22 OUTLINES OF EVOLUTIONARY BIOLOGY 



water. If this really be a correct account of the minute structure 

 of living protoplasm it helps us, as we have already seen, to 

 explain its characteristic movements in terms of well known 

 physical phenomena. </ 



Other competent observers, however, maintain that the appear- 

 ance of foam-structure is a delusion and that what Biitschli 

 interprets as thin sheets separating the droplets from one another 

 are in reality very delicate fibres arranged in a network. These 

 fibrillae are supposed to be contractile and thus to be responsible 

 for the movements of the protoplasm as a whole. But whence 

 comes the contraction of the fibrillse ? 



Various considerations, again, and especially the phenomena 

 of heredity, oblige us to postulate for protoplasm an even more 

 minute fundamental structure than the microscope is capable of 

 revealing to us. It is, in all probability, made up of ultra- 

 microscopic material units, each composed of a group of molecules, 

 which units, or "biophors," must themselves be regarded as living 

 bodies capable of nourishing themselves, growing and multiplying 

 by division. 



It is difficult to form a satisfactory idea of the chemical com- 

 position of protoplasm because it is impossible to analyze it in the 

 living condition ; indeed, in the living condition it is constantly 

 undergoing chemical change, and the moment it dies it ceases to 

 be protoplasm. It is certain, however, that it is not a definite 

 chemical compound, but a mixture of several distinct sub- 

 stances : proteids, mineral salts and water. Moreover, different 

 samples of protoplasm, taken from different organisms or from 

 different parts of the same organism, may differ widely from one 

 another in chemical composition. Thus the difference between 

 nucleoplasm and cytoplasm is largely a chemical one, depending 

 to some extent upon the relatively large amount of phosphorus 

 present in the former. 



By far the most characteristic and important of the chemical 

 constituents of protoplasm are the proteids (proteins). These 

 form a remarkable class of substances which do not occur in 

 nature except in the bodies of plants and animals. They are 

 definite chemical compounds containing the elements carbon, 

 hydrogen, oxygen, nitrogen, sulphur and frequently phosphorus, 

 and they have an extremely complex and unstable constitution, 

 readily splitting up on oxidation into simpler and more stable 

 compounds and thereby liberating kinetic energy. Many 



