180 THE PHILOSOPHY OF BIOLOGY 



outer environment. Their activities are inherent in 

 themselves, are rhjrthmic, and become functional 

 only on the assumption by the soma of the phase of 

 sexual maturity. From the point of the species the 

 soma is only the envelope of the germ-cells. It is 

 affected by every change of the environment, and 

 being usually cumulatively affected by the latter it 

 becomes at length an unfit envelope. Somatic death 

 then follows as a natural consummation, but the germ- 

 cells are, in a sense, immortal in that they remain 

 capable of indefinite growth by division. 



In the sexual reproduction of the higher organism 

 a part of the germ-plasm becomes detached, under- 

 goes growth, and develops into an organism exhibiting 

 the parental organisation. But in the development 

 of the offspring, part of the germ-plasm received from 

 the parent persists unchanged, is transmitted to 

 another generation, and so on without apparent limit. 

 Something is transmitted from parent to offspring. 

 This something we regard as a cell exhibiting a definite 

 chemical and physical structure ; but while the germ- 

 cell differs in certain respects from an ordinary somatic 

 cell, these structural and chemical differences are 

 insignificant when they are compared with the differ- 

 ences in the potentialities of the cells. The somatic 

 cells are, in general, capable of reproducing only the 

 general character of the tissues of which they form 

 part. Some of them, the cells of the grey matter of 

 the central nervous system, for instance, appear to be 

 incapable of division and growth. But again the 

 facts of regeneration appear to point to the possession 

 by the somatic cells of more than this restricted power 

 of reproducing the tissues of which they form part : 

 to this extent the regeneration experiments tend to 

 remove the essential distinction between the somatic 



