n employed where exact estimates are unobtainable.
Thus Darwin, having found certain modifications of animals in form,
coloration and habits, that were not clearly derivable from their
struggle for existence in relation to other species or to external
conditions, suggested that they were due to Sexual Selection.

The 'vestiges' and 'survivals' so common in Biology and Sociology are
residuary phenomena. It is a general inference from the doctrine of
Natural Selection that every organ of a plant, animal, or society is in
some way useful to it. There occur, however, organs that have at present
no assignable utility, are at least wasteful, and sometimes even
injurious. And the explanation is that formerly they were useful; but
that, their uses having lapsed, they are now retained by the force of
heredity or tradition. Either they are not injurious enough to be
eliminated by natural selection; or they are correlated with other
organs, whose utility outweighs their disutility.

CHAPTER XVII

COMBINATION OF INDUCTION WITH DEDUCTION

Sec. 1. We have now reviewed Mill's five Canons of Inductive Proof. At
bottom, as he observes, there are only two, namely, Agreement and
Difference: since the Double Method, Variations and Residues are only
special forms of the other two. Indeed, in their function of _proof_,
they are all reducible to one, namely, Difference; for the cogency of
the method of Agreement (as distinguished from a simple enumeration of
instances agreeing in the coincidence of a supposed cause and its
effect), depends upon the omission, in one instance after another, of
all other circumstances; which omission is a point of difference.

The Canons are an analysis of the conditions of proving directly (where
possible), by means of observation or experiment, any proposition that
predicates causation. But if we say 'by means of observation or
experiment,' it is not to be understood that these are the only means
and that nothing else is involved; for it has been shown that the Law of
Causation is itself an indispensable foundation of the evidence. In fact
Inductive Logic may be considered as having a purely formal character.
It consists (1) in a statement of the Law of Cause and Effect; (2) in
certain immediate inferences from this Law, expanded into the Canons;
(3) in the syllogistic application of the Canons to special predications
of causation by means of minor premises, showing that certain instances
satisfy