306 The Influence of Environment 



mentally, through variation of temperature, dimor- 

 phism of form in Daphnia. 



5. Next or equal in importance with the tempera- 

 ture is the nature of the medium in which the cells 

 are living. 



[ It has often been pointed out that the marine animals 

 and the cells of the body of metazoic animals are 

 surrounded by a medium of similar constitution, the 

 sea water and the blood or lymph, both media be- 

 ing salt solutions differing in concentration but con- 

 taining the three salts NaCl, KC1, and CaCl a in about 

 the same relative concentration, namely 100 mole- 

 cules NaCl : 2.2 molecules of KC1 : 1.5 molecules of 

 CaCl 2 . This has suggested to some authors the poetical 

 dream that our home was once the ocean, but we can- 

 not test the idea since unfortunately we cannot experi- 

 ment with the past. Plants, unicellular fresh-water 

 algae, and bacteria do not demand such a medium for 

 their existence. 



Herbst had shown that when sea-urchin larva; were 

 raised in a medium in which only one of the constitu- 

 ents of the sea water was lacking (not only NaCl, KC1, 

 or CaCl 2 , but also Na 2 SO„, NaHC0 3 , or Na 2 HP0 4 ), 

 the eggs could not develop into plutei; from which he 

 concluded that every constituent of the sea water was 

 necessary. This would indicate a case of extreme 

 adaptation to all the minutiae of the external medium. 



Experiments on a much more favourable animal 



