"Good-bye." He held out his hand, and she gave him hers a moment--a moment long enough for him to bend his handsome bared head and kiss it.

Then, still agitating, in his mastered emotion, his implement of the chase, he walked rapidly away.

He was evidently much upset.
Isabel herself was upset, but she had not been affected as she would have imagined.

What she felt was not a great responsibility, a great difficulty of choice; it appeared to her there had been no choice in the question.

She couldn't marry Lord Warburton; the idea failed to support any enlightened prejudice in favour of the free exploration of life that she had hitherto entertained or was now capable of entertaining.
She must write this to him, she must convince him, and that duty was comparatively simple.