BIOLOGY OF THE SEA-HOW THEY LIVE 27 



drawn from the sea's organic "soup." Before the beginnings of the Cambrian 

 Period, about a half a bilhon years ago, all the major animal phyla had 

 evolved, the history of which still remains largely a matter of mystery. All of 

 these phvla carry remnants of their place of origin in the sea because there is a 

 liquid (plasma) bathing the tissues of all animals which is very much like sea 

 water in its composition. The similarity of sea water and plasma indicates that 

 the seas of a billion years ago, when life arose, were very similar in chemical 

 composition to what they are today. 



Only one-fifth of all the total number of species of animals (including 

 insects) are found in the sea, whereas four-fifths live on land. This is in spite of 

 the fact that the sea's area is over twice that of land. It is also in spite of the fact 

 that of the sixty classes of animals, only four are not found in the sea, whereas 

 twenty-five are exclusively marine (Hesse, and others, 1951). Therefore the 

 sea has a greater diversity of groups since most groups originated there, but it 

 has a much smaller number of species than the land. This apparent paradox 

 can be explained by the fact that land, where barriers between habitats are 

 much more difficult to cross, which results in isolation, is a much harsher and 

 more variable environment than the sea. Only the most adaptable animals 

 are able to live on land at all, and these are subject to large and constant 

 en\'ironmental changes, with which they must keep step by evolving to meet 

 new conditions quickly. It is isolation, enforced bv barriers, and environmental 

 xariabilitv that lead to increased numbers of species. Land animals have evolved 

 to a higher psychological order in response to a relatively complex and harsh 

 environment. 



Competition is another factor that affects the rate of evolution. Evolution 

 is fastest in areas of most strenuous competition on land or sea. The greatest 

 competition in the sea occurs in the tropical reef areas, where a great number 

 of species are jammed close together in an optimum environment subject to 

 little variation. Consequently the most advanced and highly evolved species 

 of marine animals are generally found about tropical reefs. Toward the deep 

 sea or toward colder waters, environments become harsher, resulting in elimina- 

 tion of species that cannot adapt to these conditions. Thus, fewer species are 

 found in deep and cold waters than in the tropics, and these are subject to 

 lessened competition. This permits some archaic animals to survive in deep 

 or cold waters, whereas they could not survive in tropical, shallow waters. This 

 is the reason that primitive fish, such as isopondyls (herringlike fishes) are 

 mainly found in temperate or arctic zones, in the deep sea, or in fresh waters. 

 The cold and deep waters are harsh enough to reduce the number of species 

 and to reduce competition but, unlike land environments, are not variable 

 enough and have few barriers to impose isolation. The numbers of individuals 

 is great in northern and deep waters, but the number of species is small. 



Evolution is not over. Environments constantly change, and all life, including 

 man, must adapt to meet change, or else perish. 



ADAPTATION TO HABITAT 



Adaptation refers to adjustment between a living animal and the parts of its 

 environment. This is a phenomenon that is exhibited by every living thing 



