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 2 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 larvae 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 S0 4 , NaHC0 3 , or Na 2 HPO 4 ), 

 the eggs could not develop into plutei; from which he 

 concluded that every constituent of the sea water w T as 

 necessary. This would indicate a case of extreme 

 adaptation to all the minutiae of the external medium. 



Experiments on a much more favourable animal 



