148 FOOT-NOTES TO EVOLUTION. 
The accumulating results of patient study have 
totally changed the earlier conceptions of the cell. Two 
and a quarter centuries ago, by the aid of the newly in- 
vented microscope, minute cavities were discovered in 
certain plant tissues, and from their resemblance to a 
honeycomb were termed “cells.” This study of such 
substances as ordinary cork, in which the cells are dead 
and empty, easily led to the idea that the cell wall was 
the all-important feature, and it has not been until 
within the past forty years that this error has been set 
aside. The name “cell” itself is some- 
what misleading in that it implies, in the 
ordinary usage, a cavity with definite 
walls of considerable firmness, which is by no means 
always true. Great groups of cells have no solid walls 
whatever, but are soft and changeable in form, and the 
majority of cells have no cavities, but are masses of 
semifluid consistence. The appearance of empty cavi- 
ties, or clear fluid-filled spaces, is a condition which 
comes about in plant cells late in life, and scarcely ever 
in animal ones. The unwearied study of biologists, 
aided by constantly improved instruments and methods 
of research, have shown that it is the contents of the 
cell which form the essential living substance. But, 
although the cell wall has lost the significance which it 
formerly was held to possess, the term cell has become 
firmly fixed by usage, and such terms as “ Energide,” 
as proposed by Sachs, though much more happily chosen, 
are very slow of adoption. 
The simplest forms of life of which we know any- 
thing are minute microscopic organisms found in both 
fresh and salt water and under the most varied condi- 
tions. Each one of these is composed of a single cell, 
and each one carries out in a general way the varied 
functions of movement, respiration, growth and multi- 
The meaning of 
the term ‘‘ cell.” 
