in one and the same society, habits vary considerably from one man to
another. It is necessary to take account of these differences, otherwise
there is a danger of explaining the actions of artists and men of
science by the beliefs and the habits of their prince or their
tradesmen.

(2) In order to ascertain the causes of an evolution, it is necessary to
study the only beings which can evolve--men. Every evolution has for its
cause a change in the material conditions or in the habits of certain
men. Observation shows us two kinds of change. In the one case, the men
remain the same, but change their manner of acting or thinking, either
voluntarily through imitation, or by compulsion. In the other, the men
who practised the old usage disappear and are replaced by others who do
not practise it; these may be strangers, or they may be the descendants
of the first set of men, but educated in a different manner. This
renewing of the generations seems, in our day, to be the most active
cause of evolution. It is natural to suppose that the same holds good of
the past; evolution has been slower, the more exclusively each
generation has been formed by the imitation of its forerunners.

There is still one more question to ask. Are men all alike, differing
merely in the _conditions_ under which they live (education, resources,
government), and is evolution produced solely by changes in these
_conditions_? Or are there groups of men with _hereditary differences_,
born with tendencies to different activities and with aptitudes leading
to different evolutions, so that evolution may be the product, in part
at least, of the increase, the diminution, and the displacement of these
groups? Taking the extreme cases, the white, black, and yellow races of
mankind, the differences in aptitude are obvious; no black people has
ever developed a civilisation. It is thus probable that smaller
hereditary differences may have had their share in the determination of
events. If so, historical evolution would be partly produced by
physiological and anthropological causes. But history provides us with
no sure means of determining the action of these hereditary differences
between men; it goes no further than the conditions of their existence.
The last question of history remains insoluble by historical methods.

CHAPTER V

EXPOSITION

We have still to study a question whose practical interest is obvious:
What are the forms in which historical