This paper attempts (1) to weigh the evidence for and against the current distinction of simple reactions into the 'sensorial' and 'muscular' forms, and (2) to show that the final settlement of the point in dispute is psychophysically desirable. It is inferred "that the distinction is a valid one, but not obtainable from every observer. Rather is there required for the work a special kind of disposition or Anlage." That the issue is important, is proved by a consideration of the various departments of inquiry in which the reaction-method has been followed. A postscript tabulates the introspective differences between the two forms, as observed in the Leipsic Institute.
There has been far too much theorizing on questions of instinct and animal psychology generally, and these theories have, moreover, overlooked the important fact that the psychology of an animal must depend on its biological organization. Hence the distinction between vertebrate and invertebrate is probably far more logical than that between human and animal psychology. Further, the more striking and advanced, and therefore presumably more complicated, exhibitions of animal intelligence, have been studied more than the simple and normal. The peculiarity of instinct is that it seems to combine both intelligence and mechanism, and the ordinary 'explanations' merely restate the fact. But the important question is: How did this state of affairs arise? The mechanical theories, e.g. of Spencer, ascribe it all to the growing complexity of reflex action; but neither in this nor in its psychological form, do they account for the appearance of anything new. The development of instincts may be explained by natural selection, on condition that variation is not reduced to a mechanical chain of motions. Variation is a real change, a phenomenon that has essentially psychological analogies, and Darwinism merely assures us of the fact, without explaining it. Observing our own mental life, we find it oscillating between the two poles of Invention, which introduces new elements, and Habit, which preserves the old. When habit prevails entirely, action becomes wholly instinctive. And this is the case with the animals. They are effete, and have lost their mental originality.
