Darwinism (1889) by Alfred Russel Wallace

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During the latter half of the present century, however, great additions
have been made to our knowledge of fossil plants; and although there
are still indications of vast gaps in our knowledge, due, no doubt, to
the very exceptional conditions required for the preservation of plant
remains, we now possess evidence of a more continuous development of the
various types of vegetation. According to Mr. Lester F. Ward, between
8000 and 9000 species of fossil plants have been described or indicated;
and, owing to the careful study of the nervation of leaves, a large
number of these are referable to their proper orders or genera, and
therefore give us some notion--which, though very imperfect, is probably
accurate in its main outlines--of the progressive development of
vegetation on the earth.[191] The following is a summary of the facts as
given by Mr. Ward:--

The lowest forms of vegetable life--the cellular plants--have been found
in Lower Silurian deposits in the form of three species of marine algae;
and in the whole Silurian formation fifty species have been recognised.
We cannot for a moment suppose, however, that this indicates the first
appearance of vegetable life upon the earth, for in these same Lower
Silurian beds the more highly organised vascular cryptogams appear in
the form of rhizocarps--plants allied to Marsilea and Azolla,--and a
very little higher, ferns, lycopods, and even conifers appear. We have
indications, however, of a still more ancient vegetation, in the
carbonaceous shales and thick beds of graphite far down in the Middle
Laurentian, since there is no other known agency than the vegetable cell
by means of which carbon can be extracted from the atmosphere and fixed
in the solid state. These great beds of graphite, therefore, imply the
existence of abundance of vegetable life at the very commencement of the
era of which we have any geological record.[192]

Ferns, as already stated, begin in the Middle Silurian formation with
the Eopteris Morrieri. In the Devonian, we have 79 species, in the
Carboniferous 627, and in the Permian 186 species; after which fossil
ferns diminish greatly, though they are found in every formation; and
the fact that fully 3000 living species are known, while the richest
portion of the Tertiary in fossil plants--the Miocene--- has only
produced 87 species, will serve to indicate the extreme imperfection of
the geological record.

The Equisetaceae (horsetails) which also first appear in the Silurian and
reach their maximum development in the Coal formation, are, in all
succeeding formations, far less numerous than ferns, and only thirty
living species are known. Lycopodiaceae, though still more abundant in
the Coal formation, are very rarely found in any succeeding deposit,
though the living species are tolerably numerous, about 500 having been
described. As we cannot suppose them to have really diminished and then
increased again in this extraordinary manner, we have another indication
of the exceptional nature of plant preservation and the extreme and
erratic character of the imperfection of the record.

Passing now to the next higher division of plants--the gymnosperms--we
find Coniferae appearing in the Upper Silurian, becoming tolerably
abundant in the Devonian, and reaching a maximum in the Carboniferous,
from which formation more than 300 species are known, equal to the
number recorded as now living. They occur in all succeeding formations,
being abundant in the Oolite, and excessively so in the Miocene, from
which 250 species have been described. The allied family of gymnosperms,
the Cycadaceae, first appear in the Carboniferous era, but very
scantily; are most abundant in the Oolite, from which formation 116
species are known, and then steadily diminish to the Tertiary, although
there are seventy-five living species.

We now come to the true flowering plants, and we first meet with
monocotyledons in the Carboniferous and Permian formations. The
character of these fossils was long disputed, but is now believed to be
well established; and the sub-class continues to be present in small
numbers in all succeeding deposits, becoming rather plentiful in the
Upper Cretaceous, and very abundant in the Eocene and Miocene. In the
latter formation 272 species have been discovered; but the 116 species
in the Eocene form a larger proportion of the total vegetation of the
period.

True dicotyledons appear very much later, in the Cretaceous period, and
only in its upper division, if we except a single species from the
Urgonian beds of Greenland. The remarkable thing is that we here find
the sub-class fully developed and in great luxuriance of types, all the
three divisions--Apetalae, Polypetalae, and Gamopetalae--being
represented, with a total of no less than 770 species. Among them are
such familiar forms as the poplar, the birch, the beech, the sycamore,
and the oak; as well as the fig, the true laurel, the sassafras, the
persimmon, the maple, the walnut, the magnolia, and even the apple and
the plum tribes. Passing on to the Tertiary period the numbers increase,
till they reach their maximum in the Miocene, where more than 2000
species of dicotyledons have been discovered. Among these the
proportionate number of the higher gamopetalae has slightly increased,
but is considerably less than at the present day.

_Possible Cause of sudden late Appearance of Exogens._

The sudden appearance of fully developed exogenous flowering plants in
the Cretaceous period is very analogous to the equally sudden appearance
of all the chief types of placental mammalia in the Eocene; and in both
cases we must feel sure that this suddenness is only apparent, due to
unknown conditions which have prevented their preservation (or their
discovery) in earlier formations. The case of the dicotyledonous plants
is in some respects the most extraordinary, because in the earlier
Mesozoic formations we appear to have a fair representation of the flora
of the period, including such varied forms as ferns, equisetums, cycads,
conifers, and monocotyledons. The only hint at an explanation of this
anomaly has been given by Mr. Ball, who supposes that all these groups
inhabited the lowlands, where there was not only excessive heat and
moisture, but also a superabundance of carbonic acid in the
atmosphere--conditions under which these groups had been developed, but
which were prejudicial to the dicotyledons. These latter are supposed to
have originated on the high table-lands and mountain ranges, in a rarer
and drier atmosphere in which the quantity of carbonic acid gas was much
less; and any deposits formed in lake beds at high altitudes and at such
a remote epoch have been destroyed by denudation, and hence we have no
record of their existence.[193]

During a few weeks spent recently in the Rocky Mountains, I was struck
by the great scarcity of monocotyledons and ferns in comparison with
dicotyledons--a scarcity due apparently to the dryness and rarity of the
atmosphere favouring the higher groups. If we compare Coulter's _Rocky
Mountain Botany_ with Gray's _Botany of the Northern (East) United
States_, we have two areas which differ chiefly in the points of
altitude and atmospheric moisture. Unfortunately, in neither of these
works are the species consecutively numbered; but by taking the pages
occupied by the two divisions of dicotyledons on the one hand,
monocotyledons and ferns on the other, we can obtain a good
approximation. In this way we find that in the flora of the
North-Eastern States the monocotyledons and ferns are to the
dicotyledons in the proportion of 45 to 100; in the Rocky Mountains they
are in the proportion of only 34 to 100; while if we take an exclusively
Alpine flora, as given by Mr. Ball, there are not one-fifth as many
monocotyledons as dicotyledons. These facts show that even at the
present day elevated plateaux and mountains are more favourable to
dicotyledons than to monocotyledons, and we may, therefore, well suppose
that the former originated within such elevated areas, and were for long
ages confined to them. It is interesting to note that their richest
early remains have been found in the central regions of the North
American continent, where they now, proportionally, most abound, and
where the conditions of altitude and a dry atmosphere were probably
present at a very early period.

[Illustration: FIG. 34.--Diagram illustrating the Geological
Distribution of Plants.]

The diagram (Fig. 34), slightly modified from one given by Mr. Ward,
will illustrate our present knowledge of the development of the
vegetable kingdom in geological time. The shaded vertical bands exhibit
the proportions of the fossil forms actually discovered, while the
outline extensions are intended to show what we may fairly presume to
have been the approximate periods of origin, and progressive increase of
the number of species, of the chief divisions of the vegetable kingdom.
These seem to accord fairly well with their respective grades of
development, and thus offer no obstacle to the acceptance of the belief
in their progressive evolution.

_Geological Distribution of Insects._

The marvellous development of insects into such an endless variety of
forms, their extreme specialisation, and their adaptation to almost
every possible condition of life, would almost necessarily imply an
extreme antiquity. Owing, however, to their small size, their lightness,
and their usually aerial habits, no class of animals has been so
scantily preserved in the rocks; and it is only recently that the whole
of the scattered material relating to fossil insects and their allies
have been brought together by Mr. Samuel H. Scudder of Boston, and we
have thus learned their bearing on the theory of evolution.[194]

The most striking fact which presents itself on a glance at the
distribution of fossil insects, is the completeness of the
representation of all the chief types far back in the Secondary period,
at which time many of the existing families appear to have been
perfectly differentiated. Thus in the Lias we find dragonflies
"apparently as highly specialised as to-day, no less than four tribes
being present." Of beetles we have undoubted Curculionidae from the Lias
and Trias; Chrysomelidae in the same deposits; Cerambycidae in the
Oolites; Scarabaeidae in the Lias; Buprestidae in the Trias; Elateridae,
Trogositidae, and Nitidulidae in the Lias; Staphylinidae in the English
Purbecks; while Hydrophilidae, Gyrinidae, and Carabidae occur in the
Lias. All these forms are well represented, but there are many other
families doubtfully identified in equally ancient rocks. Diptera of the
families Empidae, Asilidae, and Tipulidae have been found as far back as
the Lias. Of Lepidoptera, Sphingidae and Tineidae have been found in
the Oolite; while ants, representing the highly specialised Hymenoptera,
have occurred in the Purbeck and Lias.

This remarkable identity of the families of very ancient with those of
existing insects is quite comparable with the apparently sudden
appearance of existing genera of trees in the Cretaceous epoch. In both
cases we feel certain that we must go very much farther back in order to
find the ancestral forms from which they were developed, and that at any
moment some fresh discovery may revolutionise our ideas as to the
antiquity of certain groups. Such a discovery was made while Mr.
Scudder's work was passing through the press. Up to that date all the
existing orders of true insects appeared to have originated in the
Trias, the alleged moth and beetle of the Coal formation having been
incorrectly determined. But now, undoubted remains of beetles have been
found in the Coal measures of Silesia, thus supporting the
interpretation of the borings in carboniferous trees as having been made
by insects of this order, and carrying back this highly specialised form
of insect life well into Palaeozoic times. Such a discovery renders all
speculation as to the origin of true insects premature, because we may
feel sure that all the other orders of insects, except perhaps
hymenoptera and lepidoptera, were contemporaneous with the highly
specialised beetles.

The less highly organised terrestrial arthropoda--the Arachnida and
Myriapoda--are, as might be expected, much more ancient. A fossil spider
has been found in the Carboniferous, and scorpions in the Upper Silurian
rocks of Scotland, Sweden, and the United States. Myriapoda have been
found abundantly in the Carboniferous and Devonian formations; but all
are of extinct orders, exhibiting a more generalised structure than
living forms.

Much more extraordinary, however, is the presence in the Palaeozoic
formations of ancestral forms of true insects, termed by Mr. Scudder
Palaeodictyoptera. They consist of generalised cockroaches and
walking-stick insects (Orthopteroidea); ancient mayflies and allied
forms, of which there are six families and more than thirty genera
(Neuropteroidea); three genera of Hemipteroidea resembling various
Homoptera and Hemiptera, mostly from the Carboniferous formation, a few
from the Devonian, and one ancestral cockroach (Palaeoblattina) from
the Middle Silurian sandstone of France. If this occurrence of a true
hexapod insect from the Middle Silurian be really established, taken in
connection with the well-defined Coleoptera from the Carboniferous, the
origin of the entire group of terrestrial arthropoda is necessarily
thrown back into the Cambrian epoch, if not earlier. And this cannot be
considered improbable in view of the highly differentiated land
plants--ferns, equisetums, and lycopods--in the Middle or Lower
Silurian, and even a conifer (Cordaites Robbii) in the Upper Silurian;
while the beds of graphite in the Laurentian were probably formed from
terrestrial vegetation.

On the whole, then, we may affirm that, although the geological record
of the insect life of the earth is exceptionally imperfect, it yet
decidedly supports the evolution hypothesis. The most specialised order,
Lepidoptera, is the most recent, only dating back to the Oolite; the
Hymenoptera, Diptera, and Homoptera go as far as the Lias; while the
Orthoptera and Neuroptera extend to the Trias. The recent discovery of
Coleoptera in the Carboniferous shows, however, that the preceding
limits are not absolute, and will probably soon be overpassed. Only the
more generalised ancestral forms of winged insects have been traced back
to Silurian time, and along with them the less highly organised
scorpions; facts which serve to show us the extreme imperfection of our
knowledge, and indicate possibilities of a world of terrestrial life in
the remotest Palaeozoic times.

_Geological Succession of Vertebrata._

The lowest forms of vertebrates are the fishes, and these appear first
in the geological record in the Upper Silurian formation. The most
ancient known fish is a Pteraspis, one of the bucklered ganoids or
plated fishes--by no means a very low type--allied to the sturgeon
(Accipenser) and alligator-gar (Lepidosteus), but, as a group, now
nearly extinct. Almost equally ancient are the sharks, which under
various forms still abound in our seas. We cannot suppose these to be
nearly the earliest fishes, especially as the two lowest orders, now
represented by the Amphioxus or lancelet and the lampreys, have not yet
been found fossil. The ganoids were greatly developed in the Devonian
era, and continued till the Cretaceous, when they gave way to the true
osseous fishes, which had first appeared in the Jurassic period, and
have continued to increase till the present day. This much later
appearance of the higher osseous fishes is quite in accordance with
evolution, although some of the very lowest forms, the lancelet and the
lampreys, together with the archaic ceratodus, have survived to our
time.

The Amphibia, represented by the extinct labyrinthodons, appear first in
the Carboniferous rocks, and these peculiar forms became extinct early
in the Secondary period. The labyrinthodons were, however, highly
specialised, and do not at all indicate the origin of the class, which
may be as ancient as the lower forms of fishes. Hardly any recognisable
remains of our existing groups--the frogs, toads, and salamanders--are
found before the Tertiary period, a fact which indicates the extreme
imperfection of the record as regards this class of animals.

True reptiles have not been found till we reach the Permian where
Prohatteria and Proterosaurus occur, the former closely allied to the
lizard-like Sphenodon of New Zealand, the latter having its nearest
allies in the same group of reptiles--Rhyncocephala, other forms of
which occur in the Trias. In this last-named formation the earliest
crocodiles--Phytosaurus (Belodon) and Stagonolepis occur, as well as the
earliest tortoises--Chelytherium, Proganochelys, and Psephoderma.[195]
Fossil serpents have been first found in the Cretaceous formation, but
the conditions for the preservation of these forms have evidently been
unfavourable, and the record is correspondingly incomplete. The marine
Plesiosauri and Ichthyosauri, the flying Pterodactyles, the terrestrial
Iguanodon of Europe, and the huge Atlantosaurus of Colorado--the largest
land animal that has ever lived upon the earth[196]--all belong to
special developments of the reptilian type which flourished during the
Secondary epoch, and then became extinct.

Birds are among the rarest of fossils, due, no doubt, to their aerial
habits removing them from the ordinary dangers of flood, bog, or ice
which overwhelm mammals and reptiles, and also to their small specific
gravity which keeps them floating on the surface of water till devoured.
Their remains were long confined to Tertiary deposits, where many living
genera and a few extinct forms have been found. The only birds yet known
from the older rocks are the toothed birds (Odontornithes) of the
Cretaceous beds of the United States, belonging to two distinct families
and many genera; a penguin-like form (Enaliornis) from the Upper
Greensand of Cambridge; and the well-known long-tailed Archaeopteryx
from the Upper Oolite of Bavaria. The record is thus imperfect and
fragmentary in the extreme; but it yet shows us, in the few birds
discovered in the older rocks, more primitive and generalised types,
while the Tertiary birds had already become specialised like those
living, and had lost both the teeth and the long vertebral tail, which
indicate reptilian affinities in the earlier Mammalia have been found,
as already stated, as far back as the Trias formation, in Europe in the
United States and in South Africa, all being very small, and belonging
either to the Marsupial order, or to some still lower and more
generalised type, out of which both Marsupials and Insectivora were
developed. Other allied forms have been found in the Lower and Upper
Oolite both of Europe and the United States. But there is then a great
gap in the whole Cretaceous formation, from which no mammal has been
obtained, although both in the Wealden and the Upper Chalk in Europe,
and in the Upper Cretaceous deposits of the United States an abundant
and well-preserved terrestrial flora has been discovered. Why no mammals
have left their remains here it is impossible to say. We can only
suppose that the limited areas in which land plants have been so
abundantly preserved, did not present the conditions which are needed
for the fossilisation and preservation of mammalian remains.

When we come to the Tertiary formation, we find mammals in abundance;
but a wonderful change has taken place. The obscure early types have
disappeared, and we discover in their place a whole series of forms
belonging to existing orders, and even sometimes to existing families.
Thus, in the Eocene we have remains of the opossum family; bats
apparently belonging to living genera; rodents allied to the South
American cavies and to dormice and squirrels; hoofed animals belonging
to the odd-toed and even-toed groups; and ancestral forms of cats,
civets, dogs, with a number of more generalised forms of carnivora.
Besides these there are whales, lemurs, and many strange ancestral forms
of proboscidea.[197]

The great diversity of forms and structures at so remote an epoch would
require for their development an amount of time, which, judging by the
changes that have occurred in other groups, would carry us back far into
the Mesozoic period. In order to understand why we have no record of
these changes in any part of the world, we must fall back upon some such
supposition as we made in the case of the dicotyledonous plants.
Perhaps, indeed, the two cases are really connected, and the upland
regions of the primeval world, which saw the development of our higher
vegetation, may have also afforded the theatre for the gradual
development of the varied mammalian types which surprise us by their
sudden appearance in Tertiary times.

[Illustration: GEOLOGICAL DISTRIBUTION OF MAMMALIA.]

Notwithstanding these irregularities and gaps in the record, the
accompanying table, summarising our actual knowledge of the geological
distribution of the five classes of vertebrata, exhibits a steady
progression from lower to higher types, excepting only the deficiency in
the bird record which is easily explained. The comparative perfection of
type in which each of these classes first appears, renders it certain
that the origin of each and all of them must be sought much farther back
than any records which have yet been discovered. The researches of
palaeontologists and embryologists indicate a reptilian origin for birds
and mammals, while reptiles and amphibia arose, perhaps independently,
from fishes.

_Concluding Remarks._

The brief review we have now taken of the more suggestive facts
presented by the geological succession of organic forms, is sufficient
to show that most, if not all, of the supposed difficulties which it
presents in the way of evolution, are due either to imperfections in the
geological record itself, or to our still very incomplete knowledge of
what is really recorded in the earth's crust. We learn, however, that
just as discovery progresses, gaps are filled up and difficulties
disappear; while, in the case of many individual groups, we have already
obtained all the evidence of progressive development that can reasonably
be expected. We conclude, therefore, that the geological difficulty has
now disappeared; and that this noble science, when properly understood,
affords clear and weighty evidence of evolution.

FOOTNOTES:

[Footnote 183: The reader who desires to understand this subject more
fully, should study chap. x. of the _Origin of Species_, and chap. xiv.
of Sir Charles Lyell's _Principles of Geology_.]

[Footnote 184: On "Stagonolepis Robertsoni and on the Evolution of the
Crocodilia," in _Q.J. of Geological Society_, 1875; and abstract in
_Nature_, vol. xii. p. 38.]

[Footnote 185: From a paper by Messrs. Scott and Osborne, "On the Origin
and Development of the Rhinoceros Group," read before the British
Association in 1883.]

[Footnote 186: American Addresses, pp. 73-76.]

[Footnote 187: Lecture on the Introduction and Succession of Vertebrate
Life in America, _Nature_, vol. xvi. p. 471.]

[Footnote 188: _Nature_, vol. xxv. p. 84.]

[Footnote 189: See _The Mammalia in their Relation to Primeval Times_,
p. 102.]

[Footnote 190: For a brief enumeration and description of these fossils,
see the author's _Geographical Distribution of Animals_, vol. i. p.
146.]

[Footnote 191: Sketch of Palaeobotany in Fifth Annual Report of U.S.
Geological Survey, 1883-84, pp. 363-452, with diagrams. Sir J. William
Dawson, speaking of the value of leaves for the determination of fossil
plants, says: "In my own experience I have often found determinations of
the leaves of trees confirmed by the discovery of their fruits or of the
structure of their stems. Thus, in the rich cretaceous plant-beds of the
Dunvegan series, we have beech-nuts associated in the same bed with
leaves referred to _Fagus_. In the Laramie beds I determined many years
ago nuts of the _Trapa_ or water-chestnut, and subsequently Lesquereux
found in beds in the United States leaves which he referred to the same
genus. Later, I found in collections made on the Red Deer River of
Canada my fruits and Lesquereux's leaves on the same slab. The presence
of trees of the genera _Carya_ and _Juglans_ in the same formation was
inferred from their leaves, and specimens have since been obtained of
silicified wood with the microscopic structure of the modern butternut.
Still we are willing to admit that determinations from leaves alone are
liable to doubt."--_The Geological History of Plants_, p. 196.]

[Footnote 192: Sir J. William Dawson's _Geological History of Plants_,
p. 18.]

[Footnote 193: "On the Origin of the Flora of the European Alps," _Proc.
of Roy. Geog. Society_, vol. i. (1879), pp. 564-588.]

[Footnote 194: Systematic Review of our Present Knowledge of Fossil
Insects, including Myriapods and Arachnids (_Bull. of U.S. Geol.
Survey_, No. 31, Washington, 1886).]