238 J. L. Jon<i: 



cal) times, and their organisation had been so constituted, tliat 

 they have been able successfully to withstand the strain which 

 their new environment has thrown on the lining membrane of 

 the alimentary tract and on the cutaneous structures and kid- 

 neys in the maintenance of an osmotic pressure of their body 

 fluid to which the protoplasm of their tissues had become 

 attuned in the course of evolution. 



In support of these conclusions may be brought the facts of 

 palaeontology and zoology. The fishes of the Cambrian and 

 Silurian ages were elasmobranch. These animals have a carti- 

 laginous skeleton, and do not possess a swim bladder, although 

 a few do possess a rudimentary diverticulum opening from the 

 oesophagus. In the devonian period the dipnoi appear. The 

 swim bladder is well developed, and the unused gills are degene- 

 rating. The animal is preparing for a terrestrial life, and we 

 at once see the beginning of the development of the organs 

 of phonation (the modern representatives, the mud fish of 

 Queensland (Ceratodns) possessing a glottis arrangement in con- 

 nection with the neck of the air sac), thus affording the animal 

 the best means of communication with his fellows, and thus 

 conducing towards a gregarious existence which is the basis 

 of man's civilisation. In the strata of this era, too, are the 

 fossil remains of the primitive amphibia, which most probably 

 arose from these dipnoi. Then from the dipnoi evolved the 

 holostei which existed in permian and triassic times, and 

 were prominent in Jurassic times. These no doubt passed on 

 into the ganoids, which gave rise to the primitive teleostei 

 which exist in the upper triassic and cretaceous ages and 

 reached their full development in the upper eocene. By this 

 time, no doubt, these fish had returned to the water, some to 

 the sea, others to the fresh water streams, and in this return 

 the swim bladder, now no longer needed for respiratory pur- 

 poses, degenerated until in the higher teleosts the duct, which 

 had been gradually shifting more dorsally (9), totally disap- 

 pears, and the swim bladder remains in these animals a closed 

 cavity, and is used by the animal for an altogether different 

 purpose (that of a " sounding organ " and as a float). What 

 determined the return of the evolving teleosts to the water can- 

 not be stated, but the fact that fresh water fish belong exclu- 



