I4 8 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- 

 The meaning of fa misleading in that it i mp li es , in the 

 the term "cell." 



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- 



