Read Ebook: Darwinism (1889) An exposition of the theory of natural selection with some of its applications by Wallace Alfred Russel

Font size: 10 px 15 px 20 px 25 px 30 px 35 px

Background color: BlackWhiteGrayLight GrayDark GrayDim Gray

Text color: BlackWhiteGrayLight GrayDark GrayDim Gray

Apply/Save settings

Ebook has 1296 lines and 174560 words, and 26 pages

WHAT ARE "SPECIES" AND WHAT IS MEANT BY THEIR "ORIGIN"

Definition of species--Special creation--The early transmutationists--Scientific opinion before Darwin--The problem before Darwin--The change of opinion effected by Darwin--The Darwinian theory--Proposed mode of treatment of the subject

Its importance--The struggle among plants--Among animals--Illustrative cases--Succession of trees in forests of Denmark--The struggle for existence on the Pampas--Increase of organisms in a geometrical ratio--Examples of rapid increase of animals--Rapid increase and wide spread of plants--Great fertility not essential to rapid increase--Struggle between closely allied species most severe--The ethical aspect of the struggle for existence

THE VARIABILITY OF SPECIES IN A STATE OF NATURE

Importance of variability--Popular ideas regarding it--Variability of the lower animals--The variability of insects--Variation among lizards--Variation among birds--Diagrams of bird-variation--Number of varying individuals--Variation in the mammalia--Variation in internal organs--Variations in the skull--Variations in the habits of animals--The variability of plants--Species which vary little--Concluding remarks

VARIATION OF DOMESTICATED ANIMALS AND CULTIVATED PLANTS

The facts of variation and artificial selection--Proofs of the generality of variation--Variations of apples and melons--Variations of flowers--Variations of domestic animals--Domestic pigeons--Acclimatisation--Circumstances favourable to selection by man--Conditions favourable to variation--Concluding remarks

NATURAL SELECTION BY VARIATION AND SURVIVAL OF THE FITTEST

Effect of struggle for existence under unchanged conditions--The effect under change of conditions--Divergence of character--In insects--In birds--In mammalia--Divergence leads to a maximum of life in each area--Closely allied species inhabit distinct areas--Adaptation to conditions at various periods of life--The continued existence of low forms of life--Extinction of low types among the higher animals--Circumstances favourable to the origin of new species--Probable origin of the dippers--The importance of isolation--On the advance of organisation by natural selection--Summary of the first five chapters

DIFFICULTIES AND OBJECTIONS

Difficulty as to smallness of variations--As to the right variations occurring when required--The beginnings of important organs--The mammary glands--The eyes of flatfish--Origin of the eye--Useless or non-adaptive characters--Recent extension of the region of utility in plants--The same in animals--Uses of tails--Of the horns of deer--Of the scale-ornamentation of reptiles--Instability of non-adaptive characters--Delboeuf's law--No "specific" character proved to be useless--The swamping effects of intercrossing--Isolation as preventing intercrossing--Gulick on the effects of isolation--Cases in which isolation is ineffective

ON THE INFERTILITY OF CROSSES BETWEEN DISTINCT SPECIES AND THE USUAL STERILITY OF THEIR HYBRID OFFSPRING

Statement of the problem--Extreme susceptibility of the reproductive functions--Reciprocal crosses--Individual differences in respect to cross-fertilisation--Dimorphism and trimorphism among plants--Cases of the fertility of hybrids and of the infertility of mongrels--The effects of close interbreeding--Mr. Huth's objections--Fertile hybrids among animals--Fertility of hybrids among plants--Cases of sterility of mongrels--Parallelism between crossing and change of conditions--Remarks on the facts of hybridity--Sterility due to changed conditions and usually correlated with other characters--Correlation of colour with constitutional peculiarities--The isolation of varieties by selective association--The influence of natural selection upon sterility and fertility--Physiological selection--Summary and concluding remarks

THE ORIGIN AND USES OF COLOUR IN ANIMALS

The Darwinian theory threw new light on organic colour--The problem to be solved--The constancy of animal colour indicates utility--Colour and environment--Arctic animals white--Exceptions prove the rule--Desert, forest, nocturnal, and oceanic animals--General theories of animal colour--Variable protective colouring--Mr. Poulton's experiments--Special or local colour adaptations--Imitation of particular objects--How they have been produced--Special protective colouring of butterflies--Protective resemblance among marine animals--Protection by terrifying enemies--Alluring coloration--The coloration of birds' eggs--Colour as a means of recognition--Summary of the preceding exposition--Influence of locality or of climate on colour--Concluding remarks

WARNING COLORATION AND MIMICRY

The skunk as an example of warning coloration--Warning colours among insects--Butterflies--Caterpillars--Mimicry--How mimicry has been produced--Heliconidae--Perfection of the imitation--Other cases of mimicry among Lepidoptera--Mimicry among protected groups--Its explanation--Extension of the principle--Mimicry in other orders of insects--Mimicry among the vertebrata--Snakes--The rattlesnake and the cobra--Mimicry among birds--Objections to the theory of mimicry--Concluding remarks on warning colours and mimicry

COLOURS AND ORNAMENTS CHARACTERISTIC OF SEX

Sex colours in the mollusca and crustacea--In insects--In butterflies and moths--Probable causes of these colours--Sexual selection as a supposed cause--Sexual coloration of birds--Cause of dull colours of female birds--Relation of sex colour to nesting habits--Sexual colours of other vertebrates--Sexual selection by the struggles of males--Sexual characters due to natural selection--Decorative plumage of males and its effect on the females--Display of decorative plumage by the males--A theory of animal coloration--The origin of accessory plumes--Development of accessory plumes and their display--The effect of female preference will be neutralised by natural selection--General laws of animal coloration--Concluding remarks

THE SPECIAL COLOURS OF PLANTS: THEIR ORIGIN AND PURPOSE

The general colour relations of plants--Colours of fruits--The meaning of nuts--Edible or attractive fruits--The colours of flowers--Modes of securing cross-fertilisation--The interpretation of the facts--Summary of additional facts bearing on insect fertilisation--Fertilisation of flowers by birds--Self-fertilisation of flowers--Difficulties and contradictions--Intercrossing not necessarily advantageous--Supposed evil results of close interbreeding--How the struggle for existence acts among flowers--Flowers the product of insect agency--Concluding remarks on colour in nature

THE GEOGRAPHICAL DISTRIBUTION OF ORGANISMS

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 GEOLOGICAL EVIDENCES OF EVOLUTION

What we may expect--The number of known species of extinct animals--Causes of the imperfection of the geological record--Geological evidences of evolution--Shells--Crocodiles--The rhinoceros tribe--The pedigree of the horse tribe--Development of deer's horns--Brain development--Local relations of fossil and living animals--Cause of extinction of large animals--Indications of general progress in plants and animals--The progressive development of plants--Possible cause of sudden late appearance of exogens--Geological distribution of insects--Geological succession of vertebrata--Concluding remarks

FUNDAMENTAL PROBLEMS IN RELATION TO VARIATION AND HEREDITY

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

DARWINISM APPLIED TO MAN

General identity of human and animal structure--Rudiments and variations showing relation of man to other mammals--The embryonic development of man and other mammalia--Diseases common to man and the lower animals--The animals most nearly allied to man--The brains of man and apes--External differences of man and apes--Summary of the animal characteristics of man--The geological antiquity of man--The probable birthplace of man--The origin of the moral and intellectual nature of man--The argument from continuity--The origin of the mathematical faculty--The origin of the musical and artistic faculties--Independent proof that these faculties have not been developed by natural selection--The interpretation of the facts--Concluding remarks

WHAT ARE "SPECIES," AND WHAT IS MEANT BY THEIR "ORIGIN"

Definition of species--Special creation--The early Transmutationists--Scientific opinion before Darwin--The problem before Darwin--The change of opinion effected by Darwin--The Darwinian theory--Proposed mode of treatment of the subject.

The term "species" was thus defined by the celebrated botanist De Candolle: "A species is a collection of all the individuals which resemble each other more than they resemble anything else, which can by mutual fecundation produce fertile individuals, and which reproduce themselves by generation, in such a manner that we may from analogy suppose them all to have sprung from one single individual." And the zoologist Swainson gives a somewhat similar definition: "A species, in the usual acceptation of the term, is an animal which, in a state of nature, is distinguished by certain peculiarities of form, size, colour, or other circumstances, from another animal. It propagates, 'after its kind,' individuals perfectly resembling the parent; its peculiarities, therefore, are permanent."

Professor Agassiz, one of the greatest naturalists of the last generation, went even further, and maintained not only that each species was specially created, but that it was created in the proportions and in the localities in which we now find it to exist. The following extract from his very instructive book on Lake Superior explains this view: "There are in animals peculiar adaptations which are characteristic of their species, and which cannot be supposed to have arisen from subordinate influences. Those which live in shoals cannot be supposed to have been created in single pairs. Those which are made to be the food of others cannot have been created in the same proportions as those which live upon them. Those which are everywhere found in innumerable specimens must have been introduced in numbers capable of maintaining their normal proportions to those which live isolated and are comparatively and constantly fewer. For we know that this harmony in the numerical proportions between animals is one of the great laws of nature. The circumstance that species occur within definite limits where no obstacles prevent their wider distribution leads to the further inference that these limits were assigned to them from the beginning, and so we should come to the final conclusion that the order which prevails throughout nature is intentional, that it is regulated by the limits marked out on the first day of creation, and that it has been maintained unchanged through ages with no other modifications than those which the higher intellectual powers of man enable him to impose on some few animals more closely connected with him."

It should be especially noted that all which is here claimed is now almost universally admitted, while the criticisms of Darwin's works refer almost exclusively to those numerous questions which, as he himself says, "will long remain obscure."

As it will be necessary, in the following chapters, to set forth a considerable body of facts in almost every department of natural history, in order to establish the fundamental propositions on which the theory of natural selection rests, I propose to give a preliminary statement of what the theory really is, in order that the reader may better appreciate the necessity for discussing so many details, and may thus feel a more enlightened interest in them. Many of the facts to be adduced are so novel and so curious that they are sure to be appreciated by every one who takes an interest in nature, but unless the need of them is clearly seen it may be thought that time is being wasted on mere curious details and strange facts which have little bearing on the question at issue.

The theory of natural selection rests on two main classes of facts which apply to all organised beings without exception, and which thus take rank as fundamental principles or laws. The first is, the power of rapid multiplication in a geometrical progression; the second, that the offspring always vary slightly from the parents, though generally very closely resembling them. From the first fact or law there follows, necessarily, a constant struggle for existence; because, while the offspring always exceed the parents in number, generally to an enormous extent, yet the total number of living organisms in the world does not, and cannot, increase year by year. Consequently every year, on the average, as many die as are born, plants as well as animals; and the majority die premature deaths. They kill each other in a thousand different ways; they starve each other by some consuming the food that others want; they are destroyed largely by the powers of nature--by cold and heat, by rain and storm, by flood and fire. There is thus a perpetual struggle among them which shall live and which shall die; and this struggle is tremendously severe, because so few can possibly remain alive--one in five, one in ten, often only one in a hundred or even one in a thousand.

This self-acting process which, by means of a few easily demonstrated groups of facts, brings about change in the organic world, and keeps each species in harmony with the conditions of its existence, will appear to some persons so clear and simple as to need no further demonstration. But to the great majority of naturalists and men of science endless difficulties and objections arise, owing to the wonderful variety of animal and vegetable forms, and the intricate relations of the different species and groups of species with each other; and it was to answer as many of these objections as possible, and to show that the more we know of nature the more we find it to harmonise with the development hypothesis, that Darwin devoted the whole of his life to collecting facts and making experiments, the record of a portion of which he has given us in a series of twelve masterly volumes.

These matters will be discussed in the four succeeding chapters, though in a somewhat different order--the struggle for existence and the power of rapid multiplication, which is its cause, occupying the first place, as comprising those facts which are the most fundamental and those which can be perfectly explained without any reference to the less generally understood facts of variation. These chapters will be followed by a discussion of certain difficulties, and of the vexed question of hybridity. Then will come a rather full account of the more important of the complex relations of organisms to each other and to the earth itself, which are either fully explained or greatly elucidated by the theory. The concluding chapter will treat of the origin of man and his relations to the lower animals.

FOOTNOTES:

Its importance--The struggle among plants--Among animals--Illustrative cases--Succession of trees in forests of Denmark--The struggle for existence on the Pampas--Increase of organisms in a geometrical ratio--Examples of great powers of increase of animals--Rapid increase and wide spread of plants--Great fertility not essential to rapid increase--Struggle between closely allied species most severe--The ethical aspect of the struggle for existence.

There is perhaps no phenomenon of nature that is at once so important, so universal; and so little understood, as the struggle for existence continually going on among all organised beings. To most persons nature appears calm, orderly, and peaceful. They see the birds singing in the trees, the insects hovering over the flowers, the squirrel climbing among the tree-tops, and all living things in the possession of health and vigour, and in the enjoyment of a sunny existence. But they do not see, and hardly ever think of, the means by which this beauty and harmony and enjoyment is brought about. They do not see the constant and daily search after food, the failure to obtain which means weakness or death; the constant effort to escape enemies; the ever-recurring struggle against the forces of nature. This daily and hourly struggle, this incessant warfare, is nevertheless the very means by which much of the beauty and harmony and enjoyment in nature is produced, and also affords one of the most important elements in bringing about the origin of species. We must, therefore, devote some time to the consideration of its various aspects and of the many curious phenomena to which it gives rise.

It is a matter of common observation that if weeds are allowed to grow unchecked in a garden they will soon destroy a number of the flowers. It is not so commonly known that if a garden is left to become altogether wild, the weeds that first take possession of it, often covering the whole surface of the ground with two or three different kinds, will themselves be supplanted by others, so that in a few years many of the original flowers and of the earliest weeds may alike have disappeared. This is one of the very simplest cases of the struggle for existence, resulting in the successive displacement of one set of species by another; but the exact causes of this displacement are by no means of such a simple nature. All the plants concerned may be perfectly hardy, all may grow freely from seed, yet when left alone for a number of years, each set is in turn driven out by a succeeding set, till at the end of a considerable period--a century or a few centuries perhaps--hardly one of the plants which first monopolised the ground would be found there.

Another phenomenon of an analogous kind is presented by the different behaviour of introduced wild plants or animals into countries apparently quite as well suited to them as those which they naturally inhabit. Agassiz, in his work on Lake Superior, states that the roadside weeds of the northeastern United States, to the number of 130 species, are all European, the native weeds having disappeared westwards; and in New Zealand there are no less than 250 species of naturalised European plants, more than 100 species of which have spread widely over the country, often displacing the native vegetation. On the other hand, of the many hundreds of hardy plants which produce seed freely in our gardens, very few ever run wild, and hardly any have become common. Even attempts to naturalise suitable plants usually fail; for A. de Candolle states that several botanists of Paris, Geneva, and especially of Montpellier, have sown the seeds of many hundreds of species of hardy exotic plants in what appeared to be the most favourable situations, but that, in hardly a single case, has any one of them become naturalised. Even a plant like the potato--so widely cultivated, so hardy, and so well adapted to spread by means of its many-eyed tubers--has not established itself in a wild state in any part of Europe. It would be thought that Australian plants would easily run wild in New Zealand. But Sir Joseph Hooker informs us that the late Mr. Bidwell habitually scattered Australian seeds during his extensive travels in New Zealand, yet only two or three Australian plants appear to have established themselves in that country, and these only in cultivated or newly moved soil.

In animals, it is the eggs or the very young that suffer most from their various enemies; in plants, the tender seedlings when they first appear above the ground. To illustrate this latter point Mr. Darwin cleared and dug a piece of ground three feet long and two feet wide, and then marked all the seedlings of weeds and other plants which came up, noting what became of them. The total number was 357, and out of these no less than 295 were destroyed by slugs and insects. The direct strife of plant with plant is almost equally fatal when the stronger are allowed to smother the weaker. When turf is mown or closely browsed by animals, a number of strong and weak plants live together, because none are allowed to grow much beyond the rest; but Mr. Darwin found that when the plants which compose such turf are allowed to grow up freely, the stronger kill the weaker. In a plot of turf three feet by four, twenty distinct species of plants were found to be growing, and no less than nine of these perished altogether when the other species were allowed to grow up to their full size.

But besides having to protect themselves against competing plants and against destructive animals, there is a yet deadlier enemy in the forces of inorganic nature. Each species can sustain a certain amount of heat and cold, each requires a certain amount of moisture at the right season, each wants a proper amount of light or of direct sunshine, each needs certain elements in the soil; the failure of a due proportion in these inorganic conditions causes weakness, and thus leads to speedy death. The struggle for existence in plants is, therefore, threefold in character and infinite in complexity, and the result is seen in their curiously irregular distribution over the face of the earth. Not only has each country its distinct plants, but every valley, every hillside, almost every hedgerow, has a different set of plants from its adjacent valley, hillside, or hedgerow--if not always different in the actual species yet very different in comparative abundance, some which are rare in the one being common in the other. Hence it happens that slight changes of conditions often produce great changes in the flora of a country. Thus in 1740 and the two following years the larva of a moth committed such destruction in many of the meadows of Sweden that the grass was greatly diminished in quantity, and many plants which were before choked by the grass sprang up, and the ground became variegated with a multitude of different species of flowers. The introduction of goats into the island of St. Helena led to the entire destruction of the native forests, consisting of about a hundred distinct species of trees and shrubs, the young plants being devoured by the goats as fast as they grew up. The camel is a still greater enemy to woody vegetation than the goat, and Mr. Marsh believes that forests would soon cover considerable tracts of the Arabian and African deserts if the goat and the camel were removed from them. Even in many parts of our own country the existence of trees is dependent on the absence of cattle. Mr. Darwin observed, on some extensive heaths near Farnham, in Surrey, a few clumps of old Scotch firs, but no young trees over hundreds of acres. Some portions of the heath had, however, been enclosed a few years before, and these enclosures were crowded with young fir-trees growing too close together for all to live; and these were not sown or planted, nothing having been done to the ground beyond enclosing it so as to keep out cattle. On ascertaining this, Mr. Darwin was so much surprised that he searched among the heather in the unenclosed parts, and there he found multitudes of little trees and seedlings which had been perpetually browsed down by the cattle. In one square yard, at a point about a hundred yards from one of the old clumps of firs, he counted thirty-two little trees, and one of them had twenty-six rings of growth, showing that it had for many years tried to raise its head above the stems of the heather and had failed. Yet this heath was very extensive and very barren, and, as Mr. Darwin remarks, no one would ever have imagined that cattle would have so closely and so effectually searched it for food.

Sir Charles Lyell observes that if, by the attacks of seals or other marine foes, salmon are reduced in numbers, the consequence will be that otters, living far inland, will be deprived of food and will then destroy many young birds or quadrupeds, so that the increase of a marine animal may cause the destruction of many land animals hundreds of miles away. Mr. Darwin carefully observed the effects produced by planting a few hundred acres of Scotch fir, in Staffordshire, on part of a very extensive heath which had never been cultivated. After the planted portion was about twenty-five years old he observed that the change in the native vegetation was greater than is often seen in passing from one quite different soil to another. Besides a great change in the proportional numbers of the native heath-plants, twelve species which could not be found on the heath flourished in the plantations. The effect on the insect life must have been still greater, for six insectivorous birds which were very common in the plantations were not to be seen on the heath, which was, however, frequented by two or three different species of insectivorous birds. It would have required continued study for several years to determine all the differences in the organic life of the two areas, but the facts stated by Mr. Darwin are sufficient to show how great a change may be effected by the introduction of a single kind of tree and the keeping out of cattle.

The next case I will give in Mr. Darwin's own words: "In several parts of the world insects determine the existence of cattle. Perhaps Paraguay offers the most curious instance of this; for here neither cattle nor horses nor dogs have ever run wild, though they swarm southward and northward in a feral state; and Azara and Rengger have shown that this is caused by the greater numbers, in Paraguay, of a certain fly which lays its eggs in the navels of these animals when first born. The increase of these flies, numerous as they are, must be habitually checked by some means, probably by other parasitic insects. Hence, if certain insectivorous birds were to decrease in Paraguay, the parasitic insects would probably increase; and this would lessen the number of the navel-frequenting flies--then cattle and horses would become feral, and this would greatly alter the vegetation: this again would largely affect the insects, and this, as we have just seen in Staffordshire, the insectivorous birds, and so onward in ever-increasing circles of complexity. Not that under nature the relations will ever be as simple as this. Battle within battle must be continually recurring with varying success; and yet in the long run the forces are so nicely balanced, that the face of nature remains for a long time uniform, though assuredly the merest trifle would give the victory to one organic being over another."

The following account of the struggle between trees in the forests of Denmark, from the researches of M. Hansten-Blangsted, strikingly illustrates our subject. The chief combatants are the beech and the birch, the former being everywhere successful in its invasions. Forests composed wholly of birch are now only found in sterile, sandy tracts; everywhere else the trees are mixed, and wherever the soil is favourable the beech rapidly drives out the birch. The latter loses its branches at the touch of the beech, and devotes all its strength to the upper part where it towers above the beech. It may live long in this way, but it succumbs ultimately in the fight--of old age if of nothing else, for the life of the birch in Denmark is shorter than that of the beech. The writer believes that light is the cause of the superiority of the latter, for it has a greater development of its branches than the birch, which is more open and thus allows the rays of the sun to pass through to the soil below, while the tufted, bushy top of the beech preserves a deep shade at its base. Hardly any young plants can grow under the beech except its own shoots; and while the beech can nourish under the shade of the birch, the latter dies immediately under the beech. The birch has only been saved from total extermination by the facts that it had possession of the Danish forests long before the beech ever reached the country, and that certain districts are unfavourable to the growth of the latter. But wherever the soil has been enriched by the decomposition of the leaves of the birch the battle begins. The birch still flourishes on the borders of lakes and other marshy places, where its enemy cannot exist. In the same way, in the forests of Zeeland, the fir forests are disappearing before the beech. Left to themselves, the firs are soon displaced by the beech. The struggle between the latter and the oak is longer and more stubborn, for the branches and foliage of the oak are thicker, and offer much resistance to the passage of light. The oak, also, has greater longevity; but, sooner or later, it too succumbs, because it cannot develop in the shadow of the beech. The earliest forests of Denmark were mainly composed of aspens, with which the birch was apparently associated; gradually the soil was raised, and the climate grew milder; then the fir came and formed large forests. This tree ruled for centuries, and then ceded the first place to the holm-oak, which is now giving way to the beech. Aspen, birch, fir, oak, and beech appear to be the steps in the struggle for the survival of the fittest among the forest-trees of Denmark.

It may be added that in the time of the Romans the beech was the principal forest-tree of Denmark as it is now, while in the much earlier bronze age, represented by the later remains found in the peat bogs, there were no beech-trees, or very few, the oak being the prevailing tree, while in the still earlier stone period the fir was the most abundant. The beech is a tree essentially of the temperate zone, having its northern limit considerably southward of the oak, fir, birch, or aspen, and its entrance into Denmark was no doubt due to the amelioration of the climate after the glacial epoch had entirely passed away. We thus see how changes of climate, which are continually occurring owing either to cosmical or geographical causes, may initiate a struggle among plants which may continue for thousands of years, and which must profoundly modify the relations of the animal world, since the very existence of innumerable insects, and even of many birds and mammals, is dependent more or less completely on certain species of plants.

Another illustration of the struggle for existence, in which both plants and animals are implicated, is afforded by the pampas of the southern part of South America. The absence of trees from these vast plains has been imputed by Mr. Darwin to the supposed inability of the tropical and sub-tropical forms of South America to thrive on them, and there being no other source from which they could obtain a supply; and that explanation was adopted by such eminent botanists as Mr. Ball and Professor Asa Gray. This explanation has always seemed to me unsatisfactory, because there are ample forests both in the temperate regions of the Andes and on the whole west coast down to Terra del Fuego; and it is inconsistent with what we know of the rapid variation and adaptation of species to new conditions. What seems a more satisfactory explanation has been given by Mr. Edwin Clark, a civil engineer, who resided nearly two years in the country and paid much attention to its natural history. He says: "The peculiar characteristics of these vast level plains which descend from the Andes to the great river basin in unbroken monotony, are the absence of rivers or water-storage, and the periodical occurrence of droughts, or 'siccos,' in the summer months. These conditions determine the singular character both of its flora and fauna.

"The soil is naturally fertile and favourable for the growth of trees, and they grow luxuriantly wherever they are protected. The eucalyptus is covering large tracts wherever it is enclosed, and willows, poplars, and the fig surround every estancia when fenced in.

Share on: WhatsApp @Hash Facebook Tweet