20 
POPULAR SCIENCE REVIEW. 
seat of thought, intelligence, sensation, emotion, will. He 
who owns these mighty implements in larger measure than his 
fellows has no doubt a great advantage over them in the 
struggle for existence, if he uses them. But they are no good 
to him in this respect while they lie latent or unused. A man 
does not become a match for a wild beast because he has a 
spear laid up in his armoury at home. The spear must be in 
his hand, and driven by strong muscles into the heart of his 
foe, to be of any use to him. So it is with the mental facul- 
ties. Just so much as a man uses of them would become the 
object of natural selection, and no more. All the surplusage 
goes for nothing in the battle of life. The largest gorilla 
brain that has yet been measured contains 34^- cubic inches. 
Probably mental power depends on some other conditions 
besides the mere size of the brain, and therefore we should not 
be justified in saying that a creature with 35 inches of brain 
would certainly beat this gorilla. But we know that size is a 
principal factor in the problem, and we may therefore say very 
confidently that 40 inches of brain would answer this purpose. 
How, then, does it happen that the lowest savage has more 
than 70? Natural selection might secure him the 40, because 
apes with less brain would be crushed out to make room for 
him ; but how would he get or keep the additional 30 ? If an 
individual chanced to be born, a mere monstrosity, with this 
huge addition to the normal quantity of his kind, what likeli- 
hood would there be of its being perpetuated ? He would be 
simply in the condition of the moth with its proboscis an inch 
longer than was required for any useful purpose, and the sure 
result — if natural selection were the only power that acted 
upon it — would be the rapid reversion of his descendants to the 
ordinary type. 
But, it may be asked, is all this brain so much surplusage in 
the savage ? Are we justified in assuming that the greater 
portion of it lies dormant ? Are we sure that he does not use 
it all, and that, in this use of it, there does not lie the secret of 
his superiority over the brutes around him, and the germ of 
that dominion over the whole creation which seems to be the 
goal to which he is continually tending ? The only answer to 
this can be found in the comparison of the savage as regards 
the action of mind, on the one hand, with the highest of the 
brutes beneath, and, on the other, with the civilised man above 
him. If the difference in the amount of brain corresponds in 
these three gradations with the difference in mental develop- 
ment, the inference would be that the whole brain was used in 
each case. If this correspondence does not exist, it will follow 
that the brain is unused in any case in the degree in which 
the mental development in that case falls short of its required 
