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

these birds are, as a rule, of dull colours, not superior on the average

to our grain-eating finches. Then, again, we have the grand pheasant

family, including the gold and the silver pheasants, the gorgeous

fire-backed and ocellated pheasants, and the resplendent peacock, all

feeding on the ground on grain or seeds or insects, yet adorned with the

most gorgeous colours.
There is, therefore, no adequate basis of facts for this theory to rest

upon, even if there were the slightest reason to believe that not only

birds, but butterflies and beetles, take any delight in colour for its

own sake, apart from the food-supply of which it indicates the presence.

All that has been proved or that appears to be probable is, that they

are able to perceive differences of colour, and to associate each colour

with the particular flowers or fruits which best satisfy their wants.

Colour being in its nature diverse, it has been beneficial for them to

be able to distinguish all its chief varieties, as manifested more

particularly in the vegetable kingdom, and among the different species

of their own group; and the fact that certain species of insects show

some preference for a particular colour may be explained by their having

found flowers of that colour to yield them a more abundant supply of

nectar or of pollen. In those cases in which butterflies frequent

flowers of their own colour, the habit may well have been acquired from

the protection it affords them.

colour for its own sake and the same aesthetic tastes as we ourselves

possess, we may be as far from the truth as were those writers who held

that the bee was a good mathematician, and that the honeycomb was

constructed throughout to satisfy its refined mathematical instincts;

whereas it is now generally admitted to be the result of the simple

principle of economy of material applied to a primitive cylindrical

cell.[162]

led to realise the wonderful complexity of the adaptations which bring

each species into harmonious relation with all those which surround it,

and which thus link together the whole of nature in a network of

relations of marvellous intricacy. Yet all this is but, as it were, the

outward show and garment of nature, behind which lies the inner

structure--the framework, the vessels, the cells, the circulating

fluids, and the digestive and reproductive processes,--and behind these

again those mysterious chemical, electrical, and vital forces which

constitute what we term Life. These forces appear to be fundamentally

the same for all organisms, as is the material of which all are

constructed; and we thus find behind the outer diversities an inner

relationship which binds together the myriad forms of life.
Each species of animal or plant thus forms part of one harmonious whole,

carrying in all the details of its complex structure the record of the

long story of organic development; and it was with a truly inspired

insight that our great philosophical poet apostrophised the humble

weed--

Flower in the crannied wall,

I hold you here, root and all, in my hand,

Little flower--but _if_ I could understand

What you are, root and all, and all in all,

I should know what God and man is.

FOOTNOTES:

_Flowers, Fruits, and Leaves_, or any general botanical work.]

484.]
[Footnote 143: For a complete historical account of this subject with

full references to all the works upon it, see the Introduction to

Hermann Müller's _Fertilisation of Flowers_, translated by D'Arcy W.

Thompson.]
[Footnote 144: For the full detail of his experiments, see _Cross-and

Self-Fertilisation of Plants_, 1876.]

extraordinary and complex arrangements in these plants.]

_British Wild Flowers in Relation to Insects_, and H. Müller's great and

original work, _The Fertilisation of Flowers_.]
[Footnote 147: Müller's _Fertilisation of Flowers_, p. 248.]
[Footnote 148: "Alpenblumen," by D.H. Müller. See _Nature_, vol. xxiii.

p. 333.]
[Footnote 149: This peculiarity of local distribution of colour in

flowers may be compared, as regards its purpose, with the recognition

colours of animals. Just as these latter colours enable the sexes to

recognise each other, and thus avoid sterile unions of distinct species,

so the distinctive form and colour of each species of flower, as

compared with those that usually grow around it, enables the fertilising

insects to avoid carrying the pollen of one flower to the stigma of a

distinct species.]
[Footnote 150: See H. Müller's _Fertilisation of Flowers_, p. 18.]
[Footnote 151: The above examples are taken from Rev. G. Henslow's paper

on "Self-Fertilisation of Plants," in _Trans. Linn. Soc._ Second series,

_Botany_, vol. i. pp. 317-398, with plate. Mr. H.O. Forbes has shown

that the same thing occurs among tropical orchids, in his paper "On the

Contrivances for insuring Self-Fertilisation in some Tropical Orchids,"

_Journ. Linn. Soc._, xxi. p. 538.]

by Mr. Hemsley that many additions have been since made to the list, and

that cleistogamic flowers probably occur in nearly all the natural

orders.]
[Footnote 153: For a full account of cleistogamic flowers, see Darwin's

_Forms of Flowers_, chap. viii.]
[Footnote 154: Henslow's "Self-Fertilisation," _Trans. Linn. Soc._

Second series, _Botany_, vol. i. p. 391.]

Structures_, says: "There is little doubt but that all wind-fertilised

angiosperms are degradations from insect-fertilised flowers....

_Poterium sanguisorba_ is anemophilous; and _Sanguisorba officinalis_

presumably was so formerly, but has reacquired an entomophilous habit;

the whole tribe Poterieae being, in fact, a degraded group which has

descended from Potentilleae. Plantains retain their corolla but in a

degraded form. Junceae are degraded Lilies; while Cyperaceae and

Gramineae among monocotyledons may be ranked with Amentiferae among

dicotyledons, as representing orders which have retrograded very far

from the entomophilous forms from which they were possibly and probably

descended" (p. 266).

illustrate the change from an entomophilous to the anemophilous state.

_P. lanceolata_ has polymorphic flowers, and is visited by

pollen-seeking insects, so that it can be fertilised either by insects

or the wind. _P. media_ illustrates transitions in point of structure,

as the filaments are pink, the anthers motionless, and the pollen grains

aggregated, and it is regularly visited by _Bombus terrestris_. On the

other hand, the slender filaments, versatile anthers, powdery pollen,

and elongated protogynous style are features of other species indicating

anemophily; while the presence of a degraded corolla shows its ancestors

to have been entomophilous. _P. media_, therefore, illustrates, not a

primitive entomophilous condition, but a return to it; just as is the

case with _Sanguisorba officinalis_ and _Salix Caprea_; but these show

no capacity of restoring the corolla, the attractive features having to

be borne by the calyx, which is purplish in Sanguisorba, by the pink

filaments of Plantago, and by the yellow anthers in the Sallow willow"

(p. 271).
"The interpretation, then, I would offer of inconspicuousness and all

kinds of degradations is the exact opposite to that of conspicuousness

and great differentiations; namely, that species with minute flowers,

rarely or never visited by insects, and habitually self-fertilised, have

primarily arisen through the neglect of insects, and have in consequence

assumed their present floral structures" (p. 282).

illustrations of his views, of which the following are the most

important: "Passing to Incompletae, the orders known collectively as

'Cyclospermeae' are related to Caryophylleae; and to my mind are

degradations from it, of which Orache is anemophilous. Cupuliferae have

an inferior ovary and rudimentary calyx-limb on the top. These, as far

as I know, cannot be interpreted except as degradations. The whole of

Monocotyledons appear to me (from anatomical reasons especially) to be

degradations from Dicotyledons, and primarily through the agency of

growth in water. Many subsequently became terrestrial, but retained the

effects of their primitive habitat through heredity. The 3-merous [sic]

perianth of grasses, the parts of the flower being in whorls, point to a

degradation from a sub-liliaceous condition."
Mr. Henslow informs me that he has long held these views, but, as far as

he knows, alone. Mr. Grant Allen, however, set forth a similar theory in

his _Vignettes from Nature_ (p. 15) and more fully in _The Colours of

Flowers_ (chap. v.), where he develops it fully and uses similar

arguments to those of Mr. Henslow.]
[Footnote 156: H. Müller gives ample proof of this in his _Fertilisation

of Flowers_.]

cases of recent degradation and readaptation to insect-fertilisation are

given by Professor Henslow (see footnote, p. 324).]
[Footnote 160: _The Colour Sense_, by Grant Allen, p. 95.]
[Footnote 161: _The Colour Sense_, chap. ix.]
[Footnote 162: See _Origin of Species_, sixth edition, p. 220.]

CHAPTER XII

The facts to be explained--The conditions which have determined

distribution--The permanence of oceans--Oceanic and continental

areas--Madagascar and New Zealand--The teachings of the

thousand-fathom line--The distribution of marsupials--The

distribution of tapirs--Powers of dispersal as illustrated by

insular organisms--Birds and insects at sea--Insects at great

altitudes--The dispersal of plants--Dispersal of seeds by the

wind--Mineral matter carried by the wind--Objections to the

theory of wind-dispersal answered--Explanation of north

temperate plants in the southern hemisphere--No proof of

glaciation in the tropics--Lower temperature not needed to

explain the facts--Concluding remarks.

The theory which we may now take as established--that all the existing

forms of life have been derived from other forms by a natural process of

descent with modification, and that this same process has been in action

during past geological time--should enable us to give a rational account

not only of the peculiarities of form and structure presented by animals

and plants, but also of their grouping together in certain areas, and

their general distribution over the earth's surface.

student of the theory of evolution might naturally anticipate that all

groups of allied organisms would be found in the same region, and that,

as he travelled farther and farther from any given centre, the forms of

life would differ more and more from those which prevailed at the

starting-point, till, in the remotest regions to which he could

penetrate, he would find an entirely new assemblage of animals and

plants, altogether unlike those with which he was familiar. He would

also anticipate that diversities of climate would always be associated

with a corresponding diversity in the forms of life.

Remoteness on the earth's surface is usually an indication of diversity

in the fauna and flora, while strongly contrasted climates are always

accompanied by a considerable contrast in the forms of life. But this

correspondence is by no means exact or proportionate, and the converse

propositions are often quite untrue. Countries which are near to each

other often differ radically in their animal and vegetable productions;

while similarity of climate, together with moderate geographical

proximity, are often accompanied by marked diversities in the prevailing

forms of life. Again, while many groups of animals--genera, families,

and sometimes even orders--are confined to limited regions, most of the

families, many genera, and even some species are found in every part of

the earth. An enumeration of a few of these anomalies will better

illustrate the nature of the problem we have to solve.

proximity, we may adduce Madagascar and Africa, whose animal and

vegetable productions are far less alike than are those of Great Britain

and Japan at the remotest extremities of the great northern continent;

while an equal, or perhaps even a still greater, diversity exists

between Australia and New Zealand. On the other hand, Northern Africa

and South Europe, though separated by the Mediterranean Sea, have faunas

and floras which do not differ from each other more than do the various

countries of Europe. As a proof that similarity of climate and general

adaptability have had but a small part in determining the forms of life

in each country, we have the fact of the enormous increase of rabbits

and pigs in Australia and New Zealand, of horses and cattle in South

America, and of the common sparrow in North America, though in none of

these cases are the animals natives of the countries in which they

thrive so well. And lastly, in illustration of the fact that allied

forms are not always found in adjacent regions, we have the tapirs,

which are found only on opposite sides of the globe, in tropical America

and the Malayan Islands; the camels of the Asiatic deserts, whose

nearest allies are the llamas and alpacas of the Andes; and the

marsupials, only found in Australia and on the opposite side of the

globe, in America. Yet, again, although mammalia may be said to be

universally distributed over the globe, being found abundantly on all

the continents and on a great many of the larger islands, yet they are

entirely wanting in New Zealand, and in a considerable number of other

islands which are, nevertheless, perfectly able to support them when

introduced.

geographical and geological facts. When the productions of remote

countries resemble each other, there is almost always continuity of land

with similarity of climate between them. When adjacent countries differ

greatly in their productions, we find them separated by a sea or strait

whose great depth is an indication of its antiquity or permanence. When

a group of animals inhabits two countries or regions separated by wide

oceans, it is found that in past geological times the same group was

much more widely distributed, and may have reached the countries it

inhabits from an intermediate region in which it is now extinct. We

know, also, that countries now united by land were divided by arms of

the sea at a not very remote epoch; while there is good reason to

believe that others now entirely isolated by a broad expanse of sea were

formerly united and formed a single land area. There is also another

important factor to be taken account of in considering how animals and

plants have acquired their present peculiarities of

distribution,--changes of climate. We know that quite recently a glacial

epoch extended over much of what are now the temperate regions of the

northern hemisphere, and that consequently the organisms which inhabit

those parts must be, comparatively speaking, recent immigrants from more

southern lands. But it is a yet more important fact that, down to middle

Tertiary times at all events, an equable temperate climate, with a

luxuriant vegetation, extended to far within the arctic circle, over

what are now barren wastes, covered for ten months of the year with snow

and ice. The arctic zone has, therefore, been in past times capable of

supporting almost all the forms of life of our temperate regions; and we

must take account of this condition of things whenever we have to

speculate on the possible migrations of organisms between the old and

new continents.

_The Conditions which have determined Distribution._

distribution of organic beings, we are confronted by several preliminary

questions, upon the solution of which will depend our treatment of the

phenomena presented to us. Upon the theory of descent which we have

adopted, all the different species of a genus, as well as all the genera

which compose a family or higher group, have descended from some common

ancestor, and must therefore, at some remote epoch, have occupied the

same area, from which their descendants have spread to the regions they

now inhabit. In the numerous cases in which the same group now occupies

countries separated by oceans or seas, by lofty mountain-chains, by wide

deserts, or by inhospitable climates, we have to consider how the

migration which must certainly have taken place has been effected. It is

possible that during some portion of the time which has elapsed since

the origin of the group the interposing barriers have not been in

existence; or, on the other hand, the particular organisms we are

dealing with may have the power of overpassing the barriers, and thus

reaching their present remote dwelling-places. As this is really the

fundamental question of distribution on which the solution of all its

more difficult problems depends, we have to inquire, in the first place,

what is the nature of, and what are the limits to, the changes of the

earth's surface, especially during the Tertiary and latter part of the

Secondary periods, as it was during those periods that most of the

existing types of the higher animals and plants came into existence;

and, in the next place, what are the extreme limits of the powers of

dispersal possessed by the chief groups of animals and plants. We will

first consider the question of barriers, more especially those formed by

seas and oceans.

_The Permanence of Oceans._

great features of the earth's surface, no less than the smaller ones,

were subject to continual mutations, and that during the course of

known geological time the continents and great oceans had again and

again changed places with each other. Sir Charles Lyell, in the last

edition of his _Principles of Geology_ (1872), said: "Continents,

therefore, although permanent for whole geological epochs, shift their

positions entirely in the course of ages;" and this may be said to have

been the orthodox opinion down to the very recent period when, by means

of deep-sea soundings, the nature of the ocean bottom was made known.

The first person to throw doubt on this view appears to have been the

veteran American geologist, Professor Dana. In 1849, in the Report of

Wilke's Exploring Expedition, he adduced the argument against a former

continent in the Pacific during the Tertiary period, from the absence of

all native quadrupeds. In 1856, in articles in the _American Journal_,

he discussed the development of the American continent, and argued for

its general permanence; and in his _Manual of Geology_ in 1863 and later

editions, the same views were more fully enforced and were latterly

applied to all continents. Darwin, in his _Journal of Researches_,

published in 1845, called attention to the fact that all the small

islands far from land in the Pacific, Indian, and Atlantic Oceans are

either of coralline or volcanic formation. He excepted, however, the

Seychelles and St. Paul's rocks; but the former have since been shown to

be no exception, as they consist entirely of coral rock; and although

Darwin himself spent a few hours on St. Paul's rocks on his outward

voyage in the _Beagle_, and believed he had found some portions of them

to be of a "cherty," and others of a "felspathic" nature, this also has

been shown to be erroneous, and the careful examination of the rocks by

the Abbé Renard clearly proves them to be wholly of volcanic

origin.[163] We have, therefore, at the present time, absolutely no

exception whatever to the remarkable fact that all the oceanic islands

of the globe are either of volcanic or coral formation; and there is,

further, good reason to believe that those of the latter class in every

case rest upon a volcanic foundation.

island had any native mammals or batrachia when first discovered, this

fact constituting the test of the class to which an island belongs;

whence he argued that none of them had ever been connected with

continents, but all had originated in mid-ocean. These considerations

alone render it almost certain that the areas now occupied by the great

oceans have never, during known geological time, been occupied by

continents, since it is in the highest degree improbable that every

fragment of those continents should have completely disappeared, and

have been replaced by volcanic islands rising out of profound oceanic

abysses; but recent research into the depth of the oceans and the nature

of the deposits now forming on their floors, adds greatly to the

evidence in this direction, and renders it almost a certainty that they

represent very ancient if not primaeval features of the earth's surface.

A very brief outline of the nature of this evidence will be now given.