Osmoftc EqinUhriUlon. J^-39 



sively to the teleosts with the exception of one or two genera 

 of stingrays in the rivers of tropical America (10) (which, after 

 all, may be " accidental inclusions "), to my mind is strong 

 argument in favour of this idea. Moreover, the development of 

 the bony skeleton and the possession of a circulating fluid whose 

 physical constants could be maintained irrespective of altera- 

 tions in those of the environment enable these teleosts to migrate 

 and pass from sea water into estuaries and up rivers, as is 

 done by such a fish as the salmon in the breeding season, feats 

 which Avould be practical impossibilities for the poikilosmotic 

 elasmobranch. 



Of course in considering this problem due consideration 

 must be given to the method of " give and take " which is 

 exemplified in all evolutionary processes. Thus could be 

 explained any unaccountable differences in vascular develop- 

 ment or higher developmeiit in parts of the nervous system in 

 some of the lower animals, and as analogies may be taken the 

 loss of the alimentary canal in intestinal worms and the degene- 

 ration of the organs of smell in man when he assumed the erect 

 posture. 



The skeletons of whales appear in the tertiary periods (eocene 

 and miocene), and since that time the whales have probably 

 given in a little in the constant struggle against the concentra- 

 tion of their body fluid by the surrounding medium. The seals 

 probably appeared later than the whales, and possibly migrated 

 to the sea later. This would account for the closer connection 

 between the A of the blood of seals and the ^^ of the blood of 

 terrestrial mammals than holds in the case of the blood of 

 whales. Marine birds are, probably Iw their aerial life and 

 migratory habits (and hence ability to obtain fresh water from 

 pools and rivers on land as suggested by Rodier, 12), removed 

 from the necessity of " giving in " to the tendency of their 

 food to render the body fluids hypertonic. 



On the other hand, from reptile and bird to mammal, the 

 osmotic pressure of the circulating fluid of the terrestrial animal 

 is about the same (0.6). But why should the A of the body fluid 

 of the fresh-water crustacean be much greater than that of 

 amphibian and amphibious reptile (tortoise)? To me it appears 

 that we have here evidence of a change of habit similar to 

 what I imagine occurred to the fishes. A crustacean wandered 



