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 G. j. ROMANES'S MESIAL EVOLUTION ix AXIMALS. 293 ing" (p. 319). Not only is this attempt made to explain conscious- ness from the objective side, but an attempt is also made to show how mental states may become factors of physical changes. " Possibly, however and as a mere matter of speculation, the possi- bility is worth stating in whatever way the inconceivable connexion between Body and Mind came to be established, the primary cause of its establishment, or of the dawn of subjectivity, may have been this very need of inducing organisms to avoid the deleterious, and to seek the bene- ficial ; the raison <fetre of Consciousness may have been that of supplying the condition to the feeling of Pleasure and Pain. Be this as it may, how- ever, it seems certain, as a matter of observable fact, that the association of Pleasure and Pain with organic states and processes which are respectively beneficial and deleterious to the organism, is the most important function of Consciousness in the scheme of Evolution" (pp. 110-111). It is difficult to infer anything from this passage except that the objective factors of the process of evolution have the power of introducing subjective factors among themselves, which in turn act upon the objective factors and change the course of events. And in other places purely physical processes are spoken of as "the raw material of consciousness". Perhaps the incon- sistency in all this is only verbal ; and it must be admitted that in many passages the expressions as to the relation of Mind and Body are not open to objection. But the opposition between subject and object might have been stated with more clearness and maintained with more consistency. The selection of "the criterion of mind" is the starting-point of the investigation. The evidence of Choice having been taken as a criterion by which "the upper limit of non-mental action " may be determined, it is decided that "the physiological aspect of choice " is " the power of discriminating between stimuli, irrespective of their relative mechanical intensities". This power and " the complementary power of adaptive response " are "the root-principles of mind". There is a correlation such as might be expected a priori "between muscular and mental evo- lution or, more generally, between power of discrimination and variety of adaptive movements". " Thus, if we again take mental operations as indices whereby to studv the more refined working of nervous centres, as we take muscular move- ments to be so many indices, ' writ large,' of the less refined working of such centres, we again tind forced upon us the truth that the method of nervous evolution has everywhere been uniform ; it has everywhere con- sisted in a progressive development of the power of discriminating between stimuli, combined with the complementary power of adaptive respo?. (p. 62 . The development of mind is figured in the diagram placed at the beginning of the book by a tree-like structure having its stem assigned to Will, and a system of branches on each side to Emotion and Intellect respectively. ' Volition ' is represented as continuous with ' Reflex Action,' which is itself continuous with