LIFE AND THE CELL 5 I 



If an animal or plant occupies a niche to which it is well adapted, it may- 

 persist almost unchanged for an indefinite period of time. The lamp shell 

 Lingula has changed very slightly since the Cambrian period. For animals 

 that bury themselves in the sea beach, life some hundreds of millions of 

 years ago was probably much the same as it is to-day. All along the course 

 of organic evolution there are forms that have found their niche and have 

 stayed there, while others that were more adventurous explored new fields 

 and acquired profound changes in adaptation to different kinds of environ- 

 ment. 



There are some types of environment that favor an advance of or- 

 ganization, and life, which is ever ready to take advantage of opportunities 

 for its own increase, has developed there into higher forms. Nature ap- 

 parently strives to fill all kinds of situations with living inhabitants. What 

 she seems to be interested in is having as many children as possible. Whether 

 they are high or low in the scale is a quite secondary matter. Certainly, 

 Nature has been remarkably successful in producing offspring of the most 

 compHcated structure along many different lines, and we may now con- 

 sider some of the ways in which she has achieved this end. 



One important influence is an indirect result of mere increase in size. 

 Every student of elementary geometry has learned that as a body increases 

 in size its surface increases as the square of its diameter, while its volume in- 

 creases as the cube. When a body grows, therefore, its volume increases 

 disproportionately to its surface. This fact has very important conse- 

 quences for living organisms. In a spherical organism of i, 2, 3 or 4 inches 

 in diameter, for instance, the surface areas would be as i, 4, 9 and 16, while 

 the volumes would be as i, 8, 27, and 64. But if the Hfe processes went on 

 at the same rate in these organisms, there would be a more rapid exchange 

 through a given area of surface in the large organisms than in the small 

 ones. As size increases, absorption of nutriment, the elimination of waste 

 and exchange of gases in respiration will have to be carried on so much 

 faster through a given area of surface that further growth would be auto- 

 matically checked. Perfectly spherical organisms of homogeneous struc- 

 ture, therefore, could not attain a very large size; they never do. Where 

 any considerable size is reached in a plant or animal, it is always attended 

 with structural devices for increasing surface in relation to volume. 



A good deal of the complicated anatomy of higher animals is a result 

 of extending surfaces devoted to the fundamental vital processes of absorp- 

 tion, excretion, digestion and respiration. Small animals can obtain suf- 

 ficient free oxygen by absorbing it through the body-wall. But where 

 size increases, relatively more surface is required for respiratory exchange. 

 Aquatic animals quite generally meet the situation by pushing out the integ- 

 ument to form gills. Among terrestrial animals gills, unless protected by 

 structures by which they are kept moist, are usually replaced by organs 

 that ramify within the body and thus keep respiratory surfaces protected 



