240 J. L. Jnna: 



in comparatively recent times (Ortman (8), has shown that 

 fresh-water crabs and crayfish did not appear till the upper 

 tertiary) from the sea to the land and rivers, and finding food 

 abundant and the environment favourable, remained there. 

 In his new environment this poikilosmotic creature responded 

 to the change in concentration of the medium in which he lived, 

 and although the osmotic pressure of his body fluid has not yet 

 reached that of the land animals derived from the primitive fish, 

 it has, however, reached a veiy low figure indeed, when we take 

 into account the fact that its tissues were originally attuned 

 to an environment whose osmotic pressure was three or four 

 times its present value. A similar line of reasoning will explain 

 the lower osmotic pressure of the blood of the amphibia and to 

 a small degree some amphibious reptiles. 



The recently published work of Macallum (7) is of additional 

 interest in this connection. 



In contrasting the composition of the water of the oceans of 

 the present day with the composition that the waters of the 

 ocean at various geological epochs must have possessed, as 

 determined from geological evidences, Macallum (6) had shown 

 that the sodium content had rapidly increased, and the mag- 

 nesium content had slowly increased, while, since rivers had 

 formed, calcium and potassium had remained practically con- 

 stant. 



Macallum in his recently published paper (7) gives analyses 

 of the blood serum and body fluid of various animals, and has 

 also drawn up a table which gives the proportions of potassium, 

 calcium, and magnesium to sodium, which is taken as 100. 

 These tables I append. 



TABLE A. 



*As calculated froui Macfilliiin's figures. 



