1 88 THE STORY OF THE LIVING MACHINE. 



fore without any foundation for further advance. 

 The cells must have had a. history of construc- 

 tion, but we do not as yet conceive any forces 

 which may be looked upon as contributing to 

 that history. Whether life phenomena can be 

 manifested by any mixture of compounds simpler 

 than the cell we do not yet know. 



The great problems still remaining for solu- 

 tion, which have hardly been touched by modern 

 biology in all its endeavours to find a mechanical 

 explanation of the living machine, are, therefore, 

 three. First, the relation of mentality to the 

 general phenomena of the correlation of force; 

 second, the intelligible understanding of the 

 mechanism of protoplasm which enables it to 

 guide the blind chemical and physical forces of 

 nature so as to produce definite results; third, 

 the kind of forces which may have contributed 

 to the origin of that simplest living machine upon 

 whose activities all vital phenomena rest the 

 living cell. 



