Call G₁ and G₂ two responses which result in O₁ and O₂ the _optima_ or most satisfying state of affairs for 1 and 2. Call I₁ and I₂ two responses which result in the continuation of S₁ and S₂. The only responses possible for 1 are G₁ and I₁. The only responses possible for 2 are G₂ and I₂. Animal 1 upon the recurrence of S₁ and C₁ is little or no more likely to respond by G₁ than he was before. Animal 2 upon the recurrence of S₂ and C₂ is far more likely to respond by G₂ than he was before. The fact thus outlined might conceivably be due to an intrinsic inequality between O₁ and O₂, the power of equally satisfying _optima_ to influence, their antecedents being identical. This is not the case in the evolution of learning, however. For even if, instead of O₂, we had only a moderately satisfying state of affairs, such as the company of other chicks to (2) a 15-day-old chick, while O₁ was the optimum of darkness, dampness, coolness, etc., for (1) an earthworm, 2 would learn far, far more rapidly than 1. The fact is due, of course, to the unequal power of equally satisfying conditions to influence their antecedents. The same argument holds good for the influence of discomfort. The ability to learn,—that is, the possession of a connection-system subject to the laws of exercise and effect,—has been found in animals as ‘low’ as the starfish and perhaps in the protozoa. It is hard to tell whether the changed responses observed in Stentor by Jennings and in Paramecium by Stevenson Smith are easily forgotten learnings or long retained excitabilities. Sooner or later clear learning appears, and then, from crabs to fish and turtle, from these to various birds and mammals, from these to monkeys, and from these to man, a fairly certain increase in sheer ability to learn, in the potency of a supposedly constant degree of satisfyingness or annoyingness to influence the connection preceding it, can be assumed. We cannot, of course, define just what we mean by equal satisfyingness to a mouse and a man, but the argument is substantially the same as that whereby we assume that the gifted boy has more sheer ability to learn than the idiot, so that if the two made the same response to the same situation and were equally satisfied thereby, the former would form the habit more firmly. We may, therefore, expect that when knowledge of the structure and behavior of the neurones comprising the connection-systems of animals (or of the neurones’ predecessors in this function) progresses far enough to inform us of just what happens when a connection is made stronger or weaker and of just what effects satisfying and annoying states of affairs exert upon the connection-system (and in particular upon the connections most recently in activity) the ability to learn will show as true an evolution as the ability to sneeze, oppose the thumb, or clasp an object touched by the hand.