Lowenstein! The name brought back to me the memory of some snippet from a newspaper which spoke of an obscure scientist who was striving in some unknown way for the secret of rejuvenescence and the elixir of life. Lowenstein of Prague! Lowenstein with the wondrous strength-giving serum, tabooed by the profession because he refused to reveal its source. In a few words I said what I remembered. Bennett had taken a manual of zoology from the shelves. “ ‘Langur,’ ” he read, “ ‘the great black-faced monkey of the Himalayan slopes, biggest and most human of climbing monkeys.’ Many details are added. Well, thanks to you, Mr. Holmes, it is very clear that we have traced the evil to its source.”

“The real source,” said Holmes, “lies, of course, in that untimely love affair which gave our impetuous professor the idea that he could only gain his wish by turning himself into a younger man. When one tries to rise above Nature one is liable to fall below it. The highest type of man may revert to the animal if he leaves the straight road of destiny.” He sat musing for a little with the phial in his hand, looking at the clear liquid within. “When I have written to this man and told him that I hold him criminally responsible for the poisons which he circulates, we will have no more trouble. But it may recur. Others may find a better way. There is danger there - a very real danger to humanity. Consider, Watson, that the material, the sensual, the worldly would all prolong their worthless lives. The spiritual would not avoid the call to something higher. It would be the survival of the least fit. What sort of cesspool may not our poor world become?” Suddenly the dreamer disappeared, and Holmes, the man of action, sprang from his chair. “I think there is nothing more to be said, Mr. Bennett. The various incidents will now fit themselves easily into the general scheme. The dog, of course, was aware of the change far more quickly than you. His smell would insure that. It was the monkey, not the professor, whom Roy attacked, just as it was the monkey who teased Roy. Climbing was a joy to the creature, and it was a mere chance, I take it, that the pastime brought him to the young lady’s window. There is an early train to town, Watson, but I think we shall just have time for a cup of tea at the Chequers before we catch it.”