INTRODUCTION. 9 



energy which may be liberated by breaking the 

 molecule to pieces. The total sum of the active 

 and potential energy in the universe is thus at all 

 times the same. 



This magnificent conception has become the 

 cornerstone of modern science. As soon as con- 

 ceived it brought at once within its grasp all forms 

 of energy in nature. It is primarily a physical 

 doctrine, and has been developed chiefly in con- 

 nection with the physical sciences. But it shows 

 at once a possible connection between living and 

 non-living nature. The living organism also ex- 

 hibits 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 suggestion that the same laws control 

 the living and the non-living world ; and a sus- 

 picion that if we can find a natural explanation 

 of the burning 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 influ- 

 ence upon the development of biology was even 

 greater, was the doctrine of evolution. It is true 

 that the doctrine of evolution was no new doc- 

 trine 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 evo- 

 lution had little significance. It made little differ- 

 ence in our philosophy whether the living organ- 

 isms 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 



