The Relations between Marine Animai and Vegetable Life. 365 



From this table one may see that whilst on an aveiage larvai 

 grown in water which had been exposed to sun aud air are prac- 

 tically unaffected, those growu in water exposed to sun only are 

 increased in size by from 14.6^ to 18.0^. The percentage numbers 

 of ova developing to larvse differ to alraost as marked an extent. 

 Thus in the one case on an average only \\%i or, if the experiment 

 in which no larvai at ali were obtained be included, 8.0 9<,, developed, 

 and in the other case 62^. The individuai values obtained with 

 water exposed to sun and air vary very considerably. Thus in the 

 first two given in the table the size of the larvse was practically 

 unaffected; but in the next one it was largely increased, and in the 

 next, stili more largely diminished. In this case a layer of water 

 only two inches in depth was exposed in a fiat dish. In another 

 experiment in which was used some of the water employed in Exp. 61, 

 but which had been exposed to the sun an additional seven days, 

 no larvai at ali were obtained. It may be thought that the adverse 

 eflfect of the water thus treated was due to evaporation and conse- 

 quent iucrease of salinity. This could not bave been the case however, 

 as in the first place the jars in which the water was exposed were 

 always covered up with glass lids, and moreover it has been shown 

 in the paper already referred to above^ that moderate increase in 

 the salinity of the water has practically no influence on the growth 

 of the larvai. 



But how, on the other band, can the very favourable effect 

 produced by the exposure of the water to the sun in a flask be 

 accounted for? It is probable, as will be shown later on, that a 

 small part of this influence, probably about ^% of it, is due to the 

 heating of the water by the sun, whereby the animai life i)resent is 

 killed off. There must obviously be some other factor at work 

 however, to which these striking differences in the efifects of the 

 water are due. Everything points to this factor being of a bacterial 

 nature. Thus it was originally shown by Downes and Blint'-^ that 

 sunlight had an adverse efl'ect on bacterial growth, and in recent 

 years Marshall Ward has made numerous investigations on the 

 bactericidal action of sunlight^. His expea-iments were made with 

 pathogenic or fresh water bacteria however. Observations on marine 



1 ibid. pag. 588. 



2 Proc. R. Soc. London Voi. 27 1877 pag. 488, also Vol. 25 1878 pag. 197. 



3 Proc. R. Soc. London Voi. 52 pag. 393, also Vol. 53 pag. 23, also Vol. 56 

 pag. 315. 



