38 OUTLINES OF EVOLUTIONARY BIOLOGY 



importance to the cell-walls, which indeed are alone visible in 

 dead tissues such as dry cork and pith. 



It was not until 1846 that von Mohl first gave the name 

 protoplasm to the slimy contents of the cells in living tissues. 

 It then gradually became evident that this protoplasm was the 

 really vital constituent of the cell, and that it was identical in 

 nature with the substance of which minute naked organisms 

 such as the Amoaba are composed, and which was already 

 known by the name sarcode, given to it by Dujardin in 

 1835. 



The cell theory, first propounded in a very imperfect form by 

 Schleiden and Schwann, about the year 1838, rapidly developed, 

 during the course of the nineteenth century, into one of the most 

 fertile generalizations of natural science. At the present day the 

 term cell is extended to protoplasmic units which may have no 

 cell-walls at all, and to which therefore it is etymologically quite 

 inapplicable, and for a long time a cell has been defined simply 

 as a single nucleated mass of protoplasm. 



In accordance with the cell theory such nucleated masses of 

 protoplasm are the organic units of which the bodies of all living 

 things are built up. The simpler organisms, such as Amoeba 

 and Haematococcus, consist each of a single unit only, and are 

 therefore said to be unicellular, while the more complex forms, 

 both of animals and plants, consist each of many such units united 

 together in a muiticellular body. Moreover, we now know that cells 

 never originate de novo but multiply by division, so that each one 

 is the immediate descendant of a pre-existing cell, a very 

 important fact which was emphasized by Virchow in his often 

 quoted phrase " Omnis cellula e cellula." 



The zoologist includes under the name Protozoa all those 

 unicellular organisms which he claims as members of the animal 

 kingdom, whilst the unicellular plants are relegated to the domain 

 of the botanist under the name Protophyta, but, as we have 

 already seen, it is impossible to draw a rational line of demarcation 

 between these two groups and they are often included together 

 under Haeckel's term Protista. 



Tbe outstanding feature in all these simple forms of life is that 

 the single cell is a complete and self-supporting organism. It 

 has to perform all the necessary vital functions for itself, by means 

 of such simple organs, temporary or permanent, as can be 

 produced by differentiation within the microscopic limits of its 



