There are, however, no sharply defined classes of intelligence; just
as the mentally defective are in many grades, so ordinary men and
women vary from low or average intelligence up to outstanding cases of
genius or capacity.

By the newer methods of mental testing it has been shewn that children
of various classes of the community, as well as men and women of
different races, can be grouped according to their intellectual
capacity, and that no educational facilities will develop that
capacity beyond a certain point.

Professor W. McDougall, F.R.S., in his most useful and interesting
book on _National Welfare and National Decay_, reaches the important
conclusion "that innate capacity for intellectual growth is the
predominant factor in determining the distribution of intelligence in
adults, and that the amount and kind of education is a factor of
subordinate importance." He claims that the evidence is overwhelming
as to the validity of the results obtained by mental testing.

A few examples of experimental work given in Professor McDougall's
book will suffice to show the trend of these results.

Tests of intelligence were carried out on recruits for the American
Army, white and coloured, and they shewed marked superiority of the
white race.

A special test was carried out in Oxford by Mr. H.B. English, who
compared the capacity of boys in a school attended by children of the
intellectual classes with that of boys in a very good primary school,