^ 



10 ESSENTIALS OF BIOLOGY 



activity, we may plausibly infer that most of our main types have 

 existed since the beginnings of life. 



103a. The Materialization of Life. In this Pre- Cambrian 

 evolution life successfully manifested itself in the processes we 

 have indicated : Ng-assimilation ; the assimilation of N- 

 compounds containing oxygen ; CO 2- assimilation ; S-assimila- 

 tion ; Fe-compounds-assimilation and probably in other 

 metabolic modes. Thus life expressed itself in the chemical and 

 physical changes that were carried on, with the aid of solar 

 radiation, in compounds of carbon and nitrogen, and (to a less 

 extent doubtless) in compounds of S and Fe. Very early in the 

 history of the animate world life became an affair of the chemical 

 activities of C and N because of what we may call the enormous 

 versatility of these elements. 



103^. Structural Manifestations of Life. We have, of 

 course, to consider the matter of the evolution of organic structure. 

 To some extent structure is unessential : for instance, the simple 

 unicellular organism such as an Amoeba, a Diatom or a Peridinian, 

 carries out all the functions of life displayed by the structurally 

 complex metazoan or metaphytic organisms. Yet there has been 

 an obvious evolution of structural forms and, in so far as this 

 seems to be essential to evolutionary '' progress," we must inquire 

 as to what it means. Structural evolution, therefore, implies 

 in the main the development of greater and greater size, on the 

 one hand, and greater and greater mobility, on the other. Size 

 is well illustrated by the difference between a pelagic, minute, 

 unicellular alga and a rooted Laminaria growing up to the surface 

 of the sea from a depth of 50 fathoms. Here the essential life- 

 activities of the two plants are much the same and evolution has 

 merely added cells to cells in such a way as to build up a plant-body 

 which can hold on to the sea-bottom, may not be easily dislodged 

 from its base and which can freely float in the water. Again we 

 may compare the structurally simple fungus with a great tree, 

 when we see that the essentially nutritive and reproductive 

 functions are as efficiently carried on by the simple, as by the 

 complex plant. The structural complexity of the tree is only 

 such as is necessary for the support of a large mass of material 

 against gravity and wind stresses, and for the conduction of water 

 throughout all parts of this plant body. And we see also in the 

 large, multicellular plants the surplus assimilation of material : 



