318 COSMIC PHILOSOPHY. [pt. u. 



sultsequent variations in quantity are accompanied by no 

 changes whatever in physical structure." ^ Here again, 

 though the antithesis is a little too absolutely stated, we have 

 set before us a real distinction. Up to a certain point, the 

 brain and the rest of the body are alike alterable by natural 

 selection and such other agencies as may be concerned in the 

 slow modification of organisms. But when the brain has 

 reached a certain point in size and complexity, the rest of the 

 body ceases to change, save in a few slight particulars, and 

 the agencies concerned in forwarding the process of evolution 

 seem to confine themselves to the brain, and especially to the 

 cerebrum, — the result being marked psychical development, 

 unattended by any notable physical alteration. Here we 

 have reached a fact of prime importance. We may grant to 

 the Duke of Argyll that when those eleven additional cubic 

 inches of brain had been acquired, some kind of a Eubicon 

 had been crossed, and a new state of things inaugurated. 

 What was that Eubicon ? 



The answer has been furnished by Mr. Wallace, and must 

 rank as one of the most brilliant contributions ever yet made 

 to the Doctrine of Evolution. Since inferior animals respond 

 chiefly by physical changes to changes in their environment, 

 natural selection deals chiefly with such changes, to the 

 visible modification of their bodily structure. In the case of 

 sheep or bears, for instance, increased cold can only select for 

 preservation the individuals most warmly coated ; or if a race 

 of lions, which has hitherto subsisted upon small and sluggish 

 ruminants until these have been nearly exterminated, is at 

 la&t obliged to attack antelopes and buffaloes, natural selection 

 can only preserve the swiftest and strongest or most ferocious 

 lions. But when an animal has once appeared, endowed with 

 sufficient intelligence to chip a stone tool and hurl a weapon, 

 natural selection will take advantage of variations in this 

 intelligence, to the comparative neglect of purely physical 

 * Duke of Argj'll, Primeval Man, pp. 67 — 64 



