ON THE PERSISTENT TYPES OF
ANIMAL LIFE.
The successive modifications which the views of physical geologists have
undergone since the infancy of their science, with regard to the amount
and the nature of the changes which the crust of the globe has suffered,
have all tended in one direction, viz. towards the establishment of the
belief, that throughout that vast series of ages which was occupied by
the deposition of the stratified rocks, and which may be called
"geological time," (to distinguish it from the "historical time" which
followed, and the "pre-geological time," which preceded it) the
intensity and the character of the physical forces which have been in
operation, have varied within but narrow limits; so that, even in
Silurian or Cambrian times, the aspect of physical nature must have been
much what it is now.
This uniformitarian view of telluric conditions, so far as geological
time is concerned, is, however, perfectly consistent with the notion of
a totally different state of things in antecedent epochs, and the
strongest advocate of such "physical uniformity" during the time of
which we have a record might, with perfect consistency, hold the
so-called "nebular hypothesis," or any other view involving the
conception of a long series of states very different from that which we
now know, and whose succession occupied pre-geological time.
The doctrine of physical uniformity and that of physical progression are
therefore perfectly consistent, if we regard geological time as having
the same relation to pre-geological time as historical time has to it.
The accepted doctrines of palæontology are by no means in harmony with
these tendencies of physical geology. It is generally believed that
there is a vast contrast between the ancient and the modern organic
worlds--it is incessantly assumed that we are acquainted with the
beginning of life, and with the primal manifestation of each of its
typical forms: nor does the fact that the discoveries of every year
oblige the holders of these views to change their ground, appear
sensibly to affect the tenacity of their adhesion.
Without at all denying the considerable positive differences which
really exist between the ancient and the modern forms of life, and
leaving the negative ones to be met by the other lines of argument, an
impartial examination of the facts revealed by palæontology seems to
show that these differences and contrasts have been greatly exaggerated.
Thus, of some two hundred known orders of plants, not one is exclusively
fossil. Among animals, there is not a single totally extinct class; and
of the orders, at the outside not more than seven per cent. are
unrepresented in the existing creation.
Again, certain well marked forms of living beings have existed through
enormous epochs, surviving not only the changes of physical conditions,
but persisting comparatively unaltered, while other forms of life have
appeared and disappeared. Some forms may be termed "persistent types" of
life; and examples of them are abundant enough in both the animal and
the vegetable worlds.
Among plants, for instance, ferns, club mosses, and _Coniferæ_, some of
them apparently generically identical with those now living, are met
with as far back as the carboniferous epoch; the cone of the oolitic
_Araucaria_ is hardly distinguishable from that of existing species; a
species of _Pinus_ has been discovered in the Purbecks, and a walnut
(_Juglans_) in the cretaceous rocks.[61] All these are types of
vegetable structure, abounding at the present day; and surely it is a
most remarkable fact to find them persisting with so little change
through such vast epochs.
Every sub-kingdom of animals yields instances of the same kind. The
_Globigerina_ of the Atlantic soundings is identical with the cretaceous
species of the same genus; and the casts of lower Silurian
_Foraminifera_, recently described by Ehrenberg, assure us of the very
close resemblance between the oldest and the newest forms of many of the
_Protozoa_.
Among the _Coelenterata_, the tabulate corals of the Silurian epoch
are wonderfully like the millepores of our own seas, as every one may
convince himself who compares _Heliolites_ with _Heliopora_.
Turning to the _Mollusca_, the genera _Crania_, _Discina_, _Lingula_,
have persisted from the Silurian epoch to the present day, with so
little change, that very competent malacologists are sometimes puzzled
to distinguish the ancient from the modern species. _Nautili_ have a
like range, and the shell of the liassic _Loligo_ is similar to that of
the "squid" of our own seas. Among the _Annulosa_, the carboniferous
insects are in several cases referable to existing genera, as are the
_Arachnida_, the highest group of which, the scorpions, is represented
in the coal by a genus differing from its living congeners only in the
disposition of its eyes.
The vertebrate sub-kingdom furnishes many examples of the same kind. The
_Ganoidei_ and _Elasmobranchii_ are known to have persisted from at
least the middle of the Palæozoic epoch to our own times, without
exhibiting a greater amount of deviation from the typical characters of
these orders, than may be found within their limits at the present day.
Among the _Reptilia_, the highest group, that of the _Crocodilia_, was
represented at the beginning of the Mesozoic epoch, if not earlier, by
species identical in the essential character of their organization with
those now living, and presenting differences only in such points as the
form of the articular faces of their vertebræ, in the extent to which
the nasal passages are separated from the mouth by bone, and in the
proportions of the limbs. Even such imperfect knowledge as we possess of
the ancient mammalian fauna leads to the belief that certain of its
types, such as that of the _Marsupialia_, have persisted with no greater
change through as vast a lapse of time.
It is difficult to comprehend the meaning of such facts as these, if we
suppose that each species of animal and plant, or each great type of
organization, was formed and placed upon the surface of the globe at
long intervals by a distinct act of creative power; and it is well to
recollect that such an assumption is as unsupported by tradition or
revelation as it is opposed to the general analogy of Nature.
If, on the other hand, we view "Persistent Types," in relation to that
hypothesis which supposes the species of living beings living at any
time to be the result of the gradual modification of pre-existing
species--a hypothesis which though unproven, and sadly damaged by some
of its supporters, is yet the only one to which physiology lends any
countenance--their existence would seem to show, that the amount of
modification which living beings have undergone during geological time
is but very small in relation to the whole series of changes which they
have suffered. In fact, palæontology and physical geology are in perfect
harmony, and coincide in indicating that all we know of the conditions
in our world during geological time, is but the last term of a vast and,
so far as our present knowledge reaches, unrecorded progression.