(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.
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