32 C. HEDLEY. 



supply of sea water perpetuated by heredity. Every 

 vertebrate thus maintains its viscera in a bath of warm 

 sea water and its very life depends on that aquarium being 

 kept unbroken. So in this way we are all sea creatures 

 still. 



If it had been upon the land that an animal first made 

 bones there would have been at its disposal such hard sub- 

 stances as iron or aluminium, while to a marine animal the 

 softer calcium was the best material available. 1 Hence 

 for the building of a firm frame it was lime that was 

 employed by all sea dwellers, the worms, the crabs, the 

 snails, and finally the vertebrates. The suggestion is that 

 this choice of lime for a skeleton was made by vertebrates 

 before migrating from the sea to the shore, after which its 

 use became a fixed habit. 



At times the land has repaid the sea, as in the case for 

 instance, of the whales, dugong, seals or sea-snakes which 

 had terrestrial origins. There are two roads from the sea 

 landwards, the easiest and most gradual way is to ascend 

 a river and undergo the transformation in marshes. The 

 other is to make the beach a changing ground, where may 

 be learned how to endure a greater change of temperature, 

 to support the body in a thinner medium and to breathe air. 

 Where a sea was tideless, there would be little opportunity 

 for transformation, and in proportion as the tide had a larger 

 range, so would be the facilities for change. Probably the 

 process of acclimatisation from sea to land was chiefly 

 through the estuaries, and an animal has rarely grown 

 independent of the water by traversing the beach. The 

 ocean existed, of course, before the rivers. Both these 

 routes are practised here. In the streams, fish like 

 Galaxiasattenuatus and the Blue-eye, Pseudomugil signifer 

 pass from salt to fresh water, and back again. 2 



1 Johnstone, Conditions of life in the Sea, 1908, p. 301. 



2 McCulloch, The Australian Zoologist, i, 1915, p. 47. 



