Suppose that there were really some huge hound loose upon it; that would go far to explain everything.

But where could such a hound lie concealed, where did it get its food, where did it come from, how was it that no one saw it by day?
It must be confessed that the natural explanation offers almost as many difficulties as the other.

And always, apart from the hound, there is the fact of the human agency in London, the man in the cab, and the letter which warned Sir Henry against the moor.

This at least was real, but it might have been the work of a protecting friend as easily as of an enemy.

Where is that friend or enemy now?
Has he remained in London, or has he followed us down here?
Could he--could he be the stranger whom I saw upon the tor?
It is true that I have had only the one glance at him, and yet there are some things to which I am ready to swear.