178 THE EVOLUTION THEORY 



would have been preserved through such a vast period of time, as we 

 shall see later, for useless organs disappear in the course of ages. 

 As the cavity has not yet disappeared, we may assume with some 

 probability that it is useful to the plant, whether by means of the 

 Anabcena, or in some other unknown way. To draw an argument 

 against the reality of the processes of selection from our lack of 

 knowledge of what this advantage may be would be as unreasonable 

 as if, notwithstanding our experience that stones sink in the water, 

 we were to assume of a particular stone which we did not see sink, 

 because it was hidden from our sight by bushes, that perhaps it had 

 not sunk, but was capable of floating. 



