40 A NEW THEORY OF EVOLUTION. 



times occur between a new type and its 

 immediate (known) antecessor. 



Comparing engines of the same variety, 

 the expert might observe that no two engines 

 are absolutely identical, but differ in re- 

 spect of quality of materials or of work- 

 manship, or in efficiency, just as from anal- 

 ogous causes two animals of the same race 

 differ in expression of type or in energy. 



Reviewing the development as a whole, 

 the expert might explain how some of the 

 earlier types of engine have, like certain 

 types of organisms, survived, while others, 

 intermediate between them and the engine 

 of to-day, have disappeared. 



The development of the steam-engine pro- 

 gressed in accordance with man's necessities. 

 Many of our present engines would have 

 been useless until the necessity for them 

 arose or conditions necessary for their use 

 were provided, and if the scheme of evolu- 

 tion were fully disclosed, we should prob- 

 ably understand the reason for the suc- 

 cessive developments of life why some types 

 persist through ages unchanged, and why 

 others disappeared, or became more or less 

 highly specialised. 



Are all these parallels between the evolu- 



