When advanced up to any given
point, there is no necessity, on the theory of natural selection, for their
further continued process; though they will, during each successive age,
have to be slightly modified, so as to hold their places in relation to
slight changes in their conditions. The foregoing objections hinge on the
question whether we really know how old the world is, and at what period
the various forms of life first appeared; and this may well be disputed.
The problem whether organisation on the whole has advanced is in many ways
excessively intricate. The geological record, at all times imperfect, does
not extend far enough back to show with unmistakable clearness that within
the known history of the world organisation has largely advanced. Even at
the present day, looking to members of the same class, naturalists are not
unanimous which forms ought to be ranked as highest: thus, some look at
the selaceans or sharks, from their approach in some important points of
structure to reptiles, as the highest fish; others look at the teleosteans
as the highest. The ganoids stand intermediate between the selaceans and
teleosteans; the latter at the present day are largely preponderant in
number; but formerly selaceans and ganoids alone existed; and in this case,
according to the standard of highness chosen, so will it be said that
fishes have advanced or retrograded in organisation.
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