Sentence:  The acquisition of a useless part can hardly be said to raise an organism in the natural scale; and in the case of the imperfect, closed flowers, above described, if any new principle has to be <WSD>invoked</WSD>, it must be one of retrogression rather than of progression; and so it must be with many parasitic and degraded animals. 
Meaning of the word marked with <WSD> tags:  To call to mind (something) for some purpose.