Osmotic Eqiiilihvation. 237 



(5) — Of marine mammals, seals (belonginiij to carnivora) give A 

 very little above that of land mammals, and whales (cetacea) a 

 somewhat larger A than seals and land mammals. 



(6) — All terrestrial vertebrates examined, viz., reptiles 

 (lizard), birds, echidna, mammals, possess a circulating fluid 

 which gives a A of about C.G^C. 



(7) — Of fresh-water animals the crustacean gives a A of 

 0.6 (0.8 according to Fredericq for Astacus fluviatilis), amphibia 

 about 0.45, reptiles (tortoises) about 0.55OC., and fishes 

 (teleosts) 0.5— 0.69OC. 



There are thus seen to be certain " jumps '' in the figures 

 given, which afford interesting food for reflection. 



Firstly, with regard to marine animals, the question arises, 

 why should the A of the blood of the teleosts dift'er in such 

 a remarkable manner from that of the elasmobranchs? The 

 acquisition of a bony skeleton by the teleost at once proclaims 

 him a superior individual to the soft cartilaginous elasmo- 

 branch. This fact combined with the indubitable fact that 

 the environment of terrestrial life conduces in every way to a 

 higher evolution, and the existence of the swimming bladder 

 in these fishes, to my mind, lead to the conclusion that a carti- 

 laginous fish passed through the stages of a mud fish, and ulti- 

 mately lived in swamp, where it developed a bony skeleton, 

 and meanwhile also developed the mechanism of maintain- 

 ing the osmotic pressure of its blood constant. Some 

 of these new individuals (bony fish) then wandered back to the 

 sea, their original home, and the osmotic pressure of their 

 blood gradually rose, but still has remained fairly constant 

 near the original A^alue. Others betook themselves to the fresh- 

 water streams, and have maintained through the ages a blood 

 Avhose osmotic pressure is approximately (possibly a little lower 

 than) that of the teleost when it (as I presume) wandered back 

 to the water. The sea turtle we can imagine to be a descendant 

 of an individual which was evolved from this paludial teleost, 

 and which wandered back to the sea at a later period than that 

 in which the teleost returned to the sea. 



The blood of marine birds and mammals is naturally ap- 

 proximately that of the land birds and mammals, since these 

 animals had taken to the sea in comparatively recent (geologi- 



