We cannot omit doing this; because the psychical changes which accompany many of the physical changes in the organism are biological factors in two ways. Those feelings, classed as sensations, which, directly initiated in the bodily framework, go along with certain states of the vital organs and more conspicuously with certain states of the external organs, now serve mainly as guides to the performance of functions, but partly as stimuli, and now serve mainly as stimuli, but in a smaller degree as guides. Visual sensations which, as co-ordinated, enable us to direct our movements, also, if vivid, raise the rate of respiration; while sensations of cold and heat, greatly depressing or raising the vital actions, serve also for purposes of discrimination. So, too, the feelings classed as emotions, which are not localizable in the bodily framework, act in more general ways, alike as guides and stimuli--having influences over the performance of functions more potent even than have most sensations. Fear, at the same time that it urges flight and evolves the forces spent in it, also affects the heart and the alimentary canal; while joy, prompting persistence in the actions bringing it, simultaneously exalts the visceral processes. Hence, in treating of conduct under its biological aspect, we are compelled to consider that interaction of feelings and functions which is essential to animal life in all its more developed forms. § 33. In the _Principles of Psychology_, § 124, it was shown that necessarily, throughout the animate world at large, "pains are the correlatives of actions injurious to the organism, while pleasures are the correlatives of actions conducive to its welfare;" since "it is an inevitable deduction from the hypothesis of Evolution, that races of sentient creatures could have come into existence under no other conditions." The argument was as follows: