n] BIOLOGICAL FOUNDATIONS 59 



the secondary one of protection is often added. Its 

 surface-tension and its solubility must be such that 

 bodies which adhere to it and dissolve in it are useful 

 to the whole cell as food. Within this outer sheath 

 are two further substances. One, called chromatin 

 on account of its affinity for many dyes, is chiefly 

 concerned with assimilation, with the constructive 

 part of the protoplasm's chemical cycle. Usually it is 

 all massed to form (together with other substances) a 

 definite body or nucleus, but in various primitive forms, 

 such as some bacteria and some flagellates, it appears 

 in the shape of minute granules scattered at random 

 in the mass of the third substance, which, constituting 

 the bulk of the cell, is called the cytoplasm. This 

 has as its special duty the destructive part of meta- 

 bolism ; it liberates energy, and uses that energy in 

 doing work, such as locomotion, for the good of the 

 cell as a whole. 



All forms of life now living must have had an 

 ancestor which existed under this double form of 

 a cell and an individual, and it is our business now 

 to trace the main lines of this development. Here 

 is no necessity to enter into the causes of change; 

 whether we believe in Natural Selection or Lamarck- 

 ism, are driven back to Bergson's elan vital, or even 

 to a complete confession of ignorance, is immaterial as 

 long as we accept change as a fact Then our task 

 is merely to trace the change itself in its course and 



