by Means of Natural Selection,
or The Preservation of Favoured Races in the Struggle for Life.
by Charles Darwin
The original, squashed down to read in about 50 minutes
This scientific treatise not only revolutionized every branch of the natural sciences, but has profoundly influenced the literary, philosophical and religious thinkers who followed. With it the established Western view that creatures had been created independently by a God, and indeed the whole supernatural explanation of the universe, had competition. At first, denunciation by the likes of Bishop Wilberforce was complete. But religious views gradually evolved, through Philip Grosse's theory that fossils had been planted by God to give the earth a coherent history, to today's position where only a remnant population of creationists remains.
This is what the modern philosopher Daniel Dennett called 'Darwin's dangerous idea'- that natural selection governs, not only the world's flora and fauna, but its history, economics and beliefs. Even religious ideas, it seems, are subject to the same laws of advancement as all other things, "multiply, vary, let the strongest live and the weakest die."
The Origin was banned from Trinity College, Cambridge in 1859; in Yugoslavia (1935) and Greece (1937). In 1925 in the USA, John Scopes was famously convicted of teaching from it, and it remained banned from some American schools until 1967.
When on board H.M.S. 'Beagle,' as naturalist, I was struck with certain facts, which seemed to throw light on the origin of species, that mystery of mysteries, as it has been called by one of our greatest philosophers. On my return home in 1837, I began patiently accumulating and reflecting on all sorts of facts which could have any bearing on it. After five years' work I drew up some short notes; enlarged in 1844 into a sketch of the conclusions. From that period to the present I have steadily pursued this object. I have not been hasty in coming to a decision.
VARIATION UNDER DOMESTICATION
When we look at individuals of cultivated plants and animals, we are struck by how they differ much more from each other, than do individuals in nature. Our oldest cultivated plants, such as wheat, still yield new varieties.
Some organisms breed freely under most unnatural conditions (for instance, rabbits or ferrets kept in hutches), showing that their reproductive system has not been affected; so will some animals and plants withstand domestication or cultivation, and vary very slightly- perhaps hardly more than in a state of nature.
Gardeners are aware of ‘sporting' buds, which suddenly assume a new and sometimes very different character from that of the rest of the plant. Such buds can be propagated by grafting, &c., and sometimes by seed, showing that variation is not necessarily connected, as some authors have supposed, with the act of generation.
He who breeds animals knows how strong is the tendency to inheritance: like produces like is his fundamental belief. Breeders know too that one characteristic accompanies another; long limbs go with an elongated head, hairless dogs have imperfect teeth; pigeons with feathered feet have skin between their toes. But the number and diversity of inheritable deviations of structure, whether of slight or considerable physiological importance, is endless. The laws governing inheritance are quite unknown; no one can say why the child often reverts in certain characters to its remote ancestor; why a peculiarity is often transmitted from one sex to both sexes or to one sex alone.
Here I may comment on the matter of reversion, that domestic varieties, when run wild, gradually but certainly revert to their aboriginal character. Hence it has been argued that no deductions can be drawn from domestic races to species in a state of nature. Certainly, when under nature the conditions of life change, variations and reversions of character probably do occur; but natural selection, as will hereafter be explained, will determine how far the new characters thus arising shall be preserved.
Believing that it is always best to study some special group, I have joined two of the London Pigeon Clubs. The diversity of the breeds is something astonishing. The short-faced tumbler has a beak like a finch; the runt is a bird of great size; the turbit has a line of reversed feathers down the breast. Such are the variations that an ornithologist would certainly rank them as well-defined species. Yet I am fully convinced that the common opinion of naturalists is correct, namely, that all have descended from the wild rock-pigeon (Columba livia).
Such variability may be attributed to the conditions of life, to use and disuse. But I am convinced that Selection is by far the predominant Power.
VARIATION UNDER NATURE
Before considering variation in nature, we must briefly discuss whether variation occurs in that state.
The terms ‘species' and ‘variety' are difficult to define, but a distinct act of creation and a community of descent is implied. We have also what are called monstrosities; but they graduate into varieties. Again, we have the slight, individual differences as appear in offspring from the same parents. These individual differences generally affect what naturalists consider unimportant parts; but I could show by a long catalogue of facts, that even important parts sometimes vary in the individuals of the same species.
There is one extremely perplexing point connected with individual differences: those 'protean' or 'polymorphic,' genera which present such an inordinate amount of variation that hardly two naturalists can agree which forms are species and which varieties. We may instance, among the plants, Rubus and Rosa. I am inclined to suspect that we see in these polymorphic genera variations which are of no service or disservice to the species, so have not been seized on and rendered definite by natural selection, as hereafter will be explained.
Compare the floras of Great Britain, of France or of the United States, drawn up by different botanists, and see what a surprising number of forms have been ranked by one botanist as species, and by another as mere varieties. Certainly no clear line of demarcation has as yet been drawn between species and sub-species. I look at varieties which are in any degree distinct and permanent, as steps leading to more strongly marked and more permanent varieties; and at these latter, as leading to sub-species, and to species. The passage from one stage to another may sometimes be due to long-continued different physical conditions in different regions; but I am more inclined to attribute the changes to the action of natural selection.
Hence it is the most flourishing, or, as they may be called, the dominant species,- those which range widely over the world, are the most diffused in their own country, and are the most numerous in individuals,- which oftenest produce well-marked varieties. It seems to me that only natural selection can account for this. On the other hand, if we look at each species as a special act of creation, there is no apparent reason why more varieties should occur in a group having many species, than in one having few.
STRUGGLE FOR EXISTENCE
It has never been disputed that there is variation amongst organic beings in a state of nature. It is immaterial for us whether a multitude of doubtful forms be called species or sub-species or varieties. But how have all those exquisite adaptations been perfected?
All these results, as we shall more fully see in the next chapter, follow inevitably from the struggle for life. The elder De Candolle and Lyell have philosophically shown that all organic beings are exposed to severe competition. Nothing is easier than to admit in words the truth of the universal struggle for life. We behold the face of nature bright with gladness, we often see superabundance of food; we do not see, or we forget, that the birds which are idly singing round us mostly live on insects or seeds, and are thus constantly destroying life; or we forget how largely these songsters, or their eggs, or their nestlings, are destroyed by birds and beasts of prey; we do not always bear in mind, that though food may be now superabundant, it is not so at all seasons of each recurring year. Two canine animals in a time of dearth, may be truly said to struggle with each other which shall get food and live. A plant which annually produces a thousand seeds, of which on an average only one comes to maturity, may be more truly said to struggle with the plants of the same and other kinds which already clothe the ground. The missletoe depends on other trees, but can only in a far-fetched sense be said to struggle with these trees, for if too many of these parasites grow on the same tree, it will languish and die. But several seedling missletoes, growing close together on the same branch, may truly be said to struggle with each other. This struggle for life must be most severe between individuals and varieties of the same species who compete for precisely the same resources.
Linnaeus has calculated that if an annual plant produced only two seeds- and there is no plant so unproductive- and their seedlings next year produced two, and so on, then in twenty years there would be a million plants. One fly deposits hundreds of eggs, and another, like the hippobosca, a single one; but this difference does not determine how many individuals of the two species can be supported in a district. Climate plays an important part in determining the average numbers of a species, and periodical seasons of extreme cold or drought, I believe to be the most effective of all checks.
Can the principle of selection, so potent in the hands of man, apply in nature? I think it can. Let it be borne in mind how infinitely complex and close-fitting are the mutual relations of all organic beings to each other and to their physical conditions of life. Since variations useful to man have undoubtedly occurred, could not other variations useful in some way to each being in the great and complex battle of life, have sometimes occurred in the course of thousands of generations? This preservation of favourable variations and the rejection of injurious variations, I call Natural Selection. Variations neither useful nor injurious would not be affected by natural selection, and would be left a fluctuating element, as perhaps we see in the species called polymorphic.
We know that changes in a creature's neighbours, and changes in climate can most seriously affect survival. Man keeps creatures of many climates in the same country; yet seldom treats each in their accustomed manner; he feeds long and short beaked pigeons the same food; he exposes sheep with long and short wool to the same climate. He does not allow the most vigorous males to struggle for the females. In such ways man preserves differences which would run out in nature.
In plants, the down on the fruit skin, and its colour, are considered by botanists as of trifling importance: yet that excellent horticulturist, Downing, shows that smooth-skinned fruits suffer more from beetle; that purple plums suffer more from disease than yellow plums. If such slight differences make a great difference in cultivating varieties, then assuredly, in a state of nature, where trees have to struggle with other trees and a host of enemies, such differences would effectually settle which variety should succeed. In all cases, natural selection will ensure that modifications shall not be in the least degree injurious: for if they became so, they would cause the extinction of the species.
In social animals, natural selection will adapt the structure of each individual for the benefit of the community. This depends, not on a struggle for existence, but on a struggle between the males for possession of the females; the result is not death to the unsuccessful competitor, but few or no offspring.
Consider the case of wolves. It is possible that a cub might be born with a slight innate tendency to pursue a particular prey, and we know from Mr St.John that the tendency to capture particular prey is inherited. If that were followed by some slight change in the climate and in the availability of prey, then that animal would be advantaged. Some of its young would probably inherit the same habits, and by the repetition of this process, a new variety might be formed. Indeed we know, according to Mr. Pierce, that in the Catskill Mountains in the United States, there exists a greyhound-like wolf, which pursues deer, and another more bulky one which attacks sheep.
In man's methodical selection, a breeder selects for some definite object. Thus it will be in nature; when some place is not so perfectly occupied as might be, natural selection will tend to preserve all the individuals varying in the right direction, so as better to fill the vacancy.
The affinities of creatures have sometimes been represented by a great tree. I believe this simile largely speaks the truth. In the diagram, each horizontal line may be supposed to represent a thousand, or a million or hundred million generations, and likewise a section of the strata of the earth's crust containing extinct remains. The green and budding twigs may represent existing species; and those produced during each former year may represent the long succession of extinct species.
LAWS OF VARIATION
I have hitherto sometimes spoken as if the variations in organic beings had been due to chance. Some authors believe it to be as much the function of the reproductive system to produce individual differences. But the greater variability, as well as the greater frequency of monstrosities, under domestication, than under nature, leads me to believe that deviations are in some way due to the conditions of life to which the parents have been exposed.
How much direct effect difference of climate, food, &c., produces on any being is extremely doubtful. My impression is, that the effect is extremely small in the case of animals, but perhaps rather more in that of plants. The fact that varieties of one species, when they range into the habitation of other species, often acquire in a very slight degree some of the characters of such species, accords with our view that species are merely well-marked varieties.
Natural selection will accumulate all profitable variations, however slight, until they become plainly developed and appreciable by us. For instance, when a new insect first arrived on an island, selection will enlarge or reduce the wings, depending on whether a greater number of individuals were saved by battling with the winds, or by rarely or never flying.
When a part has been developed in an extraordinary manner in any one species, we may conclude that this part has undergone an extraordinary amount of modification, since the period when the species branched off from the common progenitor.
Distinct species present analogous variations; and a variety of one species often assumes some of the characters of an allied species, or reverts to some of the characters of an early progenitor. After twelve generations, the proportion of blood, to use a common expression, of any one ancestor, is only 1 in 2048; and yet it seems that a tendency to reversion is retained by this very small proportion of foreign blood.
DIFFICULTIES ON THEORY
Difficulties and objections may be classed under the following heads:- Firstly, why, if species have descended from other species by minuscule gradations, do we not everywhere see innumerable transitional forms? Secondly, is it possible that an animal having, for instance, the structure and habits of a bat, could have been formed by the modification of some animal with wholly different habits? Can we believe that natural selection could produce, on the one hand, organs of trifling importance, such as the tail of a giraffe, which serves as a fly-flapper, and, on the other hand, such wonderful structures as the eye, of which we hardly as yet understand the inimitable perfection? Thirdly, can instincts be acquired and modified through natural selection?
As to the rarity of transitional varieties, as natural selection acts solely by the preservation of profitable modifications, each new form will tend to take the place of, and finally to exterminate, its less improved parent. Hence, if we look at each species as descended from some other unknown form, both the parent and all the transitional varieties will generally have exterminated by the very process of formation and perfection of the new form. Furthermore, forms existing in larger numbers will always have a better chance, within any given period, of presenting further favourable variations for natural selection to seize on, than will the rarer forms which exist in lesser numbers. So that, in any one region and at any one time, we ought only to see a few species presenting slight modifications of structure in some degree permanent; and this assuredly we do see. Consequently evidence of the existence of earlier forms could be found only amongst fossil remains which are preserved, and as we shall in a future chapter attempt to show, these form an extremely imperfect and intermittent record.
In considering how animals of precise and perfected structure might arise from quite different forms, look at the family of squirrels. Here we have fine gradation from animals with their tails only slightly flattened, and others, as Sir J. Richardson has remarked, with the posterior part of their bodies and the skin on their flanks rather full, to the so-called flying squirrels with their limbs united by a broad expanse of skin, which serves as a parachute and allows them to glide through the air from tree to tree. I can see no difficulty in the continued preservation of individuals with fuller and fuller flank-membranes, each useful modification being propagated, until the accumulated effects of this process of natural selection produced a perfect so-called flying squirrel. When we see any structure perfected for any particular habit, as the wings of a bird for flight, we should bear in mind that animals displaying transitional grades of the structure will seldom continue to exist, having been supplanted by the very process of perfection through natural selection.
He who believes in separate and innumerable acts of creation will say that it has pleased the Creator to cause a being of one type to take the place of one of another type. He who believes in the struggle for existence and in the principle of natural selection, will acknowledge that every organic being is constantly endeavouring to increase in numbers; and that if any one being varies ever so little in habits or structure, and thus gains an advantage over some other inhabitant of the country, it will seize on the place of that inhabitant. Hence, it will cause him no surprise that there should be geese and frigate-birds with webbed feet living away from water; long-toed corncrakes living in meadows instead of in swamps; or woodpeckers where not a tree grows.
Organs of extreme perfection and complication. - To suppose that the eye could have been formed by natural selection, seems absurd. Yet reason tells me, that if gradations from a perfect and complex eye to one very imperfect and simple, can be shown to exist; if further, the eye does vary ever so slightly, and the variations be inherited, which is certainly the case; and if any variation or modification in the organ be ever useful to an animal under changing conditions of life, then the difficulty of believing that a perfect and complex eye could be formed by natural selection, though insuperable by our imagination, can hardly be considered real.
In the Articulata we see an optic nerve merely coated with pigment, and without any other mechanism. In certain crustaceans, there is a double cornea, with a lens-shaped swelling. In other crustaceans the transparent cones which are coated by pigment, are convex at their upper ends and must act by convergence; and at their lower ends there seems to be an imperfect vitreous substance. These facts, here far too briefly and imperfectly given, show that there is much graduated diversity in the eyes of living crustaceans.
He who will go thus far ought not to hesitate to go further, and to admit that a structure even as perfect as the eye of an eagle might be formed by natural selection. His reason ought to conquer his imagination. It is scarcely possible to avoid comparing the eye to a telescope. We know that this instrument has been perfected by the long-continued efforts of the highest human intellects; and we naturally infer that the eye has been formed by a somewhat analogous process. Is it not presumptuous to assume that the Creator works by intellectual powers like those of man?
I will treat the subject of instinct separately, especially as such wonderful instincts as those of the hive-bee will probably have occurred to many readers. I am not explaining the origin of mental powers, any more than of life itself. ‘Instinct' embraces many mental actions; but every one understands what is meant when it is said that instinct impels the cuckoo to migrate and to lay her eggs in other birds' nests. A little dose, as Huber says, of judgement or reason, is apparent, even in the lowliest of animals.
Instincts are certainly as important as corporeal structure for the welfare of each species. Under changed conditions of life, it is at least possible that slightly changed instincts might be profitable; and if it can be shown that instincts do vary ever so little, then I can see no difficulty in natural selection preserving and accumulating their variations.
But, as with corporeal structures, we ought to find in nature, not the actual transitional gradations by which each complex instinct has been acquired, but only the collateral lines of descent; or we ought at least to be able to show that gradations are possible. But I am well aware that these general statements, without detailed facts, can produce but a feeble effect on the reader's mind.
Mozart played the pianoforte at three years old with wonderfully little practice, only if he had played with no practice at all could we say he did so instinctively.
Familiarity alone prevents our seeing how universally the minds of our domestic animals have been modified by domestication. How rarely do our civilised dogs, even when young, require to be taught not to attack livestock! When occasionally they do attack, they are then beaten; and if not cured, are destroyed; so that habit, with some degree of selection, has probably civilised our dogs.
Domestic instincts are sometimes thought to be inherited from long-continued habit, but this, I think, is not true. No one would ever have thought of teaching, or probably could have taught, the tumbler-pigeon to tumble, an action which, as I have witnessed, is performed by young birds, that have never seen a pigeon tumble. We may believe that some one pigeon showed a slight tendency to this strange habit, and that selection in successive generations made tumblers what they now are.
Huber has observed an ant (Formica polyerges) in which both males and fertile females do no work other than capturing slaves. They are incapable of making their own nests, or of feeding their own larvae. So utterly helpless are the masters, that when Huber shut up thirty of them without a slave, but with ample food, and their larvae to stimulate them to work, they did nothing; they could not even feed themselves, and many perished of hunger. Huber then introduced a single slave (F. fusca), and she instantly set to work, fed and saved the survivors; made cells, tended the larvae, and put all to rights. Are these facts not extraordinary?
By what steps the slave-making instinct originated I will not pretend to conjecture. But as ants will, as I have seen, carry off pupae of other species, it is possible that pupae originally stored as food might become developed; and the ants thus unintentionally reared would then follow their instincts to work. If their presence proved useful to the species which had seized them, then the habit of collecting pupae originally for food might by natural selection be strengthened and rendered permanent for the purpose of raising slaves.
The hive bee has perfected the art of making its cells the proper shape to hold the greatest possible amount of honey, with the least possible consumption of precious wax. Indeed, the geometer Professor Miller, of Cambridge, tells me that the shape could not be bettered. Natural selection of each tiny modification profitable to the individual seems the only possible explanation.
Finally, it may not be a logical deduction, but I feel that such instincts as the young cuckoo ejecting its foster-brothers, ants making slaves, ichneumonidae larvae feeding within live caterpillars, are not specially created instincts, but are small consequences of one general law, leading to the advancement of all organic beings, namely; multiply, vary, let the strongest live and the weakest die.
THE IMPERFECTION OF THE GEOLOGICAL RECORD
In a previous chapter I endeavoured to show that intermediate varieties, from existing in lesser numbers than the forms which they connect, will generally be beaten out and exterminated during the course of further modification and improvement.
It is hardly possible for me even to recall to one who is not a practical geologist, the facts leading the mind feebly to comprehend the lapse of time. It is not enough to study the principles of geology, a man must for years examine for himself great piles of superimposed strata, and watch the sea at work grinding down old rocks, before he can hope to comprehend anything of the lapse of time, the monuments of which we see around us.
To give a small example, consider the denudation of the Weald, though this is but a mere trifle. Standing on the North Downs one can safely picture to oneself the great dome of rocks which must have covered up the Weald. The distance from the northern to the southern Downs is about 22 miles, and the thickness of the several formations is on an average about 1100 feet, says Prof. Ramsay. If their denudation had proceeded at a rate of one inch per century, which would be an ample allowance, the denudation of the Weald must have required 306,662,400years; or say three hundred million years.
That our palaeontological collections are very imperfect, is admitted by every one. The late Edward Forbes noted that many of our fossil species are known and named from single and often broken specimens, or from specimens collected on one spot. Only a small portion of the surface of the earth has been geologically explored, and no part with sufficient care. Further even shells and bones will decay and disappear when left on the bottom of the sea. Such as is preserved is held only when sediment is deposited over it, yet the bright purity of much of the seas tells that this but rarely happens. For instance, only one single land shell is known in the whole carboniferous strata of North America. The consideration of these facts impresses my mind almost in the same manner as does the vain endeavour to grapple with the idea of eternity.
Those who think the natural geological record in any degree perfect, and who do not attach much weight to the facts and arguments given in this volume, will reject my theory. For my part, following Lyell's metaphor, I look at the natural geological record, as a history of the world imperfectly written in a changing dialect; of this history we possess the last volume alone, and that of only two or three countries. Of this volume, only here and there a short chapter has been preserved; and of each page, only here and there a few lines.
ON THE GEOLOGICAL SUCCESSION
Let us now see whether the facts of geological succession better accord with the common view of the immutability of species, or with that of gradual modification.
In the oldest tertiary beds a few living shells may still be found among a multitude of extinct forms. But, when a species has once disappeared from the face of the earth, we have reason to believe that the same identical form never reappears.
This whole subject of extinction has been involved in the most gratuitous mystery. Some authors have even supposed that as the individual has a definite length of life, so have species a definite duration. When I found in La Plata the tooth of a horse along with the remains of Mastodon, Megatherium, and other extinct monsters, which all co-existed, I was filled with astonishment. Seeing that the horse, since its introduction by the Spaniards into South America, has run wild over the whole country, I asked myself what could so recently have exterminated the former horse under conditions of life apparently so favourable. My astonishment was groundless, for we cannot tell what unfavourable conditions checked its increase.
It is worth noting that the improved and modified descendants of a species will generally cause the extermination of their nearest allies. Therefore, the utter extinction of a group is generally a slower process than its production.
On the Affinities of extinct Species to each other, and to living forms- As Buckland remarked, all fossils belong either in still existing groups, or between them. Some writers have objected to extinct species being considered intermediate between living species. This objection may have some validity, yet if we compare the older Reptiles and Batrachians, the older Fish, the older Cephalopods, and the eocene Mammals, with the more recent, we must admit that there is some truth in the remark.
On this theory, it is evident that the fauna of any great period in the earth's history will be intermediate in general character between that which preceded and that which succeeded it. I need give only one instance, namely, the manner in which the fossils of the Devonian system were at once recognised by palaeontologists as intermediate in character between those of the overlying carboniferous, and underlying Silurian system.
On the state of Development of Ancient Forms-. In one particular sense the more recent forms must, on my theory, be higher than the more ancient; for each new species is formed by having had some advantage in the struggle for life over preceding forms. If the eocene inhabitants were put into competition with the existing inhabitants, the eocene fauna or flora would certainly be beaten and exterminated; as would a secondary fauna by an eocene, and a palaeozoic fauna by a secondary fauna.
Mr. Agassiz insists that ancient animals partly resemble recent animals of the same classes. The embryo seems a sort of picture, preserved by nature, of the ancient and less modified form of each animal. This view may be true, and yet it may never be capable of full proof.
On the Succession of the same Types within the same areas - Mr. Clift many years ago showed that fossil mammals from the Australian caves were closely allied to the living marsupials. In South America, a similar relationship is manifest, even to an uneducated eye, in the gigantic pieces of armadillo-like armour, found in La Plata. On the theory of descent with modification, the great law of the long enduring, but not immutable, succession of the same types within the same areas, is at once explained.
If then the geological record be as imperfect as I believe it to be, and it cannot be proved more perfect, then the main objections to the theory of natural selection are greatly diminished or disappear.
In considering the distribution of organic beings over the face of the globe, the first fact which strikes us is, that the similarity and dissimilarity of the inhabitants of various regions cannot be accounted for by their climatal and other physical conditions. The case of America alone would almost suffice to prove its truth. If we travel over the vast American continent, we find deserts, mountains, grassy plains, forests, marshes, lakes, and great rivers, under almost every temperature. There is hardly a climate or condition in the Old World which cannot be paralleled in the New. Yet how widely different are their living productions!
A second great fact which strikes us is, that barriers of any kind, or obstacles to free migration, are related in a close and important manner to the differences between the productions of various regions. On each continent we find different productions in different regions; though as mountain chains, deserts, &c., are not as impassable as the oceans separating continents, the differences are less than those of distinct continents.
Beyond the westward shores of America, a wide space of open ocean extends, with not an island as a halting-place for emigrants. Here we have a barrier of another kind, and as soon as this is passed we meet in the eastern islands of the Pacific, with another and totally distinct fauna. Proceeding still further westward we come to the shores of Africa, where we meet with quite different productions. The plains near the Straits of Magellan are inhabited by a species of Rhea (American ostrich), and northward the plains of La Plata by another species of the same genus; but not by the true ostrich or emeu found in Africa and Australia. We see in these facts some deep organic bond, prevailing throughout space and time, over the same areas of land and water, and independent of their physical conditions.
We are thus brought to the question as to whether species have been created at one or more points of the earth's surface. The conditions of life are so nearly the same that a multitude of European animals and plants have become naturalised in America and Australia; and some of the aboriginal plants are identical at distant points of the northern and southern hemispheres. If the existence of similar species at distant and isolated points of the earth's surface can in many instances be explained by species having migrated from a single birthplace we must then ask if such transport is possible.
Seeds may be occasionally transported on drift timber, in the carcasses of birds or, indeed, through living birds.
For myself, I am disposed to the view that much dispersal occurred during the Glacial period. The very wide distribution of alpine species seems to attest to this. We have evidence of almost every kind, organic and inorganic, that within a very recent geological period, central Europe and North America suffered an Arctic climate. The ruins of a house burnt by fire do not tell their tale more plainly, than do the mountains of Scotland, with their scored flanks, and perched boulders, of the icy streams with which their valleys were lately filled. As each southern zone became fitted for arctic beings and ill-fitted for their former more temperate inhabitants, the latter would be supplanted by arctic productions. The inhabitants of the temperate regions would at the same time travel southward where they could.
The many cases we find of relationship, without identity, of the inhabitants of seas now disjoined, and likewise of the past and present inhabitants of the temperate lands of North America and Europe, are inexplicable on the theory of creation.
As lakes and river-systems are separated from each other by barriers of land, it might have been thought that fresh-water productions would not have ranged widely within the same country, yet alone would have extended to distant countries. But not only have many fresh-water species, an enormous range, but allied species prevail in a remarkable manner throughout the world. I well remember, when first collecting in the fresh waters of Brazil, feeling much surprise at the similarity of the fresh-water insects, shells, &c., and at the dissimilarity of the surrounding terrestrial beings, compared with those of Britain.
But this wide range, can, I think, in most cases be explained by their having become fitted, in a manner highly useful to them, for short and frequent migrations from pond to pond, or from stream to stream. Sir Charles Lyell informs me that a Dyticus has been caught with an Ancylus (a shell like a limpet) firmly adhering to it; and a Colymbetes water-beetle once flew on board the ‘Beagle,' when forty-five miles distant from the nearest land. I have myself germinated 537 plants from as much pond-mud as would be contained in a breakfast cup! Nature, like a careful gardener, thus takes her seeds from a bed of a particular nature, and drops them in another equally well fitted for them.
On the Inhabitants of Oceanic Islands- The most striking and important fact for us in regard to the inhabitants of islands, is their affinity to those of the nearest mainland, without being actually the same species. I will give the example of the Galapagos Archipelago, situated under the equator, some 550 miles off South America.
There is nothing in the land or climate of the islands, which resembles closely the conditions of the South American coast: in fact there is a considerable dissimilarity. On the other hand, there is considerable resemblance between the Galapagos and Cape de Verde Archipelagos: but what an entire and absolute difference in their inhabitants! The inhabitants of the Cape de Verde Islands are related to those of Africa, those of the Galapagos to America. I believe this grand fact can receive no sort of explanation on the ordinary view of independent creation; whereas on the view here maintained, it is obvious that the Galapagos Islands would be likely to receive colonists, whether by occasional means of transport or by formerly continuous land, from America; and the Cape de Verde Islands from Africa; and that such colonists would be liable to modification;- the principle of inheritance still betraying their original birthplace. Many facts could be given to support an almost universal rule that the endemic productions of islands are related to those of the nearest continent, or near islands.
MUTUAL AFFINITIES OF ORGANIC BEINGS
From the first dawn of life, all organic beings are found to resemble each other in descending degrees, so that they can be classed in groups. The existence of groups would have been of simple signification, if one group had been exclusively fitted to inhabit the land, and another the water; one to feed on flesh, another on vegetable matter, and so on; but the case is that even the same subgroup have different habits.
Morphology is the very soul of natural history. What can be more curious than that the hand of a man, formed for grasping, that of a mole for digging, the leg of the horse, the paddle of the porpoise, and the wing of the bat, should all be constructed on the same pattern, of the same bones, in the same relative positions?
We see the same great law in the construction of the mouths of insects: what can be more different than the long spiral proboscis of a sphinx-moth, the curious folded one of a bee or bug, and the great jaws of a beetle? - yet all these organs, serving for such different purposes, are formed by modifications of an upper lip, mandibles and maxillae. So it is with the flowers of plants, with the limbs of crustaceans and many others.
The ordinary view of creation can only say that;- it has so pleased the Creator to construct each animal and plant. On the theory of the natural selection, it can be said that each modification has little or no tendency to modify the original pattern, or to transpose parts. Bones might be shortened and widened, become gradually enveloped in thick membrane, so as to serve as a fin; or a webbed foot might have its bones lengthened, and the membrane connecting them increased, so as to serve as a wing: yet in all this great amount of modification there will be no tendency to alter the framework of bones or the relative connexion of the several parts.
Embryology- How can we explain these several facts in embryology,- namely of the striking similarity between embryos of different species. I believe that all these facts can be explained only on the view of descent with modification.
Rudimentary, atrophied, or aborted organs- These are extremely common throughout nature. For instance, rudimentary mammae are very general in the males of mammals. Nothing can be plainer than that wings are formed for flight, yet in how many insects do we see wings so reduced in size as to be utterly incapable of flight, and not rarely lying under wing-cases, firmly soldered together!
Every one must be struck with astonishment: for the same reasoning which tells us that most parts and organs are exquisitely adapted for certain purposes, tells us with equal plainness that these rudimentary or atrophied organs, are imperfect and useless. In works on natural history rudimentary organs are generally said to have been created ‘for the sake of symmetry,' or in order ‘to complete the scheme of nature;' but this seems to me no explanation, merely a restatement of the fact. On the view of descent with modification, we may conclude that the existence of organs in a rudimentary, imperfect, and useless condition, far from presenting a strange difficulty, as they assuredly do on the ordinary doctrine of creation, might even have been anticipated and accounted for by the laws of inheritance.
Finally, the several classes of facts which have been considered in this chapter, seem to me to proclaim so plainly, that the innumerable species, genera, and families of organic beings, with which this world is peopled, have all descended, each within its own class or group, from common parents, and have all been modified in the course of descent.
RECAPITULATION AND CONCLUSION
That many and grave objections may be advanced against the theory of descent with modification through natural selection, I do not deny. Nothing at first can appear more difficult to believe than that complex organs and instincts should have been perfected, not by means analogous with human reason, but by the accumulation of innumerable slight variations. Nevertheless, this difficulty cannot be considered real if we admit the following propositions, namely; that there are gradations in the perfection of any organ or instinct, - that all organs and instincts are, in ever so slight a degree, variable, - and, lastly, that there is a struggle for existence leading to the preservation of each profitable deviation of structure or instinct. The truth of these propositions cannot, I think, be disputed.
In the distant future I see this understanding opening fields for far more important researches. Psychology will be based on a new foundation, that of the necessary acquirement of each mental power and capacity by gradation. Light will be thrown on the origin of man and his history.
Authors of the highest eminence seem to be fully satisfied with the view that each species has been independently created. To my mind it accords better with what we know of the laws impressed on matter by the Creator, that the production and extinction of the past and present inhabitants of the world should have been due to secondary causes, like those determining the birth and death of the individual. When I view all beings not as special creations, but as the lineal descendants of some few beings which lived long before the first Silurian age, they seem to me to become ennobled. And as natural selection works solely by and for the good of each being, all corporeal and mental endowments will tend to progress towards perfection.
It is interesting to contemplate an entangled bank, clothed with many plants of many kinds, with birds singing on the bushes, insects flitting about and worms crawling through the damp earth, and to reflect that these forms, so different yet so dependent on each other in so complex a manner, have all been produced by simple laws. There is grandeur in this view of life, with its several powers, having been originally breathed by the Creator into a few forms or into one; and that, whilst this planet has gone circling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being evolved.
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