one main
phenomenon, viz. Animal Life.

This arrangement of the instances, whence the law is to be collected, in
a series, is that which is always implied in and is a condition of _any_
application of the method, viz. that of Concomitant Variations, which
must be used when conjoined circumstances cannot easily be separated by
experiment. But sometimes (and it is so in Zoology) the law of the
subject of the special enquiry (e.g. Animal Life) has such influence
over the general character of the objects, that all other differences
among them seem mere modifications of it; and then the classification
required for the special purpose becomes the determining principle of
the classification of the same objects for general purposes.

To recognise the identity of phenomena which thus differ only in degree,
we must assume a type-species. This will be that _kind_ which has the
class-properties in their greatest intensity (and, therefore, most
easily studied with all their effects); and we must conceive the other
varieties as instances of degeneracy from that type.

The divisions of the series must be determined by the principles of
_natural_ grouping in general (that is, in effect, by natural affinity);
in subordination, however, to the principle of a natural series; that
is, in the same group must not be placed things which ought to occupy
different points of the general scale.

Zoology affords the only _complete_ example of the true principles of
rational classification, whether as to the formation of groups or of
series. Yet the same principles are applicable to all cases (to art and
business as well as science) where the various parts of a wide subject
have to be brought into mental co-ordination.

BOOK V.

FALLACIES.

CHAPTER I.

FALLACIES IN GENERAL.

The habit of reasoning well is the only complete safeguard against
reasoning ill, that is, against drawing conclusions with insufficient
evidence, a practice which the various contradictory opinions,
particularly about the phenomena relating to Man, show to be even now
common, and that too among the most enlightened. But, to be able to
explain an error is a necessary condition of seeing the truth; for,
'Contrariorum eadem est Scientia.' Consequently, a work on Logic must
classify Fallacies, that is, the varieties of Apparent Evidence; for
they _can_ be classified, though not in respect of their negative
quality of being either not evidence at all, or i