haracters, and
therefore advantages in common, so the species, whose place the new or
more favoured ones are seizing, from partaking of a common inferiority
(whether in any particular point of structure, or of general powers of
mind, of means of distribution, of capacity for variation, &c., &c.),
will be apt to be allied. Consequently species of the same genus will
slowly, one after the other, _tend_ to become rarer and rarer in
numbers, and finally extinct; and as each last species of several allied
genera fails, even the family will become extinct. There may of course
be occasional exceptions to the entire destruction of any genus or
family. From what has gone before, we have seen that the slow and
successive formation of several new species from the same stock will
make a new genus, and the slow and successive formation of several other
new species from another stock will make another genus; and if these two
stocks were allied, such genera will make a new family. Now, as far as
our knowledge serves, it is in this slow and gradual manner that groups
of species appear on, and disappear from, the face of the earth.

The manner in which, according to our theory, the arrangement of species
in groups is due to partial extinction, will perhaps be rendered clearer
in the following way. Let us suppose in any one great class, for
instance in the Mammalia, that every species and every variety, during
each successive age, had sent down one unaltered descendant (either
fossil or living) to the present time; we should then have had one
enormous series, including by small gradations every known mammiferous
form; and consequently the existence of groups{450}, or chasms in the
series, which in some parts are in greater width, and in some of less,
is solely due to former species, and whole groups of species, not having
thus sent down descendants to the present time.

{450} The author probably intended to write "groups separated by
chasms."

With respect to the "analogical" or "adaptive" resemblances between
organic beings which are not really related{451}, I will only add, that
probably the isolation of different groups of species is an important
element in the production of such characters: thus we can easily see, in
a large increasing island, or even a continent like Australia, stocked
with only certain orders of the main classes, that the conditions would
be highly favourable for species from these orders to become adap