In this paper there is presented a series of characteristic examples of vital agency, followed by some considerations bearing on the extent to which, in a physiological point of view, it can be admitted that the physical forces, under exchangeable forms, are concerned in organic phenomena. At the same time, one of the particular objects of inquiry is, whether there be ground for the belief that vital agency, being not a force but a directive principle, may accomplish all the parts assigned to it, solely by means of the physical forces variously modified through organic structure.