That is his mark.

Yes, you will find some very singular points about the moor, Dr.Watson.Oh, excuse me an instant! It is surely Cyclopides." A small fly or moth had fluttered across our path, and in an instant Stapleton was rushing with extraordinary energy and speed in pursuit of it.

To my dismay the creature flew straight for the great mire, and my acquaintance never paused for an instant, bounding from tuft to tuft behind it, his green net waving in the air.

His gray clothes and jerky, zigzag, irregular progress made him not unlike some huge moth himself.
I was standing watching his pursuit with a mixture of admiration for his extraordinary activity and fear lest he should lose his footing in the treacherous mire, when I heard the sound of steps and, turning round, found a woman near me upon the path.

She had come from the direction in which the plume of smoke indicated the position of Merripit House, but the dip of the moor had hid her until she was quite close.
I could not doubt that this was the Miss Stapleton of whom I had been told, since ladies of any sort must be few upon the moor, and I remembered that I had heard someone describe her as being a beauty.