What characterizes tropisms is a directive orientation which automatically leads the organism in the same axis of the stimulating force, this being invariable as long as the stimulation lasts. What will be the response of the organism if one abruptly varies the intensity of the stimulating force? Will the response still be a directive orientation, a tropic displacement.
It is quite otherwise, as Jacques Loeb observed in the course of his first experiments upon tropisms. Led by a remark of Wheeler's, the learned biologist had no trouble in showing that a worm, Planaria torva, having no phototropism, shows a greater activity when the luminous intensity increases and quiets down when it diminishes, so that if one puts a certain number of these Plana-rias into a cylindrical vessel in front of a window, they all gather at the points in the vessel which are lighted the most poorly,-that is to say, at the two extremities of the diameter parallel to the lighted glass. More sensitive still to variations in intensity is Serpula uncinata, a little tube-inhabiting annelid studied by Loeb in the laboratory at Naples. The branchiae of this insect are spread out to the light of the sun, and form a graceful dis-coidal plume at the orifice of the tube, but if but the shadow of one's finger falls upon them, immediately the plume is withdrawn. It is drawn in, also, when the blinds of the window are closed. In these two cases the animal resumes little by little its first position, and when the branchiae are reestablished they are not drawn in if the light returns. Positively phototropic, the Serpula reacts only to negative radiations of the luminous intensity.
These phenomena do not enter into the formula of tropisms, because they are independent of the direction of the luminous rays, and do not provoke a directive orientation. Loeb gives them (1893) the barbaric name of UnterschiedenipfidlichJceit, which Georges Bohn has happily translated " differential sensitiveness."
The phenomena of differential sensitiveness have been subjected to a minute study by the latter author, who has the honor of having established certain laws. It will be well to examine these rather closely, for they are almost always connected with tropisms and indicate a serious advance in the animal activity.