18 THE STORY OF LIFE'S MECHANISM. 



all forms of energy in nature. It is primarily 

 a physical doctrine, and has been developed 

 chiefly in connection with the physical sciences. 

 But it shows at once a possible connection 

 between living and non-living nature. The 

 living organism also exhibits motion and heat, 

 and, if the doctrine of the conservation of 

 energy be true, this energy must be correlated 

 with other forms of energy. Here is a sugges- 

 tion that the same laws control the living and 

 the non-living world ; and a suspicion that if 

 we can find a natural explanation of the burn- 

 ing of a piece of coal and the motion of a 

 locomotive, so, too, we may find a natural 

 explanation of the motion of a living machine. 



EVOLUTION. 



A second conception, whose influence upon 

 the development of biology was even greater, 

 was the doctrine of evolution. It is true that 

 the doctrine of evolution was no new doctrine 

 with the middle of this century, for it had been 

 conceived somewhat vaguely before. But until 

 historical geology had been formulated, and 

 until the idea of the unity of nature had 

 dawned upon the minds of scientists, the 

 doctrine of evolution had little significance. 

 It made little difference in our philosophy 

 whether the living organisms were regarded as 

 independent creations or as descended from each 

 other, so long as they were looked upon as a 

 distinct realm of nature without connection with 

 the rest of nature's activity. If they are distinct 



