i8o The Science of Life. 



by diurnal and seasonal changes. It is the scene of an 

 intense struggle for existence, and has been the happy 

 hunting-ground of many of our greatest naturalists. 



The lower boundary of the littoral area has been 

 called the " mud-line ", where the minute organic and 

 inorganic particles derived from the land and surface 

 waters find a resting-place, or form the food-supply of 

 crowds of animals. Sir John Murray regards this line 

 as "the great feeding ground in the ocean ", and as the 

 primary haunt from which animals migrated to the deep 

 sea. 



The study of the fauna and flora of the open sea 

 has not been long begun. For although the marvellous 

 p la i i Johannes Muller, who found time for all 

 sorts of researches, experimented about 1845 

 in "open-sea fishing with a fine net", and Eschscholtz 

 was another pioneer, little was done before the Chal- 

 lenger expedition, and even then attention was mainly 

 concentrated on the great depths. 



From his Challenger experience Murray was led to 

 conclude (1876) that there was an intermediate pelagic 

 fauna between the surface and the depths. This was 

 denied by Agassiz (1878, 1891) below 200 fathoms; but 

 the later work of Chun (1888-1889) has confirmed 

 Murray's conclusion. 



A great step was taken by Hensen (1887), who im- 

 proved the appliances, instituted a more systematic 

 survey, and introduced the quantitative method of 

 estimating the volume of floating organisms in different 

 waters and at different depths, and the proportions in 

 which different species occur. He is responsible for the 

 term " Plankton ", applied to floating organisms, and 

 his theory of its uniformity over wide areas gave rise to 

 a lively controversy between him and Haeckel, who 

 strongly maintained its oscillating and extremely vari- 

 able character. Improvement of plankton-methods, 

 e.g. the use of the pump and self-closing tow-nets (still 

 far from practical perfection), their application to lakes 

 and even rivers (e.g. by Zacharias); the taking of 

 observations at different seasons throughout the year; 

 and a combination of zoologists and botanists in the 



