WILHELM WUNDT, Grundzilge d. Physiologischcn Psychologie. 245 of vicarious functioning of nervous structures afforded by instances of restoration of motor function after destruction of nervous tissue ; but it has not been shown that after destruction of a sensory area of the cortex the corresponding sensations are ever regained, and nothing short of this will support Wundt's view. Equally ineffec- tive is his argument from the impossibility of conceiving the mode of origin of innate specific functions of cortical elements. Wundt's own principle of adaptation, combined with that of the inheritance of acquired characters, which he explicitly accepts, would neces- sarily result in the establishment of innate specific functions in the whole of each sensory tract. He scoffs at the view (accepted by Prof. James) that the origin of specific sensory functions may be explained by the principle of natural selection of spontaneous varia- tions, yet this principle may be applied to explain the origin of these functions with just as great, or as little, difficulty as in the case of any other bodily function. He gives us a hypothetical sketch of the evolution of the senses and sense-organs by differentiation of a primitive cutaneous sensibility, and we may accept this as probably in the main a true account of the stages of development ; yet the acceptance of this historical description by no means compels us, as Wundt claims (i., 449), to accept his principle of the adaptation of sensory organs and functions to the stimuli, for it is equally compatible with the belief that the principal part in the evolution has been played by spontaneous variation and natural selection, especially now that the reality of discontinuous variation has been established. And there are serious objections to Wundt's principle of specialisation by use of originally indifferent functions. It is bound up with the assumption of protoplasmic continuity through- out the sensory tract and, therefore, with the rejection of the neurone-theory which, as was pointed out above, is so superior in many respects to the older network doctrine ; it assumes that the impulses carried by sensory nerves are as varied as the elementary qualities of sensation, whereas we have no direct evidence of any differences in the characters of the impulses in either sensory or motor nerves, while the success of experiments in joining together nerves of different functions implies uniformity of nature. It would be easier to imagine a variety of impulses in sensory nerves corresponding to varieties of physical stimuli if we supposed the impulses to be purely physical changes, such as various forms of regular vibration ; but if, with Wundt, we assume that the impulse consists in the propagation of an explosive decomposition, it is hardly conceivable that the character of this chemical change can be determined by the nature of the physical shock which initiates it, rather than by the molecular constitution of the substance in which the change propagates itself. A further serious objection appears when we attempt to define more nearly the nervous process that is the immediate correlate of the psychical element or unit of sensation. Wundt writes, " Hence every conscious content, though it be as in these instances quite 17
