On the hypothesis of monism, the nature of which, so far as it bears on our inquiry, was briefly indicated in the foregoing section, the conscious situation is the psychical or mental expression of that which for the physiologist is what we may term a neural situation. As such it does not enter into the chain of physical causation; nor do physical events as such--that is to say, save as experienced--enter into the chain of mental causation. For mental development they have no independent existence, and are negligible except in so far as they enter as items of experience into the conscious situation. But altogether apart from the way or ways in which we may attempt to explain the fact, most of us believe, with unquestioning confidence, that the growth of practical experience, somehow associated with nervous changes in the brain or sensorium, is of real value in the guidance of behaviour in such manner as to secure biological ends. Conscious experience must therefore, in the animal world, serve its biological purpose, or it will be of no avail. If there be not a pre-established harmony, there must be an evolved harmony; and how such a harmony could be evolved if consciousness be not by some means in vital touch with behaviour, influenced by and in turn influencing it, we cannot conceive. The steam-whistle theory of consciousness leaves the matter, for the evolutionist, in this inconceivable position. We need not, however, flog a dead horse. We need not ask how, on the steam-whistle theory, those states of feeling which we broadly classify as pleasurable could become associated with behaviour conducing to welfare, and those which we group as hurtful with behaviour which is biologically harmful. It is more important, again, to notice that, associated and consonant with the biological end, there arises a psychological end of behaviour--what we may term, with the qualifications before considered,[199] the getting of pleasure and the avoidance of pain. This is the purpose of behaviour as viewed from the psychological aspect. The biological end of animal conation is racial survival; its psychological end is individual satisfaction. And the two ends are, in the main and broadly speaking, consonant--a result which would unquestionably be secured by natural selection, but is on any other naturalistic hypothesis difficult of explanation.