Alfred Russel Wallace
February, 1855
(also known as the Sarawack Law. ed.)
Electronically Enhanced Text (c) Copyright 1991, World Library, Inc.
Wallace, Alfred Russel (1823-1913) - A largely self-educated English naturalist and philosopherwho developed a theory of evolution by natural selection independently of and concurrently withCharles Darwin. On the Law Which Has Regulated the Introduction of New Species (1855) -Published before any of Darwin’s theories were made public, this paper states Wallace’s theory,“every species has come into existence coincident both in time and space with a pre-existingclosely allied species.” ON THE LAW WHICH HAS REGULATED NEW SPECIES
Geographical Distribution dependent on Geologic Changes.
EVERY naturalist who has directed his attention to the subject of the geographical distribution ofanimals and plants, must have been interested in the singular facts which it presents. Many ofthese facts are quite different from what would have been anticipated, and have hitherto beenconsidered as highly curious, but quite inexplicable. None of the explanations attempted from thetime of Linnaeus are now considered at all satisfactory; none of them have given a causesufficient to account for the facts known at the time, or comprehensive enough to include all thenew facts which have since been, and are daily being added. Of late years, however, a great lighthas been thrown upon the subject by geological investigations, which have shown that thepresent state of the earth and of the organisms now inhabiting it, is but the last stage of a longand uninterrupted series of changes which it has undergone, and consequently, that to endeavourto explain and account for its present condition without any reference to those changes (as hasfrequently been done) must lead to very imperfect and erroneous conclusions.
The facts proved by geology are briefly these:- That during an immense, but unknown period,the surface of the earth has undergone successive changes; land has sunk beneath the ocean,while fresh land has risen up from it; mountain chains have been elevated; islands have beenformed into continents, and continents submerged till they have become islands; and thesechanges have taken place, not once merely, but perhaps hundreds, perhaps thousands of times:-That all these operations have been more or less continuous, but unequal in their progress, andduring the whole series the organic life of the earth has undergone a corresponding alteration.
This alteration also has been gradual, but complete; after a certain interval not a single speciesexisting which had lived at the commencement of the period. This complete renewal of the formsof life also appears to have occurred several times:- That from the last of the geological epochsto the present or historical epoch, the change of organic life has been gradual: the firstappearance of animals now existing can in many cases be traced, their numbers graduallyincreasing in the more recent formations, while other species continually die out and disappear,so that the present condition of the organic world is clearly derived by a natural process ofgradual extinction and creation of species from that of the latest geological periods. We maytherefore safely infer a like gradation and natural sequence from one geological epoch to another.
Now, taking this as a fair statement of the results of geological inquiry, we see that the presentgeographical distribution of life upon the earth must be the result of all the previous changes,both of the surface of the earth itself and of its in- habitants. Many causes, no doubt, have operated of which we must ever remain in ignorance, andwe may, therefore, expect to find many details very difficult of explanation, and in attempting togive one, must allow ourselves to call into our service geological changes which it is highlyprobable may have occurred, though we have no direct evidence of their individual operation.
The great increase of our knowledge within the last twenty years, both of the present and pasthistory of the organic world, has accumulated a body of facts which should afford a sufficientfoundation for a comprehensive law embracing and explaining them all, and giving a direction tonew researches. It is about ten years since the idea of such a law suggested itself to the writer ofthis essay, and he has since taken every opportunity of testing it by all the newly-ascertainedfacts with which he has become acquainted, or has been able to observe himself.
These have all served to convince him of the correctness of his hypothesis. Fully to enter intosuch a subject would occupy much space, and it is only in consequence of some views havingbeen lately promulgated, he believes, in a wrong direction, that he now ventures to present hisideas to the public, with only such obvious illustrations of the arguments and results as occur tohim in a place far removed from all means of reference and exact information.
A Law deduced from well-known Geographical and Geological Facts.
The following propositions in Organic Geography and Geology give the main facts on which thehypothesis is founded.
1. Large groups, such as classes and orders, are generally spread over the whole earth, whilesmaller ones, such as families and genera, are frequently confined to one portion, often to a verylimited district.
2. In widely distributed families the genera are often limited in range; in widely distributedgenera, well marked groups of species are peculiar to each geographical district.
3. When a group is confined to one district, and is rich in species, it is almost invariably the casethat the most closely allied species are found in the same locality or in closely adjoininglocalities, and that therefore the natural sequence of the species by affinity is also geographical.
4. In countries of a similar climate, but separated by a wide sea or lofty mountains, the families,genera and species of the one are often represented by closely allied families, genera and speciespeculiar to the other.
5. The distribution of the organic world in time is very similar to its present distribution in space.
6. Most of the larger and some small groups extend through several geological periods.
7. In each period, however, there are peculiar groups, found nowhere else, and extending throughone or several formations.
8. Species of one genus, or genera of one family occurring in the same geological time, are moreclosely allied than those separated in time.
9. As generally in geography no species or genus occurs in two very distant localities withoutbeing also found in intermediate places, so in geology the life of a species or genus has not beeninterrupted. In other words, no group or species has come into existence twice.
10. The following law may be deduced from these facts:- Every species has come into existencecoincident both in space and time with a pre-existing closely allied species.
This law agrees with, explains and illustrates all the facts connected with the following branchesof the subject:- 1st. The system of natural affinities. 2nd. The distribution of animals and plants in space. 3rd. The same in time, including all the phaenomenaof representative groups, and those which Professor Forbes supposed to manifest polarity. 4th.
The phaenomena of rudimentary organs. We will briefly endeavour to show its bearing uponeach of these.
The Form of a true system of Classification determined by this Law.
If the law above enunciated be true, it follows that the natural series of affinities will alsorepresent the order in which the several species came into existence, each one having had for itsimmediate antitype a closely allied species existing at the time of its origin. It is evidentlypossible that two or three distinct species may have had a common antitype, and that each ofthese may again have become the antitypes from which other closely allied species were created.
The effect of this would be, that so long as each species has had but one new species formed onits model, the line of affinities will be simple, and may be represented by placing the severalspecies in direct succession in a straight line. But if two or more species have been independentlyformed on the plan of a common antitype, then the series of affinities will be compound, and canonly be represented by a forked or many branched line. Now, all attempts at a Naturalclassification and arrangement of organic beings show, that both these plans have obtained increation.
Sometimes the series of affinities can be well represented for a space by a direct progressionfrom species to species or from group to group, but it is generally found impossible so tocontinue. There constantly occur two or more modifications of an organ or modifications of twodistinct organs, leading us on to two dis- tinct series of species, which at length differ so much from each other as to form distinct generaor families. These are the parallel series or representative groups of naturalists, and they oftenoccur in different countries, or are found fossil in different formations. They are said to have ananalogy to each other when they are so far removed from their common antitype as to differ inmany important points of structure, while they still preserve a family resemblance. We thus see how difficult it is to determine in every case whether a given relation is an analogy or an affinity,for it is evident that as we go back along the parallel or divergent series, towards the commonantitype, the analogy which existed between the two groups becomes an affinity. We are alsomade aware of the difficulty of arriving at a true classification, even in a small and perfectgroup;- in the actual state of nature it is almost impossible, the species being so numerous andthe modifications of form and structure so varied, arising probably from the immense number ofspecies which have served as antitype for the existing species, and thus produced a complicatedbranching of the lines of affinity, as intricate as the twigs of a gnarled oak or the vascular systemof the human body. Again, if we consider that we have only fragments of this vast system, thestem and main branches being represented by extinct species of which we have no knowledge,while a vast mass of limbs and boughs and minute twigs and scattered leaves is what we have toplace in order, and determine the true position each originally occupied with regard to the others,the whole difficulty of the true Natural System of classification becomes apparent to us.
We shall thus find ourselves obliged to reject all those systems of classification which arrangespecies or groups in circles, as well as those which fix a definite number for the divisions of eachgroup. The latter class have been very generally rejected by naturalists, as contrary to nature,notwithstanding the ability with which they have been advocated; but the circular system ofaffinities seems to have obtained a deeper hold, many eminent naturalists having to some extentadopted it. We have, however, never been able to find a case in which the circle has been closedby a direct and close affinity. In most cases a palpable analogy has been substituted, in others theaffinity is very obscure or altogether doubtful.
The complicated branching of the lines of affinities in extensive groups must also afford greatfacilities for giving a show of probability to any such purely artificial arrangements. Their death-blow was given by the admirable paper of the lamented Mr. Strickland, published in the “Annalsof Natural History,” in which he so cleverly showed the true synthetical method of discoveringthe Natural System.
Geographical Distribution of Organisms.
If we now consider the geographical distribution of animals and plants upon the earth, we shallfind all the facts beautifully in accordance with, and readily explained by, the present hypothesis.
A country having species, genera, and whole families peculiar to it, will be the necessary resultof its having been isolated for a long period, sufficient for many series of species to have beencreated on the type of pre-existing ones, which, as well as many of the earlier-formed species,have become extinct, and thus made the groups appear isolated. If in any case the antitype had an extensive range, two or more groups of species might have been formed, eachvarying from it in a different manner, and thus producing several representative or analogousgroups. The Sylviadae of Europe and the Sylvicolidae of North America, the Heliconidae of South America and the Euploeas of the East, the group of Trogons inhabiting Asia, and thatpeculiar to South America, are examples that may be accounted for in this manner.
Such phaenomena as are exhibited by the Galapagos Islands, which contain little groups ofplants and animals peculiar to themselves, but most nearly allied to those of South America, havenot hitherto received any, even a conjectural explanation. The Galapagos are a volcanic group ofhigh antiquity, and have probably never been more closely connected with the continent thanthey are at present. They must have been first peopled, like other newly-formed islands, by theaction of winds and currents, and at a period sufficiently remote to have had the original speciesdie out, and the modified prototypes only remain. In the same way we can account for theseparate islands having each their peculiar species, either on the supposition that the sameoriginal emigration peopled the whole of the islands with the same species from whichdifferently modified prototypes were created, or that the islands were successively peopled fromeach other, but that new species have been created in each on the plan of the pre-existing ones.
St. Helena is a similar case of a very ancient island having obtained an entirely peculiar, thoughlimited, flora. On the other hand, no example is known of an island which can be provedgeologically to be of very recent origin (late in the Ter- tiary, for instance), and yet possess generic or family groups, or even many species peculiar toitself.
When a range of mountains has attained a great elevation, and has so remained during a longgeological period, the species of the two sides at and near their bases will be often very different,representative species of some genera occurring, and even whole genera being peculiar to oneside, as is remarkably seen in the case of the Andes and Rocky Mountains. A similarphaenomena occurs when an island has been separated from a continent at a very early period.
The shallow sea between the Peninsula of Malacca, Java, Sumatra and Borneo was probably acontinent or large island at an early epoch, and may have become submerged as the volcanicranges of Java and Sumatra were elevated. The organic results we see in the very considerablenumber of species of animals common to some or all of these countries, while at the same time anumber of closely allied representative species exist peculiar to each, showing that aconsiderable period has elapsed since their separation. The facts of geographical distribution andof geology may thus mutually explain each other in doubtful cases, should the principles hereadvocated be clearly established.
In all those cases in which an island has been separated from a continent, or raised by volcanic orcoralline action from the sea, or in which a mountain-chain has been elevated in a recentgeological epoch, the phaenomena of peculiar groups or even of single representative specieswill not exist. Our own island is an example of this, its separation from the continent beinggeologically very re- cent, and we have consequently scarcely a species which is peculiar to it; while the Alpine range,one of the most recent mountain elevations, separates faunas and floras which scarcely differmore than may be due to climate and latitude alone.
The series of facts alluded to in Proposition (3), of closely allied species in rich groups beingfound geographically near each other, is most striking and important. Mr. Lovell Reeve has wellexemplified it in his able and interesting paper on the Distribution of the Bulimi. It is also seen inthe Hummingbirds and Toucans, little groups of two or three closely allied species being oftenfound in the same or closely adjoining districts, as we have had the good fortune of personallyverifying. Fishes give evidence of a similar kind: each great river has its peculiar genera, and inmore extensive genera its groups of closely allied species.
But it is the same throughout Nature; every class and order of animals will contribute similarfacts. Hitherto no attempt has been made to explain these singular phaenomena, or to show howthey have arisen. Why are the genera of Palms and of Orchids in almost every case confined toone hemisphere? Why are the closely allied species of brownbacked Trogons all found in theEast, and the greenbacked in the West? Why are the Macaws and the Cockatoos similarlyrestricted? Insects furnish a countless number of analogous examples;- the Goliathi of Africa, theOrnithopterae of the Indian Islands, the Heliconidae of South America, the Danaidae of the East,and in all, the most closely allied species found in geographical proximity. The question forcesitself upon every thinking mind,- why are these things so? They could not be as they are had no law regulated their creation anddispersion. The law here enunciated not merely explains, but necessitates the facts we see toexist, while the vast and long-continued geological changes of the earth readily account for theexceptions and apparent discrepancies that here and there occur. The writer’s object in puttingforward his views in the present imperfect manner is to submit them to the test of other minds,and to be made aware of all the facts supposed to be inconsistent with them. As his hypothesis isone which claims acceptance solely as explaining and connecting facts which exist in nature, heexpects facts alone to be brought to disprove it, not a priori arguments against its probability.
Geological Distribution of the Forms of Life.
The phaenomena of geological distribution are exactly analogous to those of geography. Closelyallied species are found associated in the same beds, and the change from species to speciesappears to have been as gradual in time as in space. Geology, however, furnishes us with positiveproof of the extinction and production of species, though it does not inform us how either hastaken place.
The extinction of species, however, offers but little difficulty, and the modus operandi has beenwell illustrated by Sir C. Lyell in his admirable “principles.” Geological changes, howevergradual, must occasionally have modified external conditions to such an extent as to haverendered the existence of certain species impossible. The extinction would in most cases be effected by a gradual dyingout, but in some instances there might have been a sudden destructionof a spe- cies of limited range. To discover how the extinct species have from time to time been replacedby new ones down to the very latest geological period, is the most difficult, and at the same timethe most interesting problem in the natural history of the earth. The present inquiry, which seeksto eliminate from known facts a law which has determined, to a certain degree, what speciescould and did appear at a given epoch, may, it is hoped, be considered as one step in the rightdirection towards a complete solution of it.
High Organization of very ancient Animals consistent with this Law.
Much discussion has of late years taken place on the question, whether the succession of lifeupon the globe has been from a lower to a higher degree of organization. The admitted factsseem to show that there has been a general, but not a detailed progression. Mollusca and Radiataexisted before Vertebrata, and the progression from Fishes to Reptiles and Mammalia, and alsofrom the lower mammals to the higher, is indisputable. On the other hand, it is said that theMollusca and Radiata of the very earliest periods were more highly organized than the greatmass of those now existing, and that the very first fishes that have been discovered are by nomeans the lowest organised of the class. Now it is believed the present hypothesis will harmonizewith all these facts, and in a great measure serve to explain them; for though it may appear tosome readers essentially a theory of progression, it is in reality only one of gradual change. It is,however, by no means difficult to show that a real progression in the scale of organization is perfectly consistent with all the appearances, and even with apparent retrogression, should suchoccur.
Returning to the analogy of a branching tree, as the best mode of representing the naturalarrangement of species and their successive creation, let us suppose that at an early geologicalepoch any group (say a class of the Mollusca) has attained to a great richness of species and ahigh organization. Now let this great branch of allied species, by geological mutations, becompletely or partially destroyed. Subsequently a new branch springs from the same trunk, thatis to say, new species are successively created, having for their antitypes the same lowerorganized species which had served as the antitypes for the former group, but which havesurvived the modified conditions which destroyed it. This new group being subject to thesealtered conditions, has modifications of structure and organization given to it, and becomes therepresentative group of the former one in another geological formation. It may, however, happen,that though later in time, the new series of species may never attain to so high a degree oforganization as those preceding it, but in its turn become extinct, and give place to yet anothermodification from the same root, which may be of higher or lower organization, more or lessnumerous in species, and more or less varied in form and structure than either of those whichpreceded it. Again, each of these groups may not have become totally extinct, but may have left a few species, the modified prototypes of which have existed in each succeeding period, a faintmemorial of their former grandeur and luxuriance. Thus every case of apparent retrogressionmay be in re- ality a progress, though an interrupted one: when some monarch of the forest loses a limb, it maybe replaced by a feeble and sickly substitute. The foregoing remarks appear to apply to the caseof the Mollusca, which, at a very early period, had reached a high organization and a greatdevelopment of forms and species in the testaceous Cephalopoda. In each succeeding agemodified species and genera replaced the former ones which had become extinct, and as weapproach the present aera, but few and small representatives of the group remain, while theGasteropods and Bivalves have acquired an immense preponderance. In the long series ofchanges the earth has undergone, the process of peopling it with organic beings has beencontinually going on, and whenever any of the higher groups have become nearly or quiteextinct, the lower forms which have better resisted the modified physical conditions have servedas the antitypes on which to found the new races. In this manner alone, it is believed, can therepresentative groups at successive periods, and the rising and fallings in the scale oforganization, be in every case explained.
Objections to Forbes’ Theory of Polarity.
The hypothesis of polarity, recently put forward by Professor Edward Forbes to account for theabundance of generic forms at a very early period and at present, while in the intermediateepochs there is a gradual diminution and impoverishment, till the minimum occurred at theconfines of the Palaeozoic and Secondary epochs, appears to us quite unnecessary, as the factsmay be readily accounted for on the principles already laid down. Between the Palaeozoic andNeo- zoic periods of Professor Forbes, there is scarcely a species in common, and the greater part ofthe genera and families also disappear to be replaced by new ones.
It is almost universally admitted that such a change in the organic world must have occupied avast period of time. Of this interval we have no record; probably because the whole area of theearly formations now exposed to our researches was elevated at the end of the Palaeozoic period,and remained so through the interval required for the organic changes which resulted in the faunaand flora of the Secondary period. The records of this interval are buried beneath the oceanwhich covers three-fourths of the globe. Now it appears highly probable that a long period ofquiescence or stability in the physical conditions of a district would be most favourable to theexistence of organic life in the greatest abundance, both as regards individuals and also as tovariety of species and generic group, just as we now find that the places best adapted to the rapidgrowth and increase of individuals also contain the greatest profusion of species and the greatestvariety of forms,- the tropics in comparison with the temperate and arctic regions. On the otherhand, it seems no less probable that a change in the physical conditions of a district, even small in amount if rapid, or even gradual if to a great amount, would be highly unfavourable to theexistence of individuals, might cause the extinction of many species, and would probably beequally unfavourable to the creation of new ones. In this too we may find an analogy with thepresent state of our earth, for it has been shown to be the violent extremes and rapid changes ofphysical conditions, rather than the actual mean state in the temperate and frigid zones, whichrenders them less prolific than the tropical regions, as exemplified by the great distance beyond the tropics to which tropical forms penetrate whenthe climate is equable, and also by the richness in species and forms of tropical mountain regionswhich principally differ from the temperate zone in the uniformity of their climate. However thismay be, it seems a fair assumption that during a period of geological repose the new specieswhich we know to have been created would have appeared, that the creations would then exceedin number the extinctions, and therefore the number of species would increase. In a period ofgeological activity, on the other hand, it seems probable that the extinctions might exceed thecreations, and the number of species consequently diminish. That such effects did take place inconnexion with the causes to which we have imputed them, is shown in the case of the Coalformation, the faults and contortions of which show a period of great activity and violentconvulsions, and it is in the formation immediately succeeding this that the poverty of forms oflife is most apparent. We have then only to suppose a long period of somewhat similar actionduring the vast unknown interval at the termination of the Palaeozoic period, and then adecreasing violence or rapidity through the Secondary period, to allow for the gradualrepopulation of the earth with varied forms, and the whole of the facts are explained. We thushave a clue to the increase of the forms of life during certain periods, and their decrease duringothers, without recourse to any causes but these we know to have existed, and to effects fairlydeducible from them. The precise manner in which the geological changes of the earlyformations were effected is so extremely obscure, that when we can explain important facts by aretardation at one time and an acceleration at another of a process which we know from its nature and from observation to have been unequal,-a cause so simple may surely be preferred to one so obscure and hypothetical as polarity.
I would also venture to suggest some reasons against the very nature of the theory of ProfessorForbes. Our knowledge of the organic world during any geological epoch is necessarily veryimperfect. Looking at the vast numbers of species and groups that have been discovered bygeologists, this may be doubted; but we should compare their numbers not merely with those thatnow exist upon the earth, but with a far larger amount. We have no reason for believing that thenumber of species on the earth at any former period was much less than at present; at all eventsthe aquatic portion, with which geologists have most acquaintance, was probably often as greator greater. Now we know that there have been many complete changes of species; new sets oforganisms have many times been introduced in place of old ones which have become extinct, sothat the total amount which have existed on the earth from the earliest geological period musthave borne about the same proportion to those now living, as the whole human race who have lived and died upon the earth, to the population at the present time. Again, at each epoch, thewhole earth was no doubt, as now, more or less the theatre of life, and as the successivegenerations of each species died, their exuviae and preservable parts would be deposited overevery portion of the then existing seas and oceans, which we have reason for supposing to havebeen more, rather than less, extensive than at present. In order then to understand our possi- ble knowledge of the early world and its inhabitants, we must compare, not the area of the wholefield of our geological researches with the earth’s surface, but the area of the examined portionof each formation separately with the whole earth. For example, during the Silurian period all theearth was Silurian, and animals were living and dying, and depositing their remains more or lessover the whole area of the globe, and they were probably (the species at least) nearly as varied indifferent latitudes and longitudes as at present. What proportion do the Silurian districts bear tothe whole surface of the globe, land and sea (for far more extensive Silurian districts probablyexist beneath the ocean than above it), and what portion of the known Silurian districts has beenactually examined for fossils? Would the area of rock actually laid open to the eye be thethousandth or the ten-thousandth part of the earth’s surface? Ask the same question with regardto the Oolite or the Chalk, or even to particular beds of these when they differ considerably intheir fossils, and you may then get some notion of how small a portion of the whole we know.
But yet more important is the probability, nay almost the certainty, that whole formationscontaining the records of vast geological periods are entirely buried beneath the ocean, and forever beyond our reach. Most of the gaps in the geological series may thus be filled up, and vastnumbers of unknown and unimaginable animals, which might help to elucidate the affinities ofthe numerous isolated groups which are a perpetual puzzle to the zoologist, may there be buried,until future revolutions may raise them in their turn above the waters, to af- ford materials for the study of whatever race of intelligent beings may then have succeeded us.
These considerations must lead us to the conclusion, that our knowledge of the whole series ofthe former inhabitants of the earth is necessarily most imperfect and fragmentary,- as much so asour knowledge of the present organic world would be, were we forced to make our collectionsand observations only in spots equally limited in area and in number with those actually laidopen for the collection of fossils. Now, the hypothesis of Professor Forbes is essentially one thatassumes to a great extent the completeness of our knowledge of the whole series of organicbeings which have existed on the earth. This appears to be a fatal objection to it, independentlyof all other considerations. It may be said that the same objections exist against every theory onsuch a subject, but this is not necessarily the case. The hypothesis put forward in this paperdepends in no degree upon the completeness of our knowledge of the former condition of theorganic world, but takes what facts we have as fragments of a vast whole, and deduces fromthem something of the nature and proportions of that whole which we can never know in detail.
It is founded upon isolated groups of facts, recognizes their isolation, and endeavours to deducefrom them the nature of the intervening portions.
Another important series of facts, quite in accordance with, and even necessary deductions from,the law now developed, are those of rudimentary organs.
That these really do exist, and in most cases have no special function in the ani- mal economy, is admitted by the first authorities in comparative anatomy. The minute limbshidden beneath the skin in many of the snake-like lizards, the anal hooks of the boa constrictor,the complete series of jointed finger-bones in the paddle of the Manatus and whale, are a few ofthe most familiar instances. In botany a similar class of facts has long been recognised. Abortivestamens, rudimentary floral envelopes and undeveloped carpels, are of the most frequentoccurrence. To every thoughtful naturalist the question must arise, What are these for? Whathave they to do with the great laws of creation? Do they not teach us something of the system ofNature? If each species has been created independently, and without any necessary relations withpre-existing species, what do these rudiments, these apparent imperfections mean? There must bea cause for them; they must be the necessary results of some great natural law. Now, if, as it hasbeen endeavoured to be shown, the great law which has regulated the peopling of the earth withanimal and vegetable life is, that every change shall be gradual; that no new creature shall beformed widely differing from anything before existing; that in this, as in everything else inNature, there shall be gradation and harmony,- then these rudimentary organs are necessary, andare an essential part of the system of Nature. Ere the higher Vertebrata were formed, for instance,many steps were required, and many organs had to undergo modifications from the rudimentalcondition in which only they had as yet existed. We still see remaining an antitypal sketch of awing adapted for flight in the scaly flapper of the penguin, and limbs first concealed beneath theskin, and then weakly protruding from it, were the necessary gradations before others should beformed fully adapted for locomotion. Many more of these modifications should we behold, and morecomplete series of them, had we a view of all the forms which have ceased to live. The greatgaps that exist between fishes, reptiles, birds, and mammals would then, no doubt, be softeneddown by intermediate groups, and the whole organic world would be seen to be an unbroken andharmonious system.
It has now been shown, though most briefly and imperfectly, how the law that “Every specieshas come into existence coincident both in time and space with a pre-existing closely alliedspecies,” connects together and renders intelligible a vast number of independent and hithertounexplained facts. The natural system of arrangement of organic beings, their geographicaldistribution, their geological sequence, the phaenomena of representative and substituted groupsin all their modifications, and the most singular peculiarites of anatomical structure, are allexplained and illustrated by it, in perfect accordance with the vast mass of facts which the researches of modern naturalists have brought together, and, it is believed, not materiallyopposed to any of them. It also claims a superiority over previous hypotheses, on the ground thatit not merely explains, but necessitates what exists. Granted the law, and many of the mostimportant facts in Nature could not have been otherwise, but are almost as necessary deductionsfrom it, as are the elliptic orbits of the planets from the law of gravitation.

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