22o PHYSICAL BASIS OF CIVILIZATION 



protoplasm within the cell, from moment to moment, 

 expose different portions of it to external influences. 

 Thus are the opportunities of the externality to 

 react on the internal cell substance increased, and 

 the conclusion reached in the last paragraph, there- 

 fore, must not be applied too rigorously. 



By substituting the word "organism" in the above 

 argument wherever the word "cell" is used, and the 

 word "cell" or "cells" where "particles," "atoms," or 

 molecules are mentioned, it will appear that this line 

 of reasoning applies with equal force to metaphita 

 and metazoa; for the external cells of these are 

 subject to modifications from external influences, 

 which, if they reach the central cells at all, do so to 

 an unappreciable extent only. This explains the 

 existence of somatic cells, in multicellular organisms. 



These are cells originally external which have 

 been modified by environmental influences during 

 so many generations that they have lost the general 

 characteristics of the organisms from which they 

 spring, and which they are therefore unable to repro- 

 duce as a whole. They do, however, reproduce 

 their own peculiar kind of cells and tissues by fission, 

 etc. Out of these somatic cells the various spe- 

 cialized organs of a more or less complex body arise. 



Variation, then, in the asexual metaphita and 

 metazoa is practically confined to the production 

 of organs out of somatic cells, while the central cells, 

 which are the reproductive cells, are the reproducers 

 and preservers of hereditary traits. 



