In "The Descent of Man," published twelve years later than "The Origin of Species," and "The Variation of Plants and Animals under Domestication," which appeared yet three years later, Darwin's views on instinct and habit are still further elaborated, and a definition of the relation of these to reason, pleasure, pain, and the moral sense, attempted. In Vol. I. of the former work, Darwin devotes two chapters to these subjects. Instinct he calls, pages 116-122, "inherited habit"; and on page 168 he says: "But as love, sympathy, and self-command became strengthened by habit, and as the power of reasoning becomes clearer, so that man can value justly the judgments of his fellows, he will feel himself impelled, apart from any transitory pleasure or pain, to certain lines of conduct." Here, I take it, the word "habit" cannot be interpreted as referring to one generation of men, but to the race as a whole, a general continuity being thus ascribed to the inheritance of mental characteristics, and the important concept of progress as adaptation acquired. In contrasting reason with instinct, Darwin thinks that instinct and intelligence do not, as Cuvier maintained, stand in inverse ratio to each other, but that a high degree of intelligence is compatible with complex instincts--as in the case of the beaver; "yet it is not improbable that there is a certain amount of interference between the development of free intelligence and of instinct,--which latter implies some inherited modification of the brain. Little is known about the functions of the brain, but we can perceive that, as the intellectual powers become highly developed, the various parts of the brain must be connected by very intricate channels of the freest intercommunication; and as a consequence, each separate part would perhaps tend to be less well fitted to answer to particular sensations or associations in a definite and inherited--that is, instinctive--manner. There seems even to exist some relation between a low degree of intelligence and a strong tendency to the formation of fixed, though not inherited habits; for, as a sagacious physician remarked to me, persons who are slightly imbecile tend to act in everything by routine or habit; and they are rendered much happier if this is encouraged."[10] Darwin thinks instinctive action and action from habit may not be connected with either pleasure or pain, though he would seem to contradict this view in the latter part of the passage just quoted, and again where he says: "Although a habit may be blindly and implicitly followed, independently of any pleasure or pain felt at the moment, yet if it be forcibly and abruptly checked, a vague sense of dissatisfaction is generally experienced."[11]