28 DOCTRINE OF EVOLUTION 



to regard life and vitality as anything but actuating 

 principles that exist apart from the materials into 

 which they enter, and which they seem to make alive. 

 According to this general conception, ''life is something 

 like an engineer who climbs into the cab of the locomo- 

 tive and pulls the levers which make it go," as health 

 might supposedly be regarded as something that does 

 not inhere in well-being, but gets into the body to alter 

 it. But is this conception really justified by the facts 

 of animal structure and physiology? Let us recall 

 the steps of our analysis. The living organism is a 

 collection of differentiated parts, the organs ; the 

 life of an organism is a series of activities of the several 

 organic systems and organs. If we could take away 

 one organ after another, there would be nothing left 

 after the last part had been subtracted. In a similar 

 manner, the activities of organs prove to be the com- 

 bined activities of the tissue-cells, and again the truth 

 of this statement will be clear when we imagine the 

 result of taking away one cell after another from 

 organisms like the frog or tree. When the last cell 

 had been withdrawn, there would be nothing left of 

 the frog's structure, and there would be no element 

 of the frog's life. It is true that the particular way the 

 tissue-cells are combined is of primary importance, but 

 it is none the less true that the life of a cell is the kind 

 of element out of which the life of even the most com- 

 plex organism is built. And we have seen that the 

 essential substance of a cell is a complex chemical 

 compound we call protoplasm, whose elements are 

 identical with chemical substances outside the living 

 world. Is there any ground for supposing that the 



