54 ECOLOGICAL FOUNDATIONS OF ZOOGEOGRAPHY 



other existing genera in the Eocene. In contrast with these, the 

 reptiles have undergone great changes since the Cretaceous, and the 

 genera of mammals rarely extend as far back as the Miocene. 



Thanks to the advantageous characters acquired in connection with 

 terrestrial life, the water affords a new sphere of action for air-breathers 

 in which they have no competitors on their own plane. This explains 

 the readaptation of terrestrial forms to life in the water, which thus 

 become secondarily aquatic animals. In the ocean it is chiefly the 

 vertebrates that have returned to aquatic life, in part poikilothermal 

 forms, such as the sea turtles and sea snakes, in the warmer seas, and 

 in part homoiothermal animals, such as auks and penguins, sea otters, 

 seals, whales, and sirenians. There are only a very few marine insects, 

 and the marine arachnids are limited in number. In fresh water, on 

 the other hand, great numbers of insects have taken up their existence, 

 either for their entire life or at least for their larval period, and there 

 are some spiders and all hydrachnids, and many pulmonate snails. 

 There are a number of reptiles (crocodiles, turtles, and numerous 

 snakes) and a few mammals, such as the otter and beaver. The breath- 

 ing of air, however, which is the chief source of advantage for the 

 terrestrial forms, is almost always retained. Only the water mites and 

 the insect larvae with tracheal gills reacquire the ability to obtain 

 oxygen from the water. Others are in the transition stage: witness 

 Limnaea from the deeper waters of Lake Geneva, and the aquatic 

 turtles with accessory anal breathing organs. The larvae of amphibians 

 breathe water, and in the axolotl and the perennibranchiate sala- 

 manders reach sexual maturity as gill-breathing forms. It is perhaps 

 a question as to whether these larvae are secondarily aquatic in a strict 

 sense. Sometimes, as in the sea snakes and sea turtles, there is a super- 

 ficial network of blood capillaries in the mouth and on the jaws. It 

 must not be forgotten that most terrestrial salamanders have entirely 

 lost their lungs. 



With the exception of a few viviparous forms such as the sea 

 snakes, the whales, and the sirenians, the secondarily aquatic verte- 

 brates above the Amphibia retain their terrestrial breeding habits. 

 Crocodiles and turtles lay their eggs on the shore, the penguins lay 

 and brood their eggs on land, and even such thoroughly aquatic 

 mammals as seals seek the land at the breeding period. 



BIBLIOGRAPHY 



1) Doderlein, 1912, Zool. Anz., 40. p. 85-93.-2) Fredericq, 1904, Arch. Biol., 

 20, p. 709-737.— 3) Bottazzi, 1901, Arch. Anat. Physiol., 1901, Suppl., p. 109.— 



