52 EVOL UTION AND NA TUP A L THEOLOGY. 



are liable to be transmitted to its progeny, and 

 perhaps intensified. If any particular modifica- 

 tion conferred an advantage on its possessor in 

 its reactions on other species, or on other indi- 

 viduals of the same species, which would have 

 a tendency to prolong its life, the new variety 

 would be more likely to be perpetuated than 

 the old one, while the original form of the 

 species, not having equal advantages, would 

 have a greater tendency than before to become 

 extinct. These slight modifications might be 



o o 



useful in a variety of ways, as for example, by 

 enabling their possessors to resist changes or 

 extremes of climate, to procure food more 

 easily, to conceal themselves better from their 

 enemies, to multiply more rapidly, etc. 



There appears to have been ample time since 

 the origin of life on the earth for the origination 

 of all organic forms from some primeval form 

 of very low position. Sir W. Thomson, indeed, 

 calculates* on astronomical data, that life 

 cannot have existed on the earth longer than 

 about 100,000,000 years. But this calculation 

 is rendered doubtful by the consideration that 



* " Trans. Geol. Soc. Glasgow," vol. iii. 



