The Project Gutenberg ebook of Darwinism (1889), by Alfred Russel Wallace

kangaroo the size of a rhinoceros or small elephant; and a quite

different animal, the Nototherium, nearly as large. The carnivorous

Thylacinus of Tasmania is also found fossil; and a huge phalanger,

Thylacoleo, the size of a lion, believed by Professor Owen and by

Professor Oscar Schmidt to have been equally carnivorous and

destructive.[189] Besides these, there are many other species more

resembling the living forms both in size and structure, of which they

may be, in some cases, the direct ancestors. Two species of extinct

Echidna, belonging to the very low Monotremata, have also been found in

New South Wales.
Next to Australia, South America possesses the most remarkable

assemblage of peculiar mammals, in its numerous Edentata--the sloths,

ant-eaters, and armadillos; its rodents, such as the cavies and

chinchillas; its marsupial opossums, and its quadrumana of the family

Cebidae. Remains of extinct species of all these have been found in the

caves of Brazil, of Post-Pliocene age; while in the earlier Pliocene

deposits of the pampas many distinct genera of these groups have been

found, some of gigantic size and extraordinary form. There are

armadillos of many types, some being as large as elephants; gigantic

sloths of the genera Megatherium, Megalonyx, Mylodon, Lestodon, and many

others; rodents belonging to the American families Cavidae and

Chinchillidae; and ungulates allied to the llama; besides many other

extinct forms of intermediate types or of uncertain affinities.[190] The

extinct Moas of New Zealand--huge wingless birds allied to the living

Apteryx--illustrate the same general law.
The examples now quoted, besides illustrating and enforcing the general

fact of evolution, throw some light on the usual character of the

modification and progression of animal forms. In the cases where the

geological record is tolerably complete, we find a continuous

development of some kind--either in complexity of ornamentation, as in

the fossil Paludinas of the Hungarian lake-basins; in size and in the

specialisation of the feet and teeth, as in the American fossil horses;

or in the increased development of the branching horns, as in the true

deer. In each of these cases specialisation and adaptation to the

conditions of the environment appear to have reached their limits, and

any change of these conditions, especially if it be at all rapid or

accompanied by the competition of less developed but more adaptable

forms, is liable to cause the extinction of the most highly developed

groups. Such we know was the case with the horse tribe in America, which

totally disappeared in that continent at an epoch so recent that we

cannot be sure that the disappearance was not witnessed, perhaps caused,

by man; while even in the Eastern hemisphere it is the smaller

species--the asses and the zebras--that have persisted, while the larger

and more highly developed true horses have almost, if not quite,

disappeared in a state of nature. So we find, both in Australia and

South America, that in a quite recent period many of the largest and

most specialised forms have become extinct, while only the smaller types

have survived to our day; and a similar fact is to be observed in many

of the earlier geological epochs, a group progressing and reaching a

maximum of size or complexity and then dying out, or leaving at most but

few and pigmy representatives.

_Cause of Extinction of Large Animals._
Now there are several reasons for the repeated extinction of large

rather than of small animals. In the first place, animals of great bulk

require a proportionate supply of food, and any adverse change of

conditions would affect them more seriously than it would smaller

animals. In the next place, the extreme specialisation of many of these

large animals would render it less easy for them to be modified in any

new direction suited to changed conditions. Still more important,

perhaps, is the fact that very large animals always increase slowly as

compared with small ones--the elephant producing a single young one

every three years, while a rabbit may have a litter of seven or eight

young two or three times a year. Now the probability of favourable

variations will be in direct proportion to the population of the

species, and as the smaller animals are not only many hundred times more

numerous than the largest, but also increase perhaps a hundred times as

rapidly, they are able to become quickly modified by variation and

natural selection in harmony with changed conditions, while the large

and bulky species, being unable to vary quickly enough, are obliged to

succumb in the struggle for existence. As Professor Marsh well observes:

"In every vigorous primitive type which was destined to survive many

geological changes, there seems to have been a tendency to throw off

lateral branches, which became highly specialised and soon died out,

because they were unable to adapt themselves to new conditions." And he

goes on to show how the whole narrow path of the persistent Suilline

type, throughout the entire series of the American tertiaries, is

strewed with the remains of such ambitious offshoots, many of them

attaining the size of a rhinoceros; "while the typical pig, with an

obstinacy never lost, has held on in spite of catastrophes and

evolution, and still lives in America to-day."

_Indications of General Progression in Plants and Animals._
One of the most powerful arguments formerly adduced against evolution

was, that geology afforded no evidence of the gradual development of

organic forms, but that whole tribes and classes appeared suddenly at

definite epochs, and often in great variety and exhibiting a very

perfect organisation. The mammalia, for example, were long thought to

have first appeared in Tertiary times, where they are represented in

some of the earlier deposits by all the great divisions of the class

fully developed--carnivora, rodents, insectivora, marsupials, and even

the perissodactyle and artiodactyle divisions of the ungulata--as

clearly defined as at the present day. The discovery in 1818 of a single

lower jaw in the Stonesfield Slate of Oxfordshire hardly threw doubt on

the generalisation, since either its mammalian character was denied, or

the geological position of the strata, in which it was found, was held

to have been erroneously determined. But since then, at intervals of

many years, other remains of mammalia have been discovered in the

Secondary strata, ranging from the Upper Oolite to the Upper Trias both

in Europe and the United States, and one even (Tritylodon) in the Trias

of South Africa. All these are either marsupials, or of some still lower

type of mammalia; but they consist of many distinct forms classed in

about twenty genera. Nevertheless, a great gap still exists between

these mammals and those of the Tertiary strata, since no mammal of any

kind has been found in any part of the Cretaceous formation, although in

several of its subdivisions abundance of land plants, freshwater shells,

and air-breathing reptiles have been discovered. So with fishes. In the

last century none had been obtained lower than the Carboniferous

formation; thirty years later they were found to be very abundant in the

Devonian rocks, and later still they were discovered in the Upper Ludlow

and Lower Ludlow beds of the Silurian formation.

fact, only what we ought to expect from the known imperfection of the

geological record. The conditions favourable to the fossilisation of any

group of animals occur comparatively rarely, and only in very limited

areas; while the conditions essential for their permanent preservation

in the rocks, amid all the destruction caused by denudation or

metamorphism, are still more exceptional. And when they are thus

preserved to our day, the particular part of the rocks in which they lie

hidden may not be on the surface but buried down deep under other

strata, and may thus, except in the case of mineral-bearing deposits, be

altogether out of our reach. Then, again, how large a proportion of the

earth consists of wild and uncivilised regions in which no exploration

of the rocks has been yet made, so that whether we shall find the

fossilised remains of any particular group of animals which lived during

a limited period of the earth's history, and in a limited area, depends

upon at least a fivefold combination of chances. Now, if we take each of

these chances separately as only ten to one against us (and some are

certainly more than this), then the actual chance against our finding

the fossil remains, say of any one order of mammalia, or of land plants,

at any particular geological horizon, will be about a hundred thousand

to one.
It may be said, if the chances are so great, how is it that we find such

immense numbers of fossil species exceeding in number, in some groups,

all those that are now living? But this is exactly what we should

expect, because the number of species of organisms that have ever lived

upon the earth, since the earliest geological times, will probably be

many hundred times greater than those now existing of which we have any

knowledge; and hence the enormous gaps and chasms in the geological

record of extinct forms is not to be wondered at. Yet, notwithstanding

these chasms in our knowledge, if evolution is true, there ought to have

been, on the whole, progression in all the chief types of life. The

higher and more specialised forms should have come into existence later

than the lower and more generalised forms; and however fragmentary the

portions we possess of the whole tree of life upon the earth, they ought

to show us broadly that such a progressive evolution has taken place. We

have seen that in some special groups, already referred to, such a

progression is clearly visible, and we will now cast a hasty glance over

the entire series of fossil forms, in order to see if a similar

progression is manifested by them as a whole.

_The Progressive Development of Plants._
Ever since fossil plants have been collected and studied, the broad fact

has been apparent that the early plants--those of the Coal

formation--were mainly cryptogamous, while in the Tertiary deposits the

higher flowering plants prevailed. In the intermediate secondary epoch

the gymnosperms--cycads and coniferae--formed a prominent part of the

vegetation, and as these have usually been held to be a kind of

transition form between the flowerless and flowering plants, the

geological succession has always, broadly speaking, been in accordance

with the theory of evolution. Beyond this, however, the facts were very

puzzling. The highest cryptogams--ferns, lycopods, and

equisetaceae--appeared suddenly, and in immense profusion in the Coal

formation, at which period they attained a development they have never

since surpassed or even equalled; while the highest plants--the

dicotyledonous and monocotyledonous angiosperms--which now form the bulk

of the vegetation of the world, and exhibit the most wonderful

modifications of form and structure, were almost unknown till the

Tertiary period, when they suddenly appeared in full development, and,

for the most part, under the same generic forms as now exist.

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.

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.

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]

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.

Distribution of Plants.]

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