A minute examination of the circumstances served only to make the case more complex.  In the first place, no reason could be given why the young man should have fastened the door upon the inside.  There was the possibility that the murderer had done this, and had afterwards escaped by the window.  The drop was at least twenty feet, however, and a bed of crocuses in full bloom lay beneath.  Neither the flowers nor the earth showed any sign of having been disturbed, nor were there any marks upon the narrow strip of grass which separated the house from the road.  Apparently, therefore, it was the young man himself who had fastened the door.  But how did he come by his death?  No one could have climbed up to the window without leaving traces.  Suppose a man had fired through the window, he would indeed be a remarkable shot who could with a revolver inflict so deadly a wound.  Again, Park Lane is a frequented thoroughfare; there is a cab stand within a hundred yards of the house.  No one had heard a shot.  And yet there was the dead man and there the revolver bullet, which had mushroomed out, as soft-nosed bullets will, and so inflicted a wound which must have caused instantaneous death.  Such were the circumstances of the Park Lane Mystery, which were further complicated by entire absence of motive, since, as I have said, young Adair was not known to have any enemy, and no attempt had been made to remove the money or valuables in the room.

All day I turned these facts over in my mind, endeavouring to hit upon some theory which could reconcile them all, and to find that line of least resistance which my poor friend had declared to be the starting-point of every investigation.  I confess that I made little progress.  In the evening I strolled across the Park, and found myself about six o’clock at the Oxford Street end of Park Lane.  A group of loafers upon the pavements, all staring up at a particular window, directed me to the house which I had come to see.  A tall, thin man with coloured glasses, whom I strongly suspected of being a plain-clothes detective, was pointing out some theory of his own, while the others crowded round to listen to what he said.  I got as near him as I could, but his observations seemed to me to be absurd, so I withdrew again in some disgust.  As I did so I struck against an elderly, deformed man, who had been behind me, and I knocked down several books which he was carrying.  I remember that as I picked them up, I observed the title of one of them, the origin of tree worship, and it struck me that the fellow must be some poor bibliophile, who, either as a trade or as a hobby, was a collector of obscure volumes.  I endeavoured to apologize for the accident, but it was evident that these books which I had so unfortunately maltreated were very precious objects in the eyes of their owner.  With a snarl of contempt he turned upon his heel, and I saw his curved back and white side-whiskers disappear among the throng.