330 ANIMAL DEFENCES 



out it is difficult to understand. One reptilian experiment in this 

 direction, however, has been more successful, for Marine Snakes 

 thrive in the Indian Ocean. Among Invertebrates the lung- 

 breathing Snails and Slugs (Pulmonata) are very much special- 

 ized to fit them for a life on land, yet certain slugs (species of 

 Oncidium, &c.) have taken to a shore -life, though they can 

 scarcely be called marine. Insects are perhaps the most charac- 

 teristic of land-animals, yet a few of them are found living on 

 the surface of the sea. Certain allies of the Earth-Worm have 

 been found burrowing- along- the shore. 



Rivers and Lakes as a Haven of Refuge. — The waters of 

 the land have afforded protection to a number of hard-pressed 

 marine forms, estuarine conditions affording a sort of half-way 

 house. Fishes afford the most striking illustration, for all of those 

 inhabiting rivers and lakes have probably sprung from a marine 

 stock. At the present time, for example, Lung- Fishes (Dipnoi) 

 are limited to some of the rivers of Africa, South America, and 

 Australia, though originally the group to which they belong 

 included only marine species, as shown by geological evidence. 

 All the marine species have now become extinct, having been 

 unable to cope with the competition offered by other forms, but 

 those which have taken to fresh water have so far been able 

 to hold their own. Amphibians appear to have been evolved 

 from ancestors resembling Lung- Fishes in many ways, while 

 Reptiles, Birds, and Mammals probably sprang from an am- 

 phibian stock. 



Floating Aquatic Forms (Plankton). — The surface layers of 

 the sea swarm with animal life, and have very likely (though this 

 is not quite certain) received their population from shallower 

 water. The gulf-weed which floats on the Sargasso Sea in the 

 North Atlantic has quite a fauna of its own, and no doubt plants 

 took to floating life before animals, which were attracted by the 

 food-supply thus offered. There is reason to believe that in very 

 remote geological times far larger areas of the ocean surface were 

 covered by drifting sea-weed than is the case at present. Such 

 floating animals are also found in lakes. 



Deep-water Life (Benthos). — Profound ocean depths are 

 peopled by a large and strange collection of animal forms, many 

 of which seem to have been driven out of shallower water by 

 the force of competition. Otherwise they would most likely have 



