176 THE STORY OF THE LIVING MACHINE. 



other. Now the difference between a machine 

 and a mass of matter is simply in the adaptation 

 of parts to act harmoniously for definite ends. 

 Hence if .we are allowed these three factors, we 

 can say that nature does possess forces adequate to 

 the manufacture of machines. These forces are not 

 chemical forces, and the construction of the ma- 

 chine has thus been brought about by forces en- 

 tirely different from those which produced the 

 chemical molecule. 



But we have plainly not reached the bottom 

 of the matter in our attempt to explain the ma- 

 chinery of living things. We have based the 

 whole process upon three factors. Reproduction, 

 variation, and heredity are the properties of all 

 living matter; but they are not, like gravity and 

 chemism, universal forces of nature. They oc- 

 cur in living organisms only. Why should they 

 occur in living organisms, and here alone ? These 

 three properties are perhaps the most marvellous 

 properties of nature; and surely we have not fin- 

 ished our task if we have based the whole process 

 of machine building upon these mysterious phe- 

 nomena, leaving them unintelligible. We must 

 therefore now ask whether we can proceed any 

 farther and find any explanation of these funda- 

 mental powers of the living machine. 



It must be confessed that here we are at present 

 forced to stop. We can proceed no further with any 

 certainty, or even probability. We may say that 

 variation and heredity are only phases of repro- 

 duction, and reproduction is a property of the liv- 

 ing cell. We may say that this power of repro- 

 duction is dependent upon the power of assimila- 

 tion and growth, for cell division is a result of 

 cell growth, We may further say that growth 



