risen higher in
the scale of civilization than another, it is now generally impossible
to assign the particular causes of the difference; much more, then, must
this be impossible in the case of still more remote conditions which
have led to the divergence of species. The requisite variations may not
have arisen in the one line of descent which did arise in the other; or
if they did arise in both, some counterbalancing disadvantages may have
attended their initial development in the one case which did not obtain
in the other. In short, where so exceedingly complex a play of
conditions are concerned, the only wonder would be if two different
lines of descent _had_ happened to present two independent and yet
perfectly parallel lines of history.

These general considerations would apply equally to the great majority
of other cases where some types have made great advances upon others,
notwithstanding that we can see no reason why the latter should not in
this respect have imitated the former. But there is yet a further
consideration which must be taken into account. The struggle for
existence is always most keen between closely allied species, because,
from the similarity of their forms, habits, needs, &c., they are in
closest competition. Therefore it often happens that the mere fact of
one species having made an advance upon others of itself precludes the
others from making any similar advance: the field, so to speak, has
already been occupied as regards that particular improvement, and where
the struggle for existence is concerned possession is emphatically nine
points of the law. For example, to return to the case of apes becoming
men, the fact of one rational species having been already evolved (even
if the rational faculty were at first but dimly nascent) must make an
enormous change in the conditions as regards the possibility of any
other such species being subsequently evolved--unless, of course, it be
by way of descent from the rational one. Or, as Sir Charles Lyell has
well put it, two rational species can never _coexist_ on the globe,
although the descendants of one rational species may in time become
_transformed_ into another single rational species[44].

[44] _Principles of Geology_, vol. ii. p. 487 (11th ed.).

In view of such considerations, another and exactly opposite objection
has sometimes been urged--viz. that we ought never to find inferior
forms of organization in company with superior, becaus