240 



ANALYSIS OF THE ENVIRONMENT 



limited in geographic range and in ecologi- 

 cal habitat by its food relations. It may be 

 added that food specialization as a direction 

 of evolution may be quite independent of 

 progressive evolution, in which lack of such 

 specialization may be one of the conditions 

 of progress. Extreme specialization may be 

 thought of as essentially irreversible (see 

 p. 679). 



As a preHminary comment on the food 

 environment, it must be stated that we re- 



the basic plant food consists of the macro- 

 scopic vascular plants. Even the vast beds 

 of the giant kelp plants, that are sometimes 

 as tall as the sequoias, form an insignificant 

 proportion of the total vegetation of the 

 sea. Thus the largest marine organisms are 

 carnivorous, dependent upon plants through 

 a chain of smaller animals, as is the case 

 even with the gigantic plankton-feeding 

 baleen whales. The largest land animals, in 

 contrast, are herbivores, directly dependent 



DIRECTION OF 

 SWIMM ING 



Fig. 64. Filter-feeding apparatus of Oikopleura. A, The animal (in stipple) in its gelatinous 

 'house," viewed from the side: S, sieve; M, mouth; N, net filaments; T, tail. B, Cast of the 

 house, viewed from above. The discovery of the marine nannoplankton was largely the result 

 of the examination of the food of Oikopleura. (After Hesse and Doflein. ) 



ject the Piitter hypothesis that an important 

 part of the energy-yielding food of aquatic 

 animals consists of dissolved organic mate- 

 rials. We are still uncertain of the extent to 

 which animals make use of particles in col- 

 loidal suspension. By way of orientation it 

 may be pointed out further that there is a 

 radical and far-reaching difference between 

 the plant base of the food pyramid in the 

 sea and that on land. In the marine habitat 

 the basic food supply consists of the micro- 

 scopic plants of the lighted zone of open 

 water, composing the major proportion of 

 the nannoplankton; while for land animals 



on vegetation composed of plants of con- 

 siderable size. The animals of fresh waters 

 include so large an element of secondarily 

 or temporarily aquatic foiins that they do 

 not fall readily into the marine-terrestrial 

 dichotomy (Sverdrup, Johnson, and Flem- 

 ing, 1942). 



The marine phytoplankton (mostly as the 

 minute nannoplankton) is fed upon directly 

 by a great number of small but still macro- 

 scopic marine animals, among which cope- 

 pods {Calaniis spp., for example) and eup- 

 hausids (Eiiphausia pellucida) are espe- 

 cially noteworthy for their vast numbers, 



