Sentence:  I stopped, and said with inly muttered voice, / 'It doth not love the shower, nor seek the cold: / This neither is its courage nor its choice, / But its <WSD>necessity</WSD> in being old. 
Meaning of the word marked with <WSD> tags:  Something which makes an act or an event unavoidable; an irresistible force; overruling power (en)