132 EVOLUTION AND NATURAL THEOLOGY. 



organisation.* The origin of life is perhaps a 

 problem analogous to the origin of gravitation ; 

 and science may ultimately teach us that life is 

 a force pervading all natuffe, and not merely so- 

 called living organisms. It is, however probable 

 that the destruction of organic life throughout 

 the world would be a break in the chain of 

 causation which would prevent its ever being 

 evolved again under similar forms. When 

 a species once becomes extinct, it can never 

 reappear, for the conditions that led to its 

 evolution can never return ; and we may reason- 

 ably suppose that the same argument would 

 apply to organic life as a whole. 



The Protista form the lowest grade of living 

 beings, and from these all others appear to have 

 arisen; but the main stems are not numerous. 

 Some of these have perhaps branched almost 

 directly from the Protista, as soon as the latter 

 had become differentiated into plants and 

 animals ; but in other cases, some of the main 

 stems seem to have been connected very far 

 down in the organic scale. Thus, the structure 

 of the Molluscoida shows us that the Mollusca 



*.H. Spencer, " Biology," vol. i. part 2, ch. 3. 



