6 NATURAL SELECTION I 
6. Most of the larger and some small 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 space and 
time 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. 2d, The distribution of 
animals and plants in space. 3d, The same in time, including 
all the phenomena of representative groups, and those which 
Professor Forbes supposed to manifest polarity. 4th, The 
phenomena of rudimentary organs. We will briefly endeavour 
to show its bearing upon each of these. 
The Form of a true system of Classification determined 
by this Law 
Tf the law above enunciated 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 antitype a closely allied species existing 
at the time of its origin. It is evidently possible that two or 
three distinct species may have had a common antitype, and 
that each of these may again have become the antitypes from 
which other closely allied species were created. The effect of 
this would be, that so long as each species has had but one 
new species formed on its model, the line of affinities will be 
simple, and may be represented by placing the several species 
in direct succession in a straight line. But if two or more 
species have been independently formed on the plan of a, 
