The Original Home of Life 9 



those that live near high-tide mark pass through their swimming 

 stages before hatching from the egg and never swim. Such a series 

 is beUeved to indicate origin in the ocean and varying degrees of 

 adjustment to Hfe on land (Colgan, 1910). Bony fishes appear to 

 have originated in fresh water, and many of them breed there. Al- 

 together considerable evidence from life histories and embryological 

 development indicates that a number of types of animals originated 

 in the ocean and gradually invaded fresh-water and land habitats. 

 The presence of gill arches in land vertebrates (reptiles, birds, 

 mammals) during embryonic development means that such animals 

 came more or less remotely from aquatic ancestors. 



While it is generally admitted by most zoologists that life prob- 

 ably originated in the ocean, there is much difference of opinion as 

 to the exact region where it appeared first. There are some (Brooks, 

 1894) who believe that life first came into existence in the open sea, 

 but most authorities (Simroth, 1891; Osborn, 1917; Johnstone, 

 1908) do not favor such a view, largely because there is a paucity 

 of available nitrogen compounds there. Manifestly the lack of nitro- 

 gen, light, and other favorable conditions does not make it seem 

 probable that life began in the depths of the ocean. Life is gen- 

 erally believed to have originated in the littoral region. 



Apparently no great groups (phyla) of animals originated except 

 in the ocean (Hesse, 1920) . The routes by which animals prob- 

 ably left the ocean and reached fresh water and land have been 

 various. Some animals probably migrated directly across sea 

 beaches; others probably ascended rivers, passed through marshes 

 and swamps, or burrowed through soil. Some animals were trans- 

 ferred from the ocean by land elevations which isolated them in 

 bodies of water which gradually became fresh. The ways animals 

 followed from sea to land will be discussed in the next chapter. 



Emigration from the sea did not take place at any one time. It 

 has occurred many times in the past and is slowly progressing on 



