I] PERIODICITY IN CONVOLUTA 35 



that the period selected for egg-laying has reference 

 to the greater security which is offered during the 

 first days of larval life. Born into the world at this 

 period, the animals and the plantlets have some days 

 of submerged grace before they become subjected 

 twice daily to such extreme environmental changes as 

 occur at other phases of the tidal sequence. 



Thus, driven back provisionally on a teleological 

 explanation, we may interpret the significance and 

 origin of periodicity of egg-laying in the following 

 way. One condition for survival of the species 

 C. paradoxa and of Dictyota is that the just-liberated 

 young shall be for some days after birth continually 

 submerged : that, for one reason or another ultimately 

 connected with nutrition, the maturing of these 

 marine organisms and the development of their 

 sexual cells requires a period of fourteen days, and 

 that the organisms fit their fortnightly periods into 

 the tidal periods in such a way that they reach their 

 climaxes at the most convenient moments. As the 

 waving flag of the guard gives the signal for the 

 train to start, so change of light intensity appears 

 to give the signal for the maturing of the sexual 

 organs and thus secures their liberation at the proper 

 moment. 



Whether such bi-lunar periods of fertility ex- 

 hibited by littoral, marine organisms have any bearing 



32 



