line of advance which has been
determined by the development of the common body of knowledge.

This general discussion suggests another factor in the working of the
human mind.

When certain vital needs or the force of circumstances compel a man to
embark upon a certain train of reasoning or invention the results to
which his investigations lead depend upon a great many circumstances.
Obviously the range of his knowledge and experience and the general
ideas he has acquired from his fellows will play a large part in shaping
his inferences. It is quite certain that even in the simplest problem of
primitive physics or biology his attention will be directed only to some
of, and not all, the factors involved, and that the limitations of his
knowledge will permit him to form a wholly inadequate conception even of
the few factors that have obtruded themselves upon his attention. But he
may frame a working hypothesis in explanation of the factors he had
appreciated, which may seem perfectly exhaustive and final, as well as
logical and rational to him, but to those who come after him, with a
wider knowledge of the properties of matter and the nature of living
beings, and a wholly different attitude towards such problems, the
primitive man's solution may seem merely a ludicrous travesty.

But once a tentative explanation of one group of phenomena has been made
it is the method of science no less than the common tendency of the
human mind to buttress this theory with analogies and fancied
homologies. In other words the isolated facts are built up into a
generalisation. It is important to remember that in most cases this
mental process begins very early; so that the analogies play a very
obtrusive part in the building up of theories. As a rule a multitude of
such influences play a part consciously or unconsciously in shaping any
belief. Hence the historian is faced with the difficulty, often quite
insuperable, of ascertaining (among scores of factors that definitely
played some part in the building up of a great generalization) the real
foundation upon which the vast edifice has been erected. I refer to
these elementary matters here for two reasons. First, because they are
so often overlooked by ethnologists; and secondly, because in these
pages I shall have to discuss a series of historical events in which a
bewildering number of factors played their part. In sifting out a
certain number of them, I want to make it clear that I d