50 
THE PRINCIPLES OE RIOLOGY. 
We have thus arrived at the proposition that excess of 
fertility is itself the cause of man’s further evolution; the 
corollary, says Mr. Spencer, is that man’s further evolution 
thus effected necessitates a decline in his fertility. The 
future progress of civilisation produced by the never ceasing 
pressure of population will be accompanied by an enhanced 
cost of Individuation, especially in nervous structure and 
function, an increase of the great nervous centres in mass, 
complexity, and activity. More emotion is the correlative 
of a larger brain, higher feeling of a more complex brain, 
more feeling and thought of an active brain, so that the 
nervous system must become a heavier tax on the organism. 
Already the brain of the civilised man is nearly thirty per¬ 
cent. larger, and is more complex in its convolutions, than 
that of the savage. Mr. Spencer concludes, therefore, that 
the particular kind of evolution, which man is hereafter to 
undergo, may be expected to cause a decline in his power of 
reproduction. He would not let us assume, however, that 
this greater expenditure in nervous action necessarily implies 
a more mentally-laborious life. The greater emotional and 
intellectual power and activity will gradually become organic, 
spontaneous, and pleasurable, just as the mental effort of an 
accomplished man is trifling compared with that of an 
illiterate one. 
What, then, is to be the limit of this progress ? As long 
as fertility exceeds mortality the population must increase ; as 
long as there is pressure on the means of subsistence further 
mental development and further diminution of fertility must 
result. The change must therefore go on until the rate of 
multiplication exactly equals that of mortality. At first sight 
this would seem to imply that eventually each pair will rarely 
have more than two offspring, but this is not so, as the 
number of premature deaths can never become so small as to 
allow the rate of multiplication to fall so low. It is manifest, 
however, that in the end pressure of population and its 
accompanying evils will disappear, and only normal and 
pleasurable activity will be required from each individual, for 
cessation in the degree of fertility implies cessation in the 
development of the nervous system, which further implies 
that the latter has become equal to the work demanded of it— 
has not more to do than is natural to it. But exercise of the 
faculties within natural bounds constitutes gratification, 
therefore in the end the obtainment of subsistence and dis¬ 
charge of all the parental and social duties will require just 
that kind and that amount of action which are needful to health 
and happiness. We see then, says Mr. Spencer, that the 
