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

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.

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._

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.

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.

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._

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.

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.]

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.]

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).]
[Footnote 195: For the facts as to the early appearance of the above

named groups of reptiles I am indebted to Mr. E. Lydekker of the

Geological Department of the Natural History Museum.]
[Footnote 196: According to Professor Marsh this creature was 50 or 60

feet long, and when erect, at least 30 feet in height. It fed upon the

foliage of the mountain forests of the Cretaceous epoch, the remains of

which are preserved with it.]

Distribution of Animals_, and Heilprin's _Geographical and Geological

Distribution of Animals_.]

CHAPTER XIV

Fundamental difficulties and objections--Mr. Herbert Spencer's

factors of organic evolution--Disuse and effects of withdrawal

of natural selection--Supposed effects of disuse among wild

animals--Difficulty as to co-adaptation of parts by variation

and selection--Direct action of the environment--The American

school of evolutionists--Origin of the feet of the

ungulates--Supposed action of animal intelligence--Semper on the

direct influence of the environment--Professor Geddes's theory

of variation in plants--Objections to the theory--On the origin

of spines--Variation and selection overpower the effects of use

and disuse--Supposed action of the environment in imitating

variations--Weismann's theory of heredity--The cause of

variation--The non-heredity of acquired characters--The theory

of instinct--Concluding remarks.

Having now set forth and illustrated at some length the most important

of the applications of the development hypothesis in the explanation of

the broader and more generally interesting phenomena presented by the

organic world, we propose to discuss some of the more fundamental

problems and difficulties which have recently been adduced by eminent

naturalists. It is the more necessary to do this, because there is now a

tendency to minimise the action of natural selection in the production

of organic forms, and to set up in its place certain fundamental

principles of variation or laws of growth, which it is urged are the

real originators of the several lines of development, and of most of the

variety of form and structure in the vegetable and animal kingdoms.

These views have, moreover, been seized upon by popular writers to throw

doubt and discredit on the whole theory of evolution, and especially on

Darwin's presentation of that theory, to the bewilderment of the general

public, who are quite unable to decide how far the new views, even if

well established, tend to subvert the Darwinian theory, or whether they

are really more than subsidiary parts of it, and quite powerless without

it to produce any effect whatever.
The writers whose special views we now propose to consider are: (1) Mr.

Herbert Spencer, on modification of structures arising from modification

of functions, as set forth in his _Factors of Organic Evolution_. (2)

Dr. E.D. Cope, who advocates similar views in detail, in his work

entitled _The Origin of the Fittest_, and may be considered the head of

a school of American naturalists who minimise the agency of natural

selection. (3) Dr. Karl Semper, who has especially studied the direct

influence of the environment in the whole animal kingdom, and has set

forth his views in a volume on _The Natural Conditions of Existence as

they Affect Animal Life_. (4) Mr. Patrick Geddes, who urges that

fundamental laws of growth, and the antagonism of vegetative and

reproductive forces, account for much that has been imputed to natural

selection.
We will now endeavour to ascertain what are the more important facts and

arguments adduced by each of the above writers, and how far they offer a

substitute for the action of natural selection; having done which, a

brief account will be given of the views of Dr. Aug. Weismann, whose

theory of heredity will, if established, strike at the very root of the

arguments of the first three of the writers above referred to.

_Mr. Herbert Spencer's Factors of Organic Evolution._
Mr. Spencer, while fully recognising the importance and wide range of

the principle of natural selection, thinks that sufficient weight has

not been given to the effects of use and disuse as a factor in

evolution, or to the direct action of the environment in determining or

modifying organic structures. As examples of the former class of

actions, he adduces the decreased size of the jaws in the civilised

races of mankind, the inheritance of nervous disease produced by

overwork, the great and inherited development of the udders in cows and

goats, and the shortened legs, jaws, and snout in improved races of

pigs--the two latter examples being quoted from Mr. Darwin,--and other

cases of like nature. As examples of the latter, Mr. Darwin is again

quoted as admitting that there are many cases in which the action of

similar conditions appears to have produced corresponding changes in

different species; and we have a very elaborate discussion of the direct

action of the medium in modifying the protoplasm of simple organisms, so

as to bring about the difference between the outer surface and the inner

part that characterises the cells or other units of which they are

formed.
Now, although this essay did little more than bring together facts which

had been already adduced by Mr. Darwin or by Mr. Spencer himself, and

lay stress upon their importance, its publication in a popular review

was immediately seized upon as "an avowed and definite declaration

against some of the leading ideas on which the Mechanical Philosophy

depends," and as being "fatal to the adequacy of the Mechanical

Philosophy as any explanation of organic evolution,"[198]--an expression

of opinion which would be repudiated by every Darwinian. For, even

admitting the interpretation which Mr. Spencer puts on the facts he

adduces, they are all included in the causes which Darwin himself

recognised as having acted in bringing about the infinitude of forms in

the organic world. In the concluding chapter of the _Origin of Species_

he says: "I have now recapitulated the facts and considerations which

have thoroughly convinced me that species have been modified during a

long course of descent. This has been effected chiefly through the

natural selection of numerous successive, slight, favourable variations;

aided in an important manner by the inherited effects of the use and

disuse of parts; and in an unimportant manner--that is, in relation to

adaptive structures whether past or present, by the direct action of

external conditions, and by variations which seem to us, in our

ignorance, to arise spontaneously." This passage, summarising Darwin's

whole inquiry, and explaining his final point of view, shows how very

inaccurate may be the popular notion, as expressed by the Duke of

Argyll, of any supposed additions to the causes of change of species as

recognised by Darwin.

that the supposed inheritance of acquired modifications--that is, of the

effects of use and disuse, or of the direct influence of the

environment--is not a fact; and if so, the very foundation is taken away

from the whole class of objections on which so much stress is now laid.

It therefore becomes important to inquire whether the facts adduced by

Darwin, Spencer, and others, do really necessitate such inheritance, or

whether any other interpretation of them is possible. I believe there is

such an interpretation; and we will first consider the cases of disuse

on which Mr. Spencer lays most stress.

diminishing the size and strength of organs are, the diminished size of