of the human body; but we must stop trying to apply the sex-ways of
birds, spiders or even cows (which are at least mammals) to human
society, which is not made up of any of these.

It is possible to be quite sure that some facts carefully observed about
mammals in a biological laboratory apply to similar structures in man,
also a mammal. Because of this relationship, the data from medicine and
surgery are priceless. Thus we are enabled to check up our systematic
experimental knowledge of animals by an ascertained fact here and there
in the human material, and to get a fairly exact idea of how great the
correspondence actually is. Gaps thus filled in are narrow enough, and
our certainty of the ground on either side sufficiently great, to give
a good deal of justifiable assurance.

If we use our general biological evidence in this way, merely to help in
clearing up points about _human_ biology, we need not be entirely
limited to mammals. Some sex phenomena are quite general, and may be
drawn from the sexual species most convenient to study and control in
experiments. When we get away from mammalian forms, however, we must be
very sure that the cases used for illustrations are of general
application, are similar in respect to the points compared, or that any
vital differences are understood and conscientiously pointed out.

Too much stress cannot be laid upon the point that such animal data,
carefully checked up with the human material, cannot safely be used for
any other purpose than to discover what the facts are about the human
body. When the discussion of human social institutions is taken up in
Part II, the obvious assumption will always be that these rest upon
human biology, and that we must not let our minds wander into vague
analogies concerning birds, spiders or crustacea.