10 Introduction 



many shores today. Among all types of animals there is a continual 

 tendency to spread into new available habitats in order to escape 

 interspecific competition. The most successful animal colonizers of 

 the land have been: (1) the arthropods, which have in many cases 

 developed book-lungs or tracheae for breathing air; (2) the verte- 

 brates, with lungs and dry skins; and (3) the snails, with slime and 

 spirally coiled shells to prevent desiccation. Certain burrowing 

 worms and many amphibians, which have little ability to conserve 

 water within their bodies, are struggling to maintain themselves on 

 land, and under favorable conditions some of these have even taken 

 up life in trees. There are at present many examples of animals 

 which are in the midst of their transformation from marine to fresh- 

 water animals, or from marine or fresh-water into land animals. 



Not only have plants and animals emigrated from sea to land, 

 but there are countless instances where migrations have taken, and 

 are taking, place in the opposite direction. Grasses, insects, reptiles, 

 birds, and mammals have left the land for the sea (Lull, 1917; 

 Pearse, 1932e). In general, primitive plants (Bews, 1923) and 

 animals (Kennedy, 1928) have remained in primitive habitats, but 

 after secondary migrations such relations may become mixed. Emi- 

 grations from water to land perhaps take place most readily in 

 moist situations in the tropics (Harms, 1929) . 



Clark (1927) disagrees somewhat with current opinion concern- 

 ing the origin of life in the ocean. He says: "The fauna of the sea 

 is the aquatic fringe of the fauna of land waters, which were more 

 extensive in the geologic past than now; the sea itself is practically 

 sterile. Life is most abundant on the land where there is the maxi- 

 mum water vapor in the air as in the moist tropics; it is most varied 

 where conditions are varied. . . . Life probably arose in marshy 

 water (non- or slightly saline) ." Against the view that life origi- 

 nated in the ocean, Macfarlane (1918) argues that many primitive 

 types of plants and animals occur today in fresh water. He states 



