262 GENERAL BIOLOGY 



(fig. 88, c), or a whale (fig. 88, d), we see that in each 

 there is a swelling, curving contour like that of a 

 boat, which offers the least resistance to movement 

 through the water. In addition, a sharp cutting 

 edge or keel is sometimes developed. Although the 

 forms mentioned have very little else in common, and 

 although their " relatives " differ much from one an- 

 other, yet in accommodation to a free-swimming 

 habit they have " adopted " each a general type 

 of structure. This sort of a resemblance between 

 organisms otherwise unrelated, in adaptation to a 

 common environment, is called Convergence. In 

 those marine forms which float semipassively in the 

 open sea (pelagic organisms) convergence may be 

 carried to a greater extreme. (Compare fig. 89.) 

 Here two factors have been of significance in each 

 type : (a) the necessity for decrease in specific 

 gravity and (6) a minimum increase of surface for 

 the maximum of bulk. A sphere is the shape that 

 ideally fits the latter condition, and we find that 

 pelagic organisms in general tend toward a spherical 

 form. The former condition has brought about 

 an elimination of a great part of the solids in the 

 organism, and the result is a form which contains a 

 very large per cent of water and is usually semi- 

 transparent. 



Aerial Adaptations. - - The development of wing- 

 like structures has enabled representatives of various 

 diverse groups of organisms to maintain themselves 

 in the air. The whole group of insects is preemi- 



