Here is a nut. To exemplify, a beautiful glossy nut, which, blessed with original strength, has outlived all the storms of autumn. Not a puncture, not a weak spot anywhere. This nut while so many of its brethren have fallen and been trodden under foot, is still in possession of all the happiness that a hazel-nut can be supposed capable of.
These lines, from Chapter Ten, are spoken by Captain Wentworth to Louisa Musgrove, and Anne Elliot overhears them. Captain Wentworth is touching on a topic that is very close to his heart: the value of constancy and strength of character. This beautiful nut has weathered the storms and stayed on the tree, unlike the others. Wentworth uses this nut as an illustration of the importance of deciveness and firmness in one's mind. The reader can clearly connect this to Wentworth's past disappointment in love; he believes that Anne Elliot broke her engagement with him because she was not strong enough to withstand the disapproval of a few people. Though she promised him her love, she reneged on her word.
Austen's characteristic irony is an important part of this passage. Captain Wentworth considers "all the happiness that a hazel-nut can be supposed capable of." By its very ridiculousness, this final line throws Wentworth's illustration into question. At this point in the novel, it is still unclear whether firmness of character does increase happiness.