THOUGHTS ON NATURAL PHILOSOPHY. 73 



also are dependent on external as well as internal 

 physical law, they are electro-chemical machines 

 connected with and controlled by the great machine 

 nature ; and perhaps the best definition of life is that 

 it consists in an arrangement of suitable materials, 

 by material motion, in suitable form, moving within 

 certain rates of speed. As we approach the border- 

 land between animate and inanimate nature it is 

 almost as difficult to separate the living from the 

 non-living as it is to distinguish between the animal 

 and vegetable ; and it is a question whether we ought 

 not to extend the definition of life to include much 

 that is now regarded as not living, although perhaps 

 much harm may not be done if we remember the 

 difficulty of drawing an abrupt dividing line between 

 the two Kingdoms. They appear to be imperceptibly 

 merged in one another. This is Nature's way and 

 may be taken to be one of the indications of the unity 

 of Nature, and her rule of natural law. You may not 

 like it, you may prefer what looks to you more clear 

 and definite; but Nature declines to work according 

 to our dictation, she has her own inimitable way of 

 gradual and imperceptible mutation. 



The great physiologist Verworn writes that : 

 " The fact stands out clearly and distinctly that life 

 from its beginning has been dependent upon the 

 external conditions of the earth's surface. In a 

 mathematical sense, life is a function of the earth's 

 development. Living substance could not exist 

 while the earth was a molten sphere without a solid, 

 cool, crust ; it was obliged to appear with the same 

 inevitable necessity as a chemical combination when 



