ECOLOGICAL 93 



the body and the sea- water outside. Thus the same kind of animals 

 may have salter blood in the Mediterranean than in the North Sea. 



The sea is the original home of living creatures — the cradle of all 

 the races; and it was from the sea that the primary freshwater 

 animals had their origin. Some of them, like the pond-mussel and 

 the river-craj^sh, betray this in the osmotic tension of their body- 

 fluids, which is much higher than that of the surrounding water. 

 There are various interesting . ways in which freshwater animals 

 preserve this marine inheritance from over-dilution, e.g. by in- 

 creasing the activity of their kidneys, or by decreasing the per- 

 meability of the surface-membranes by which the surrounding water 

 tends to seep into the body. 



Our problem is not with the indigenous animals of the sea, nor 

 with the early colonists of the fresh waters, nor with the interesting 

 land animals, like the water-spider, that have found a secondary 

 refuge in ponds and streams ; we think of the return to the sea after 

 ages of sojourning on land. Among backboned animals there are 

 several distinct groups of mammals that have followed this policy — 

 the Cetaceans (whales, dolphins, porpoises), the Sea-cows or Sireni- 

 ans (dugong and manatee), the seals and sea-lions, and the now rare 

 sea-otter (Enhydris). Even the Common Otter may swim from 

 the mainland to an island, and the polar bear may swim in open 

 water for many miles. Among birds that have become secondarily 

 marine we may rank the Antarctic penguins ; and in the North the 

 likewise flightless Great Auk, now extinct. All the members of the 

 auk family, like guillemot and razorbill, may be called pelagic birds, 

 and the petrels are even finer conquerors of the open sea. The Storm 

 Petrels — "Mother Carey's Chickens" — are the smallest of web- 

 footed birds, but they are at home in mid-ocean, and never come to 

 land except at nesting-time; and almost the same may be said of 

 the largest web-footed bird, the Albatross, which has a spread of 

 wing up to eleven feet eight inches. 



Among reptiles there are the sea-snakes, venomous fish-eaters of 

 the open ocean, which do not come ashore except to bring forth 

 their young among the rocks. The edible turtles that feed on sea- 

 weeds keep near shore ; but the fish-eaters frequent the open waters, 

 except at the breeding season, when perforce they must come to 

 sandy beaches to bury their eggs. It will be understood that whereas 

 a whale can bring forth its young one in the sea, the oviparous birds, 

 and such aquatic reptiles as are oviparous, must go back to the old 

 home to find a cradle for their eggs. The unhatched embryo of a 

 turtle, like that of a bird, breathes dry air, which diffuses through 

 the egg-shell and into the blood-vessels spread out on the embry- 

 onic membrane called the allantois. In the sea-snakes there is vivi- 

 parity; and the young ones must be able from the first to use their 

 lungs on the surface of the water. 



