale and certain other marine animals 

 afford examples of the same principle. On the other hand, the 

 extraordinary regularity among birds of the migratory movements, 

 which are correlated with changes in the size of the reproductive 

 organs and with the breeding phenomena generally, take place 

 with a sexual rhythm which is often independent of temporary 

 climatic conditions. 



In many animals there are cyclical periods of generative 

 activity occurring within the breeding season. Thus Fox states 

 that the sea-urchins at Suez breed from spring until September, 

 but that the genital products are developed in cycles correspond- 

 ing to the lunar periods, the majority of individuals spawning 

 their ova and spermatozoa into the sea about the time of the 

 full moon. Fox remarks that the variation in the size of the 

 gonads of the sea-m'chin in relation to the moon's phases has 

 long been common knowledge in the fish markets of the Mediter- 

 ranean area where the gonads are used for food. 



Another example of correlation between breeding habits and 

 lunar periodicity is afforded by certain genera of marine polychaet 

 worms called Palolos. Thus the Atlantic Palolo (Eunice fucata) 

 and the South Pacific Palolo (Eunice viridis) swarm out for 

 purposes of breeding twice a year (June and July with the first- 

 named species, and October and November for the second), 



