22 EVOLUTION OF LIVING ORGANISMS. 



us that the protoplasm is always present in the form of 

 cells. We now come to another of the great generalisa- 

 tions of biology : the so-called " cell theory," founded by 

 Schleiden and Schwann more than seventy years ago, and 

 much modified and extended since then. Briefly it may 

 be summed up as follows : The cell is a small mass of 

 protoplasm consisting of a nucleus and surrounding cell- 

 body. All plants and animals are made up of such cells, 

 either singly or in aggregates. Growth is due to the 

 increase in size and multiplication of the cells. They 

 reproduce by division into two, and all the cells of the 

 body of an individual are thus derived from a single 

 original cell. Differentiation in multicellular organisms 

 is related to the progressive division of labour among 

 the cells, comparable to the division of labour among the 

 individuals of a civilised community. The body of a 

 multicellular organism is thus an aggregation, not of 

 separate units brought together, but of a multitude of 

 related cells remaining in association, and building up its 

 tissues. As a rule the tissue-cells remain in actual proto- 

 plasmic continuity ; but in animals certain cells may 

 become free and lead a quasi-independent life, as for 

 instance the white corpuscles of the blood. The activity 

 of an organism is the sum of the activities of its com- 

 ponent cells. All living phenomena are ultimately ques- 

 tions of cell-life ; all organic products due to the action 

 of cells (Fig. 1). 



While the cell-body consists of ordinary granular 

 protoplasm (cytoplasm), the nucleus is formed of special 

 protoplasm differing from it both in structure and in 

 chemical composition. In the nucleus may be distin- 

 guished by appropriate methods, besides a more fluid 

 nuclear sap, a meshwork of substance known as linin, in 

 which are suspended masses of another substance called 

 chromatin, from the fact that it colours deeply with 

 certain stains after having been killed and coagulated 

 with appropriate reagents. 



