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Dismissing abstract difference of quality, a thing difficult to verify, the efficiency of an organ is known to depend not solely on its size but on its activity: and of this we have an approximate measure in the energy Avith which the blood circulates through it, both the stimulus and the reparative force being mainly dependent on the circulation. It would not be surprising—it is indeed an hypothesis which accords well with the differences actually observed between the mental operations of the tAVO sexes—if men on the average should have the advantage in the size of the brain, and women in activity of cerebral cir­culation. The results which conjecture, founded on analogy, Avould lead us to expect from this difference of organization, would correspond to some of those Avhicli avc most commonly see. In the first place, the mental operations of men might be expected to be slower. They Avould neither be so prompt as Avomen in thinking, nor so quick to feel. Large bodies take more time to get into full action. On the other hand, Avhen once got thoroughly into play, men's brain Avould bear more Avork. It Avould be more per­sistent in the line first taken; it aaouW have more difficulty in changing from one mode of action to another, but, in the one thing it Avas doing, it could go on longer AA'ithout loss of poAver or sense of fatigue. And do avc not find that