y similar
to its present distribution in space.

(6) Most of the larger and some of the smaller groups extend
through several geological periods.

(7) In each period, however, there are peculiar groups, found
nowhere else, and extending through one or several formations.

(8) Species of one genus, or genera of one family, occurring in
the same geological time are more closely allied than those
separated in time.

(9) As generally in geography no species or genus occurs in two
very distant localities without being also found in intermediate
places, so in geology the life of a species or genus has not been
interrupted. In other words, no group or species has come into
existence twice.

(10) The following law may be deduced from these facts: _Every
species has come into existence coincident both in time and space
with a pre-existing closely allied species_.

This law agrees with, explains and illustrates all the facts
connected with the following branches of the subject: 1st, the
system of natural affinities; 2nd, the distribution of animals and
plants in space; 3rd, the same in time, including all the
phenomena of representative groups, and those which Prof. Forbes
supposed to manifest polarity; 4th, the phenomena of rudimentary
organs. We will briefly endeavour to show its bearing upon each of
these.

If [this] law be true, it follows that the natural series of
affinities will also represent the order in which the several
species came into existence, each one having had for its immediate
antetype a clearly allied species existing at the time of its
origin.... If two or more species have been independently formed
on the plan of a common antetype, then the series of affinities
will be compound, and can only be represented by a forked or
many-branched line.... Sometimes the series of affinities can be
well represented for a space by a direct progression from species
to species or from group to group, but it is generally found
impossible so to continue. There constantly occur two or more
modifications of an organ or modifications of two distinct organs,
leading us on to two distinct series of species, which at length
differ so much from each other as to form distinct genera or
families. These are the parallel series or representative groups
of naturalists, a