434 SCIENTIFIC THOUGHT. 



we also know that the red blood corpuscles in verte- 

 brate animals convey oxygen in a concentrated form ^ 

 through all the organs, giving it up wherever it may 

 be wanted, the real chemical process concerned in the 

 action of chlorophyll is not cleared up ; ^ and " no one 

 has been able hitherto to explain, by a reference to 

 physical laws, the active functions of the heart and 

 muscular wall," by which the circulation of the blood 

 is effected.^ 



In the explanation of many physiological phenomena 

 no idea has proved more fruitful than the con- 

 ception of natural selection, introduced by Darwin to 

 explain the growing diversity and the purposeful- 

 ness of organisms. Coupled with the cellular theory, 

 which looks upon every living organism as a society 

 of self-accommodating individual units or cells, forced 

 by circumstances into differentiation of form and into 

 divided labour or function, it relieved biologists of that 

 spectre of vitalism which still survived after Lotze 

 and Du Bois-Eeymond had placed the creative and 

 formative influence outside of the mechanism — as the 

 watchmaker lives outside of the watch, which exhibits 

 only mechanical contrivances. That which puzzles the 

 spectator of the watch, as it does the spectator of every 



1 See Buuge, ' Physiological 

 Chemistry,' p. 275. 



- " Iron play.s an important part in 

 vegetable life : we know that chlo- 

 rophyll granules cannot be formed 

 without it. If plants are allowed 

 to grow in nutritive solutions free 

 from iron, the leaves are colourless, 

 but become green as soon as an 

 iron salt is added to the fluid in 

 which the roots are immersed. It 



is even sufficient merely to brush 

 the suiface of the colourless leaf 

 with a solution of an iron salt to 

 cause the ajjpearance of the green 

 colour in the part thus i>ainted. 

 Chlorophyll itself contains no iron, 

 and we do not know in what way 

 the iron is concerned in its produc- 

 tion " (Bunge, loc. cit., p. 2.5). See 

 also Hertwig, 'The Cell,' p. 1.53. 

 •* Bunge, }). 7 ; cf. also p. 275. 



