The Principles of Psychology

After the rather remarkable attempt to show the mode of " genesis " of a simple nervous system, which was discussed at some length in our preceding number, Mr. Spencer begins the simple, though not easy task of showing the mode of " gen¬ esis of compound nervous systems."After a short reference to the apparent fact, that the production of pigment in the dermal structures of various living beings, is at least partly due to the action of light, Mr. Spencer proceeds to show, how the organs of vision are produced.He says, " the rudimentary eye con¬ sists of a few pigment-grains, under the outermost dermal layer, and hence we may infer that rudimentary vision is constituted by the wave of disturbance which a sudden change in the states of these pigment-grains propagates through the body."(P.532.)Now, what is there to justify such an inference?There is absolutely no legitimate warrant for it.But this is only one from among hundreds of instances, in Mr. Spencer's works, where important steps are taken, or positions assumed, apparently without a consciousness of the illegitimacy of the procedure.But to continue." How such pigment-grains become concen¬ trated in the particular place they may most advantageously occupy, we need not consider at length.Other things being equal, they will develop most where most light falls, and where, consequently, variations of light, caused by adjacent things, are strongest; and since a close cluster of pigmentgrains, when affected, will send through the body a more effi¬ cient wave of disturbance, natural selection will further the con¬ centration-there will be a survival of individuals, in which the approximation is greatest, ending in the formation of an inte¬ grated patch.The pre-existence of a simple nervous system