These being the few facts of their lives which concern the present subject one comes, as usual, to interpretation. These tree-sloths are descended from an older form that inhabited the ground, so that the present mode of life, which is so largely arboreal, has been acquired by dint of long years of struggle and adaptation to bitter needs. It seems hardly reasonable to call in the aid of selection for the production of its singular disposition of hair though that factor ruled in the production of its arboreal habit. It is almost flying in the face of common sense to attribute this upward, or downward (according to one’s point of view) singular arrangement to anything but the effects of gravity upon its long hairs. If it be not so, it looks a remarkable likely solution of this small problem. CHAPTER XV. EXPERIMENTAL. About ten years ago I began an investigation into the results of the application by man to the domestic horse of various forms of harness, desiring to find out if these results were capable of being transmitted from one generation to another. In 1908 I had not got very far, but thought it well to bring before the Zoological Society of London the results observed up to that time and read a paper entitled, “Some observations on the effects of Pressure upon the Direction of Hair in Mammals.” It was kindly received, but was not published in their proceedings, as it appeared to the Publication Committee a paper more suited to “another place,” presumably those of a veterinary society. It was illustrated by the two figures I give here of a horse in full harness, and another with the chief results as to changes of the direction of hair, or new patterns, displayed on its coat. Progress of Inquiry. Being disposed to think that the investigation could be carried further, I proceeded to look about for any examples in horses which might show the transmission of these artificial results to their descendants, and had to wait awhile before I could see which of the regions affected by the pressure of harness were likely to afford the required phenomena. These were in due time forthcoming, and will form the chief subject of the present chapter. I look upon them as cases of an _undesigned experiment_ and will describe them later. In the present stage of science all hypotheses must be submitted to the test of experiment before they can enter the charmed circle of natural laws. For this reason one must endeavour to apply the test of experiment to the hypothesis before us. The Nature of Experiment.