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Present distribution cannot be accounted for by differences in physicalconditions -- Importance of barriers -- Affinity of the productions of thesame continent -- Centres of creation -- Means of dispersal by changes ofclimate and of the level of the land, and by occasional means -- Dispersalduring the Glacial period -- Alternate Glacial periods in the North andSouth.

In considering the distribution of organic beings over the face of theglobe, the first great fact which strikes us is, that neither thesimilarity nor the dissimilarity of the inhabitants of various regions canbe wholly accounted for by climatal and other physical conditions. Oflate, almost every author who has studied the subject has come to thisconclusion. The case of America alone would almost suffice to prove itstruth; for if we exclude the arctic and northern temperate parts, allauthors agree that one of the most fundamental divisions in geographicaldistribution is that between the New and Old Worlds; yet if we travel overthe vast American continent, from the central parts of the United States toits extreme southern point, we meet with the most diversified conditions;humid districts, arid deserts, lofty mountains, grassy plains, forests,marshes, lakes and great rivers, under almost every temperature. There ishardly a climate or condition in the Old World which cannot be paralleledin the New--at least so closely as the same species generally require. Nodoubt small areas can be pointed out in the Old World hotter than any inthe New World; but these are not inhabited by a fauna different from thatof the surrounding districts; for it is rare to find a group of organismsconfined to a small area, of which the conditions are peculiar in only aslight degree. Notwithstanding this general parallelism in the conditionsof Old and New Worlds, how widely different are their living productions!

In the southern hemisphere, if we compare large tracts of land inAustralia, South Africa, and western South America, between latitudes 25and 35 degrees, we shall find parts extremely similar in all theirconditions, yet it would not be possible to point out three faunas andfloras more utterly dissimilar. Or, again, we may compare the productionsof South America south of latitude 35 degrees with those north of 25degrees, which consequently are separated by a space of ten degrees oflatitude, and are exposed to considerably different conditions; yet theyare incomparably more closely related to each other than they are to theproductions of Australia or Africa under nearly the same climate. Analogous facts could be given with respect to the inhabitants of the sea.

A second great fact which strikes us in our general review is, thatbarriers of any kind, or obstacles to free migration, are related in aclose and important manner to the differences between the productions ofvarious regions. We see this in the great difference in nearly all theterrestrial productions of the New and Old Worlds, excepting in thenorthern parts, where the land almost joins, and where, under a slightlydifferent climate, there might have been free migration for the northerntemperate forms, as there now is for the strictly arctic productions. Wesee the same fact in the great difference between the inhabitants ofAustralia, Africa, and South America under the same latitude; for thesecountries are almost as much isolated from each other as is possible. Oneach continent, also, we see the same fact; for on the opposite sides oflofty and continuous mountain-ranges, and of great deserts and even oflarge rivers, we find different productions; though as mountain chains,deserts, etc., are not as impassable, or likely to have endured so long, asthe oceans separating continents, the differences are very inferior indegree to those characteristic of distinct continents.

Turning to the sea, we find the same law. The marine inhabitants of theeastern and western shores of South America are very distinct, withextremely few shells, crustacea, or echinodermata in common; but Dr.Gunther has recently shown that about thirty per cent of the fishes are thesame on the opposite sides of the isthmus of Panama; and this fact has lednaturalists to believe that the isthmus was formerly open. Westward of theshores of America, a wide space of open ocean extends, with not an islandas 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 Pacificwith another and totally distinct fauna. So that three marine faunas rangenorthward and southward in parallel lines not far from each other, undercorresponding climate; but from being separated from each other byimpassable barriers, either of land or open sea, they are almost whollydistinct. On the other hand, proceeding still further westward from theeastern islands of the tropical parts of the Pacific, we encounter noimpassable barriers, and we have innumerable islands as halting-places, orcontinuous coasts, until, after travelling over a hemisphere, we come tothe shores of Africa; and over this vast space we meet with no well-definedand distinct marine faunas. Although so few marine animals are common tothe above-named three approximate faunas of Eastern and Western America andthe eastern Pacific islands, yet many fishes range from the Pacific intothe Indian Ocean, and many shells are common to the eastern islands of thePacific and the eastern shores of Africa on almost exactly oppositemeridians of longitude.

A third great fact, partly included in the foregoing statement, is theaffinity of the productions of the same continent or of the same sea,though the species themselves are distinct at different points andstations. It is a law of the widest generality, and every continent offersinnumerable instances. Nevertheless, the naturalist, in travelling, forinstance, from north to south, never fails to be struck by the manner inwhich successive groups of beings, specifically distinct, though nearlyrelated, replace each other. He hears from closely allied, yet distinctkinds of birds, notes nearly similar, and sees their nests similarlyconstructed, but not quite alike, with eggs coloured in nearly the samemanner. The plains near the Straits of Magellan are inhabited by onespecies of Rhea (American ostrich), and northward the plains of La Plata byanother species of the same genus; and not by a true ostrich or emu, likethose inhabiting Africa and Australia under the same latitude. On thesesame plains of La Plata we see the agouti and bizcacha, animals havingnearly the same habits as our hares and rabbits, and belonging to the sameorder of Rodents, but they plainly display an American type of structure. We ascend the lofty peaks of the Cordillera, and we find an alpine speciesof bizcacha; we look to the waters, and we do not find the beaver ormuskrat, but the coypu and capybara, rodents of the South American type. Innumerable other instances could be given. If we look to the islands offthe American shore, however much they may differ in geological structure,the inhabitants are essentially American, though they may be all peculiarspecies. We may look back to past ages, as shown in the last chapter, andwe find American types then prevailing on the American continent and in theAmerican seas. We see in these facts some deep organic bond, throughoutspace and time, over the same areas of land and water, independently ofphysical conditions. The naturalist must be dull who is not led to inquirewhat this bond is.

The bond is simply inheritance, that cause which alone, as far as wepositively know, produces organisms quite like each other, or, as we see inthe case of varieties, nearly alike. The dissimilarity of the inhabitantsof different regions may be attributed to modification through variationand natural selection, and probably in a subordinate degree to the definiteinfluence of different physical conditions. The degrees of dissimilaritywill depend on the migration of the more dominant forms of life from oneregion into another having been more or less effectually prevented, atperiods more or less remote--on the nature and number of the formerimmigrants--and on the action of the inhabitants on each other in leadingto the preservation of different modifications; the relation of organism toorganism in the struggle for life being, as I have already often remarked,the most important of all relations. Thus the high importance of barrierscomes into play by checking migration; as does time for the slow process ofmodification through natural selection. Widely-ranging species, aboundingin individuals, which have already triumphed over many competitors in theirown widely-extended homes, will have the best chance of seizing on newplaces, when they spread out into new countries. In their new homes theywill be exposed to new conditions, and will frequently undergo furthermodification and improvement; and thus they will become still furthervictorious, and will produce groups of modified descendants. On thisprinciple of inheritance with modification we can understand how it is thatsections of genera, whole genera, and even families, are confined to thesame areas, as is so commonly and notoriously the case.

There is no evidence, as was remarked in the last chapter, of the existenceof any law of necessary development. As the variability of each species isan independent property, and will be taken advantage of by naturalselection, only so far as it profits each individual in its complexstruggle for life, so the amount of modification in different species willbe no uniform quantity. If a number of species, after having long competedwith each other in their old home, were to migrate in a body into a new andafterwards isolated country, they would be little liable to modification;for neither migration nor isolation in themselves effect anything. Theseprinciples come into play only by bringing organisms into new relationswith each other and in a lesser degree with the surrounding physicalconditions. As we have seen in the last chapter that some forms haveretained nearly the same character from an enormously remote geologicalperiod, so certain species have migrated over vast spaces, and have notbecome greatly or at all modified.

According to these views, it is obvious that the several species of thesame genus, though inhabiting the most distant quarters of the world, mustoriginally have proceeded from the same source, as they are descended fromthe same progenitor. In the case of those species which have undergone,during whole geological periods, little modification, there is not muchdifficulty in believing that they have migrated from the same region; forduring the vast geographical and climatical changes which have supervenedsince ancient times, almost any amount of migration is possible. But inmany other cases, in which we have reason to believe that the species of agenus have been produced within comparatively recent times, there is greatdifficulty on this head. It is also obvious that the individuals of thesame species, though now inhabiting distant and isolated regions, must haveproceeded from one spot, where their parents were first produced: for, ashas been explained, it is incredible that individuals identically the sameshould have been produced from parents specifically distinct.

SINGLE CENTRES OF SUPPOSED CREATION.

We are thus brought to the question which has been largely discussed bynaturalists, namely, whether species have been created at one or morepoints of the earth's surface. Undoubtedly there are many cases of extremedifficulty in understanding how the same species could possibly havemigrated from some one point to the several distant and isolated points,where now found. Nevertheless the simplicity of the view that each specieswas first produced within a single region captivates the mind. He whorejects it, rejects the vera causa of ordinary generation with subsequentmigration, and calls in the agency of a miracle. It is universallyadmitted, that in most cases the area inhabited by a species is continuous;and that when a plant or animal inhabits two points so distant from eachother, or with an interval of such a nature, that the space could not havebeen easily passed over by migration, the fact is given as somethingremarkable and exceptional. The incapacity of migrating across a wide seais more clear in the case of terrestrial mammals than perhaps with anyother organic beings; and, accordingly, we find no inexplicable instancesof the same mammals inhabiting distant points of the world. No geologistfeels any difficulty in Great Britain possessing the same quadrupeds withthe rest of Europe, for they were no doubt once united. But if the samespecies can be produced at two separate points, why do we not find a singlemammal common to Europe and Australia or South America? The conditions oflife are nearly the same, so that a multitude of European animals andplants have become naturalised in America and Australia; and some of theaboriginal plants are identically the same at these distant points of thenorthern and southern hemispheres? The answer, as I believe, is, thatmammals have not been able to migrate, whereas some plants, from theirvaried means of dispersal, have migrated across the wide and brokeninterspaces. The great and striking influence of barriers of all kinds, isintelligible only on the view that the great majority of species have beenproduced on one side, and have not been able to migrate to the oppositeside. Some few families, many subfamilies, very many genera, a stillgreater number of sections of genera, are confined to a single region; andit has been observed by several naturalists that the most natural genera,or those genera in which the species are most closely related to eachother, are generally confined to the same country, or if they have a widerange that their range is continuous. What a strange anomaly it would beif a directly opposite rule were to prevail when we go down one step lowerin the series, namely to the individuals of the same species, and these hadnot been, at least at first, confined to some one region!

Hence, it seems to me, as it has to many other naturalists, that the viewof each species having been produced in one area alone, and havingsubsequently migrated from that area as far as its powers of migration andsubsistence under past and present conditions permitted, is the mostprobable. Undoubtedly many cases occur in which we cannot explain how thesame species could have passed from one point to the other. But thegeographical and climatical changes which have certainly occurred withinrecent geological times, must have rendered discontinuous the formerlycontinuous range of many species. So that we are reduced to considerwhether the exceptions to continuity of range are so numerous, and of sograve a nature, that we ought to give up the belief, rendered probable bygeneral considerations, that each species has been produced within onearea, and has migrated thence as far as it could. It would be hopelesslytedious to discuss all the exceptional cases of the same species, nowliving at distant and separated points; nor do I for a moment pretend thatany explanation could be offered of many instances. But, after somepreliminary remarks, I will discuss a few of the most striking classes offacts, namely, the existence of the same species on the summits of distantmountain ranges, and at distant points in the Arctic and Antarctic regions;and secondly (in the following chapter), the wide distribution of freshwater productions; and thirdly, the occurrence of the same terrestrialspecies on islands and on the nearest mainland, though separated byhundreds of miles of open sea. If the existence of the same species atdistant and isolated points of the earth's surface can in many instances beexplained on the view of each species having migrated from a singlebirthplace; then, considering our ignorance with respect to formerclimatical and geographical changes, and to the various occasional means oftransport, the belief that a single birthplace is the law seems to meincomparably the safest.

In discussing this subject we shall be enabled at the same time to considera point equally important for us, namely, whether the several species of agenus which must on our theory all be descended from a common progenitor,can have migrated, undergoing modification during their migration from someone area. If, when most of the species inhabiting one region are differentfrom those of another region, though closely allied to them, it can beshown that migration from the one region to the other has probably occurredat some former period, our general view will be much strengthened; for theexplanation is obvious on the principle of descent with modification. Avolcanic island, for instance, upheaved and formed at the distance of a fewhundreds of miles from a continent, would probably receive from it in thecourse of time a few colonists, and their descendants, though modified,would still be related by inheritance to the inhabitants of that continent. Cases of this nature are common, and are, as we shall hereafter see,inexplicable on the theory of independent creation. This view of therelation of the species of one region to those of another, does not differmuch from that advanced by Mr. Wallace, who concludes that "every specieshas come into existence coincident both in space and time with apre-existing closely allied species." And it is now well known that heattributes this coincidence to descent with modification.

The question of single or multiple centres of creation differs from anotherthough allied question, namely, whether all the individuals of the samespecies are descended from a single pair, or single hermaphrodite, orwhether, as some authors suppose, from many individuals simultaneouslycreated. With organic beings which never intercross, if such exist, eachspecies, must be descended from a succession of modified varieties, thathave supplanted each other, but have never blended with other individualsor varieties of the same species, so that, at each successive stage ofmodification, all the individuals of the same form will be descended from asingle parent. But in the great majority of cases, namely, with allorganisms which habitually unite for each birth, or which occasionallyintercross, the individuals of the same species inhabiting the same areawill be kept nearly uniform by intercrossing; so that many individuals willgo on simultaneously changing, and the whole amount of modification at eachstage will not be due to descent from a single parent. To illustrate whatI mean: our English race-horses differ from the horses of every otherbreed; but they do not owe their difference and superiority to descent fromany single pair, but to continued care in the selecting and training ofmany individuals during each generation.

Before discussing the three classes of facts, which I have selected aspresenting the greatest amount of difficulty on the theory of "singlecentres of creation," I must say a few words on the means of dispersal.

MEANS OF DISPERSAL.

Sir C. Lyell and other authors have ably treated this subject. I can givehere only the briefest abstract of the more important facts. Change ofclimate must have had a powerful influence on migration. A region nowimpassable to certain organisms from the nature of its climate, might havebeen a high road for migration, when the climate was different. I shall,however, presently have to discuss this branch of the subject in somedetail. Changes of level in the land must also have been highlyinfluential: a narrow isthmus now separates two marine faunas; submergeit, or let it formerly have been submerged, and the two faunas will nowblend together, or may formerly have blended. Where the sea now extends,land may at a former period have connected islands or possibly evencontinents together, and thus have allowed terrestrial productions to passfrom one to the other. No geologist disputes that great mutations of levelhave occurred within the period of existing organisms. Edward Forbesinsisted that all the islands in the Atlantic must have been recentlyconnected with Europe or Africa, and Europe likewise with America. Otherauthors have thus hypothetically bridged over every ocean, and unitedalmost every island with some mainland. If, indeed, the arguments used byForbes are to be trusted, it must be admitted that scarcely a single islandexists which has not recently been united to some continent. This viewcuts the Gordian knot of the dispersal of the same species to the mostdistant points, and removes many a difficulty; but to the best of myjudgment we are not authorized in admitting such enormous geographicalchanges within the period of existing species. It seems to me that we haveabundant evidence of great oscillations in the level of the land or sea;but not of such vast changes in the position and extension of ourcontinents, as to have united them within the recent period to each otherand to the several intervening oceanic islands. I freely admit the formerexistence of many islands, now buried beneath the sea, which may haveserved as halting places for plants and for many animals during theirmigration. In the coral-producing oceans such sunken islands are nowmarked by rings of coral or atolls standing over them. Whenever it isfully admitted, as it will some day be, that each species has proceededfrom a single birthplace, and when in the course of time we know somethingdefinite about the means of distribution, we shall be enabled to speculatewith security on the former extension of the land. But I do not believethat it will ever be proved that within the recent period most of ourcontinents which now stand quite separate, have been continuously, oralmost continuously united with each other, and with the many existingoceanic islands. Several facts in distribution--such as the greatdifference in the marine faunas on the opposite sides of almost everycontinent--the close relation of the tertiary inhabitants of several landsand even seas to their present inhabitants--the degree of affinity betweenthe mammals inhabiting islands with those of the nearest continent, beingin part determined (as we shall hereafter see) by the depth of theintervening ocean--these and other such facts are opposed to the admissionof such prodigious geographical revolutions within the recent period, asare necessary on the view advanced by Forbes and admitted by his followers. The nature and relative proportions of the inhabitants of oceanic islandsare likewise opposed to the belief of their former continuity ofcontinents. Nor does the almost universally volcanic composition of suchislands favour the admission that they are the wrecks of sunken continents;if they had originally existed as continental mountain ranges, some atleast of the islands would have been formed, like other mountain summits,of granite, metamorphic schists, old fossiliferous and other rocks, insteadof consisting of mere piles of volcanic matter.

I must now say a few words on what are called accidental means, but whichmore properly should be called occasional means of distribution. I shallhere confine myself to plants. In botanical works, this or that plant isoften stated to be ill adapted for wide dissemination; but the greater orless facilities for transport across the sea may be said to be almostwholly unknown. Until I tried, with Mr. Berkeley's aid, a few experiments,it was not even known how far seeds could resist the injurious action ofsea-water. To my surprise I found that out of eighty-seven kinds, sixty-four germinated after an immersion of twenty-eight days, and a few survivedan immersion of 137 days. It deserves notice that certain orders were farmore injured than others: nine Leguminosae were tried, and, with oneexception, they resisted the salt-water badly; seven species of the alliedorders, Hydrophyllaceae and Polemoniaceae, were all killed by a month'simmersion. For convenience sake I chiefly tried small seeds without thecapsules or fruit; and as all of these sank in a few days, they could nothave been floated across wide spaces of the sea, whether or not they wereinjured by salt water. Afterwards I tried some larger fruits, capsules,etc., and some of these floated for a long time. It is well known what adifference there is in the buoyancy of green and seasoned timber; and itoccurred to me that floods would often wash into the sea dried plants orbranches with seed-capsules or fruit attached to them. Hence I was led todry the stems and branches of ninety-four plants with ripe fruit, and toplace them on sea-water. The majority sank quickly, but some which, whilstgreen, floated for a very short time, when dried floated much longer; forinstance, ripe hazel-nuts sank immediately, but when dried they floated forninety days, and afterwards when planted germinated; an asparagus plantwith ripe berries floated for twenty-three days, when dried it floated foreighty-five days, and the seeds afterwards germinated: the ripe seeds ofHelosciadium sank in two days, when dried they floated for above ninetydays, and afterwards germinated. Altogether, out of the ninety-four driedplants, eighteen floated for above twenty-eight days; and some of theeighteen floated for a very much longer period. So that as 64/87 kinds ofseeds germinated after an immersion of twenty-eight days; and as 18/94distinct species with ripe fruit (but not all the same species as in theforegoing experiment) floated, after being dried, for above twenty-eightdays, we may conclude, as far as anything can be inferred from these scantyfacts, that the seeds of 14/100 kinds of plants of any country might befloated by sea-currents during twenty-eight days, and would retain theirpower of germination. In Johnston's Physical Atlas, the average rate ofthe several Atlantic currents is thirty-three miles per diem (some currentsrunning at the rate of sixty miles per diem); on this average, the seeds of14/100 plants belonging to one country might be floated across 924 miles ofsea to another country; and when stranded, if blown by an inland gale to afavourable spot, would germinate.

Subsequently to my experiments, M. Martens tried similar ones, but in amuch better manner, for he placed the seeds in a box in the actual sea, sothat they were alternately wet and exposed to the air like really floatingplants. He tried ninety-eight seeds, mostly different from mine, but hechose many large fruits, and likewise seeds, from plants which live nearthe sea; and this would have favoured both the average length of theirflotation and their resistance to the injurious action of the salt-water. On the other hand, he did not previously dry the plants or branches withthe fruit; and this, as we have seen, would have caused some of them tohave floated much longer. The result was that 18/98 of his seeds ofdifferent kinds floated for forty-two days, and were then capable ofgermination. But I do not doubt that plants exposed to the waves wouldfloat for a less time than those protected from violent movement as in ourexperiments. Therefore, it would perhaps be safer to assume that the seedsof about 10/100 plants of a flora, after having been dried, could befloated across a space of sea 900 miles in width, and would then germinate. The fact of the larger fruits often floating longer than the small, isinteresting; as plants with large seeds or fruit which, as Alph. deCandolle has shown, generally have restricted ranges, could hardly betransported by any other means.

Seeds may be occasionally transported in another manner. Drift timber isthrown up on most islands, even on those in the midst of the widest oceans;and the natives of the coral islands in the Pacific procure stones fortheir tools, solely from the roots of drifted trees, these stones being avaluable royal tax. I find that when irregularly shaped stones areembedded in the roots of trees, small parcels of earth are very frequentlyenclosed in their interstices and behind them, so perfectly that not aparticle could be washed away during the longest transport: out of onesmall portion of earth thus COMPLETELY enclosed by the roots of an oakabout fifty years old, three dicotyledonous plants germinated: I amcertain of the accuracy of this observation. Again, I can show that thecarcasses of birds, when floating on the sea, sometimes escape beingimmediately devoured; and many kinds of seeds in the crops of floatingbirds long retain their vitality: peas and vetches, for instance, arekilled by even a few days' immersion in sea-water; but some taken out ofthe crop of a pigeon, which had floated on artificial sea-water for thirtydays, to my surprise nearly all germinated.

Living birds can hardly fail to be highly effective agents in thetransportation of seeds. I could give many facts showing how frequentlybirds of many kinds are blown by gales to vast distances across the ocean. We may safely assume that under such circumstances their rate of flightwould often be thirty-five miles an hour; and some authors have given a farhigher estimate. I have never seen an instance of nutritious seeds passingthrough the intestines of a bird; but hard seeds of fruit pass uninjuredthrough even the digestive organs of a turkey. In the course of twomonths, I picked up in my garden twelve kinds of seeds, out of theexcrement of small birds, and these seemed perfect, and some of them, whichwere tried, germinated. But the following fact is more important: thecrops of birds do not secrete gastric juice, and do not, as I know bytrial, injure in the least the germination of seeds; now, after a bird hasfound and devoured a large supply of food, it is positively asserted thatall the grains do not pass into the gizzard for twelve or even eighteenhours. A bird in this interval might easily be blown to the distance offive hundred miles, and hawks are known to look out for tired birds, andthe contents of their torn crops might thus readily get scattered. Somehawks and owls bolt their prey whole, and after an interval of from twelveto twenty hours, disgorge pellets, which, as I know from experiments madein the Zoological Gardens, include seeds capable of germination. Someseeds of the oat, wheat, millet, canary, hemp, clover, and beet germinatedafter having been from twelve to twenty-one hours in the stomachs ofdifferent birds of prey; and two seeds of beet grew after having been thusretained for two days and fourteen hours. Fresh-water fish, I find, eatseeds of many land and water plants; fish are frequently devoured by birds,and thus the seeds might be transported from place to place. I forced manykinds of seeds into the stomachs of dead fish, and then gave their bodiesto fishing-eagles, storks, and pelicans; these birds, after an interval ofmany hours, either rejected the seeds in pellets or passed them in theirexcrement; and several of these seeds retained the power of germination. Certain seeds, however, were always killed by this process.

Locusts are sometimes blown to great distances from the land. I myselfcaught one 370 miles from the coast of Africa, and have heard of otherscaught at greater distances. The Rev. R.T. Lowe informed Sir C. Lyell thatin November, 1844, swarms of locusts visited the island of Madeira. Theywere in countless numbers, as thick as the flakes of snow in the heaviestsnowstorm, and extended upward as far as could be seen with a telescope. During two or three days they slowly careered round and round in an immenseellipse, at least five or six miles in diameter, and at night alighted onthe taller trees, which were completely coated with them. They thendisappeared over the sea, as suddenly as they had appeared, and have notsince visited the island. Now, in parts of Natal it is believed by somefarmers, though on insufficient evidence, that injurious seeds areintroduced into their grass-land in the dung left by the great flights oflocusts which often visit that country. In consequence of this belief Mr.Weale sent me in a letter a small packet of the dried pellets, out of whichI extracted under the microscope several seeds, and raised from them sevengrass plants, belonging to two species, of two genera. Hence a swarm oflocusts, such as that which visited Madeira, might readily be the means ofintroducing several kinds of plants into an island lying far from themainland.

Although the beaks and feet of birds are generally clean, earth sometimesadheres to them: in one case I removed sixty-one grains, and in anothercase twenty-two grains of dry argillaceous earth from the foot of apartridge, and in the earth there was a pebble as large as the seed of avetch. Here is a better case: the leg of a woodcock was sent to me by afriend, with a little cake of dry earth attached to the shank, weighingonly nine grains; and this contained a seed of the toad-rush (Juncusbufonius) which germinated and flowered. Mr. Swaysland, of Brighton, whoduring the last forty years has paid close attention to our migratorybirds, informs me that he has often shot wagtails (Motacillae), wheatears,and whinchats (Saxicolae), on their first arrival on our shores, beforethey had alighted; and he has several times noticed little cakes of earthattached to their feet. Many facts could be given showing how generallysoil is charged with seeds. For instance, Professor Newton sent me the legof a red-legged partridge (Caccabis rufa) which had been wounded and couldnot fly, with a ball of hard earth adhering to it, and weighing six and ahalf ounces. The earth had been kept for three years, but when broken,watered and placed under a bell glass, no less than eighty-two plantssprung from it: these consisted of twelve monocotyledons, including thecommon oat, and at least one kind of grass, and of seventy dicotyledons,which consisted, judging from the young leaves, of at least three distinctspecies. With such facts before us, can we doubt that the many birds whichare annually blown by gales across great spaces of ocean, and whichannually migrate--for instance, the millions of quails across theMediterranean--must occasionally transport a few seeds embedded in dirtadhering to their feet or beaks? But I shall have to recur to thissubject.

As icebergs are known to be sometimes loaded with earth and stones, andhave even carried brushwood, bones, and the nest of a land-bird, it canhardly be doubted that they must occasionally, as suggested by Lyell, havetransported seeds from one part to another of the arctic and antarcticregions; and during the Glacial period from one part of the now temperateregions to another. In the Azores, from the large number of plants commonto Europe, in comparison with the species on the other islands of theAtlantic, which stand nearer to the mainland, and (as remarked by Mr. H.C.Watson) from their somewhat northern character, in comparison with thelatitude, I suspected that these islands had been partly stocked byice-borne seeds during the Glacial epoch. At my request Sir C. Lyell wroteto M. Hartung to inquire whether he had observed erratic boulders on theseislands, and he answered that he had found large fragments of granite andother rocks, which do not occur in the archipelago. Hence we may safelyinfer that icebergs formerly landed their rocky burdens on the shores ofthese mid-ocean islands, and it is at least possible that they may havebrought thither the seeds of northern plants.

Considering that these several means of transport, and that other means,which without doubt remain to be discovered, have been in action year afteryear for tens of thousands of years, it would, I think, be a marvellousfact if many plants had not thus become widely transported. These means oftransport are sometimes called accidental, but this is not strictlycorrect: the currents of the sea are not accidental, nor is the directionof prevalent gales of wind. It should be observed that scarcely any meansof transport would carry seeds for very great distances; for seeds do notretain their vitality when exposed for a great length of time to the actionof sea water; nor could they be long carried in the crops or intestines ofbirds. These means, however, would suffice for occasional transport acrosstracts of sea some hundred miles in breadth, or from island to island, orfrom a continent to a neighbouring island, but not from one distantcontinent to another. The floras of distant continents would not by suchmeans become mingled; but would remain as distinct as they now are. Thecurrents, from their course, would never bring seeds from North America toBritain, though they might and do bring seeds from the West Indies to ourwestern shores, where, if not killed by their very long immersion in saltwater, they could not endure our climate. Almost every year, one or twoland-birds are blown across the whole Atlantic Ocean, from North America tothe western shores of Ireland and England; but seeds could be transportedby these rare wanderers only by one means, namely, by dirt adhering totheir feet or beaks, which is in itself a rare accident. Even in thiscase, how small would be the chance of a seed falling on favourable soil,and coming to maturity! But it would be a great error to argue thatbecause a well-stocked island, like Great Britain, has not, as far as isknown (and it would be very difficult to prove this), received within thelast few centuries, through occasional means of transport, immigrants fromEurope or any other continent, that a poorly-stocked island, thoughstanding more remote from the mainland, would not receive colonists bysimilar means. Out of a hundred kinds of seeds or animals transported toan island, even if far less well-stocked than Britain, perhaps not morethan one would be so well fitted to its new home, as to become naturalised. But this is no valid argument against what would be effected by occasionalmeans of transport, during the long lapse of geological time, whilst theisland was being upheaved, and before it had become fully stocked withinhabitants. On almost bare land, with few or no destructive insects orbirds living there, nearly every seed which chanced to arrive, if fittedfor the climate, would germinate and survive.

DISPERSAL DURING THE GLACIAL PERIOD.

The identity of many plants and animals, on mountain-summits, separatedfrom each other by hundreds of miles of lowlands, where Alpine speciescould not possibly exist, is one of the most striking cases known of thesame species living at distant points, without the apparent possibility oftheir having migrated from one point to the other. It is indeed aremarkable fact to see so many plants of the same species living on thesnowy regions of the Alps or Pyrenees, and in the extreme northern parts ofEurope; but it is far more remarkable, that the plants on the WhiteMountains, in the United States of America, are all the same with those ofLabrador, and nearly all the same, as we hear from Asa Gray, with those onthe loftiest mountains of Europe. Even as long ago as 1747, such facts ledGmelin to conclude that the same species must have been independentlycreated at many distinct points; and we might have remained in this samebelief, had not Agassiz and others called vivid attention to the Glacialperiod, which, as we shall immediately see, affords a simple explanation ofthese facts. We have evidence of almost every conceivable kind, organicand inorganic, that, within a very recent geological period, central Europeand North America suffered under an Arctic climate. The ruins of a houseburnt by fire do not tell their tale more plainly than do the mountains ofScotland and Wales, with their scored flanks, polished surfaces, andperched boulders, of the icy streams with which their valleys were latelyfilled. So greatly has the climate of Europe changed, that in NorthernItaly, gigantic moraines, left by old glaciers, are now clothed by the vineand maize. Throughout a large part of the United States, erratic bouldersand scored rocks plainly reveal a former cold period.

The former influence of the glacial climate on the distribution of theinhabitants of Europe, as explained by Edward Forbes, is substantially asfollows. But we shall follow the changes more readily, by supposing a newglacial period slowly to come on, and then pass away, as formerly occurred. As the cold came on, and as each more southern zone became fitted for theinhabitants of the north, these would take the places of the formerinhabitants of the temperate regions. The latter, at the same time wouldtravel further and further southward, unless they were stopped by barriers,in which case they would perish. The mountains would become covered withsnow and ice, and their former Alpine inhabitants would descend to theplains. By the time that the cold had reached its maximum, we should havean arctic fauna and flora, covering the central parts of Europe, as farsouth as the Alps and Pyrenees, and even stretching into Spain. The nowtemperate regions of the United States would likewise be covered by arcticplants and animals and these would be nearly the same with those of Europe;for the present circumpolar inhabitants, which we suppose to haveeverywhere travelled southward, are remarkably uniform round the world.

As the warmth returned, the arctic forms would retreat northward, closelyfollowed up in their retreat by the productions of the more temperateregions. And as the snow melted from the bases of the mountains, thearctic forms would seize on the cleared and thawed ground, alwaysascending, as the warmth increased and the snow still further disappeared,higher and higher, whilst their brethren were pursuing their northernjourney. Hence, when the warmth had fully returned, the same species,which had lately lived together on the European and North American lowlands, would again be found in the arctic regions of the Old and NewWorlds, and on many isolated mountain-summits far distant from each other.

Thus we can understand the identity of many plants at points so immenselyremote as the mountains of the United States and those of Europe. We canthus also understand the fact that the Alpine plants of each mountain-rangeare more especially related to the arctic forms living due north or nearlydue north of them: for the first migration when the cold came on, and there-migration on the returning warmth, would generally have been due southand north. The Alpine plants, for example, of Scotland, as remarked by Mr.H.C. Watson, and those of the Pyrenees, as remarked by Ramond, are moreespecially allied to the plants of northern Scandinavia; those of theUnited States to Labrador; those of the mountains of Siberia to the arcticregions of that country. These views, grounded as they are on theperfectly well-ascertained occurrence of a former Glacial period, seem tome to explain in so satisfactory a manner the present distribution of theAlpine and Arctic productions of Europe and America, that when in otherregions we find the same species on distant mountain-summits, we may almostconclude, without other evidence, that a colder climate formerly permittedtheir migration across the intervening lowlands, now become too warm fortheir existence.

As the arctic forms moved first southward and afterwards backward to thenorth, in unison with the changing climate, they will not have been exposedduring their long migrations to any great diversity of temperature; and asthey all migrated in a body together, their mutual relations will not havebeen much disturbed. Hence, in accordance with the principles inculcatedin this volume, these forms will not have been liable to much modification. But with the Alpine productions, left isolated from the moment of thereturning warmth, first at the bases and ultimately on the summits of themountains, the case will have been somewhat different; for it is not likelythat all the same arctic species will have been left on mountain ranges fardistant from each other, and have survived there ever since; they willalso, in all probability, have become mingled with ancient Alpine species,which must have existed on the mountains before the commencement of theGlacial epoch, and which during the coldest period will have beentemporarily driven down to the plains; they will, also, have beensubsequently exposed to somewhat different climatical influences. Theirmutual relations will thus have been in some degree disturbed; consequentlythey will have been liable to modification; and they have been modified;for if we compare the present Alpine plants and animals of the severalgreat European mountain ranges, one with another, though many of thespecies remain identically the same, some exist as varieties, some asdoubtful forms or sub-species and some as distinct yet closely alliedspecies representing each other on the several ranges.

In the foregoing illustration, I have assumed that at the commencement ofour imaginary Glacial period, the arctic productions were as uniform roundthe polar regions as they are at the present day. But it is also necessaryto assume that many sub-arctic and some few temperate forms were the sameround the world, for some of the species which now exist on the lowermountain slopes and on the plains of North America and Europe are the same;and it may be asked how I account for this degree of uniformity of thesub-arctic and temperate forms round the world, at the commencement of thereal Glacial period. At the present day, the sub-arctic and northerntemperate productions of the Old and New Worlds are separated from eachother by the whole Atlantic Ocean and by the northern part of the Pacific. During the Glacial period, when the inhabitants of the Old and New Worldslived further southwards than they do at present, they must have been stillmore completely separated from each other by wider spaces of ocean; so thatit may well be asked how the same species could then or previously haveentered the two continents. The explanation, I believe, lies in the natureof the climate before the commencement of the Glacial period. At this, thenewer Pliocene period, the majority of the inhabitants of the world werespecifically the same as now, and we have good reason to believe that theclimate was warmer than at the present day. Hence, we may suppose that theorganisms which now live under latitude 60 degrees, lived during thePliocene period further north, under the Polar Circle, in latitude 66-67degrees; and that the present arctic productions then lived on the brokenland still nearer to the pole. Now, if we look at a terrestrial globe, wesee under the Polar Circle that there is almost continuous land fromwestern Europe through Siberia, to eastern America. And this continuity ofthe circumpolar land, with the consequent freedom under a more favourableclimate for intermigration, will account for the supposed uniformity of thesub-arctic and temperate productions of the Old and New Worlds, at a periodanterior to the Glacial epoch.

Believing, from reasons before alluded to, that our continents have longremained in nearly the same relative position, though subjected to greatoscillations of level, I am strongly inclined to extend the above view, andto infer that during some earlier and still warmer period, such as theolder Pliocene period, a large number of the same plants and animalsinhabited the almost continuous circumpolar land; and that these plants andanimals, both in the Old and New Worlds, began slowly to migrate southwardsas the climate became less warm, long before the commencement of theGlacial period. We now see, as I believe, their descendants, mostly in amodified condition, in the central parts of Europe and the United States. On this view we can understand the relationship with very little identity,between the productions of North America and Europe--a relationship whichis highly remarkable, considering the distance of the two areas, and theirseparation by the whole Atlantic Ocean. We can further understand thesingular fact remarked on by several observers that the productions ofEurope and America during the later tertiary stages were more closelyrelated to each other than they are at the present time; for during thesewarmer periods the northern parts of the Old and New Worlds will have beenalmost continuously united by land, serving as a bridge, since renderedimpassable by cold, for the intermigration of their inhabitants.

During the slowly decreasing warmth of the Pliocene period, as soon as thespecies in common, which inhabited the New and Old Worlds, migrated southof the Polar Circle, they will have been completely cut off from eachother. This separation, as far as the more temperate productions areconcerned, must have taken place long ages ago. As the plants and animalsmigrated southward, they will have become mingled in the one great regionwith the native American productions, and would have had to compete withthem; and in the other great region, with those of the Old World. Consequently we have here everything favourable for much modification--forfar more modification than with the Alpine productions, left isolated,within a much more recent period, on the several mountain ranges and on thearctic lands of Europe and North America. Hence, it has come, that when wecompare the now living productions of the temperate regions of the New andOld Worlds, we find very few identical species (though Asa Gray has latelyshown that more plants are identical than was formerly supposed), but wefind in every great class many forms, which some naturalists rank asgeographical races, and others as distinct species; and a host of closelyallied or representative forms which are ranked by all naturalists asspecifically distinct.

As on the land, so in the waters of the sea, a slow southern migration of amarine fauna, which, during the Pliocene or even a somewhat earlier period,was nearly uniform along the continuous shores of the Polar Circle, willaccount, on the theory of modification, for many closely allied forms nowliving in marine areas completely sundered. Thus, I think, we canunderstand the presence of some closely allied, still existing and extincttertiary forms, on the eastern and western shores of temperate NorthAmerica; and the still more striking fact of many closely alliedcrustaceans (as described in Dana's admirable work), some fish and othermarine animals, inhabiting the Mediterranean and the seas of Japan--thesetwo areas being now completely separated by the breadth of a wholecontinent and by wide spaces of ocean.

These cases of close relationship in species either now or formerlyinhabiting the seas on the eastern and western shores of North America, theMediterranean and Japan, and the temperate lands of North America andEurope, are inexplicable on the theory of creation. We cannot maintainthat such species have been created alike, in correspondence with thenearly similar physical conditions of the areas; for if we compare, forinstance, certain parts of South America with parts of South Africa orAustralia, we see countries closely similar in all their physicalconditions, with their inhabitants utterly dissimilar.

ALTERNATE GLACIAL PERIODS IN THE NORTH AND SOUTH.

But we must return to our more immediate subject. I am convinced thatForbes's view may be largely extended. In Europe we meet with the plainestevidence of the Glacial period, from the western shores of Britain to theUral range, and southward to the Pyrenees. We may infer from the frozenmammals and nature of the mountain vegetation, that Siberia was similarlyaffected. In the Lebanon, according to Dr. Hooker, perpetual snow formerlycovered the central axis, and fed glaciers which rolled 4,000 feet down thevalleys. The same observer has recently found great moraines at a lowlevel on the Atlas range in North Africa. Along the Himalaya, at points900 miles apart, glaciers have left the marks of their former low descent;and in Sikkim, Dr. Hooker saw maize growing on ancient and giganticmoraines. Southward of the Asiatic continent, on the opposite side of theequator, we know, from the excellent researches of Dr. J. Haast and Dr.Hector, that in New Zealand immense glaciers formerly descended to a lowlevel; and the same plants, found by Dr. Hooker on widely separatedmountains in this island tell the same story of a former cold period. Fromfacts communicated to me by the Rev. W.B. Clarke, it appears also thatthere are traces of former glacial action on the mountains of the south-eastern corner of Australia.

Looking to America: in the northern half, ice-borne fragments of rock havebeen observed on the eastern side of the continent, as far south aslatitude 36 and 37 degrees, and on the shores of the Pacific, where theclimate is now so different, as far south as latitude 46 degrees. Erraticboulders have, also, been noticed on the Rocky Mountains. In theCordillera of South America, nearly under the equator, glaciers onceextended far below their present level. In central Chile I examined a vastmound of detritus with great boulders, crossing the Portillo valley, which,there can hardly be a doubt, once formed a huge moraine; and Mr. D. Forbesinforms me that he found in various parts of the Cordillera, from latitude13 to 30 degrees south, at about the height of 12,000 feet, deeply-furrowedrocks, resembling those with which he was familiar in Norway, and likewisegreat masses of detritus, including grooved pebbles. Along this wholespace of the Cordillera true glaciers do not now exist even at much moreconsiderable heights. Further south, on both sides of the continent, fromlatitude 41 degrees to the southernmost extremity, we have the clearestevidence of former glacial action, in numerous immense boulders transportedfar from their parent source.

>From these several facts, namely, from the glacial action having extendedall round the northern and southern hemispheres--from the period havingbeen in a geological sense recent in both hemispheres--from its havinglasted in both during a great length of time, as may be inferred from theamount of work effected--and lastly, from glaciers having recentlydescended to a low level along the whole line of the Cordillera, it at onetime appeared to me that we could not avoid the conclusion that thetemperature of the whole world had been simultaneously lowered during theGlacial period. But now, Mr. Croll, in a series of admirable memoirs, hasattempted to show that a glacial condition of climate is the result ofvarious physical causes, brought into operation by an increase in theeccentricity of the earth's orbit. All these causes tend towards the sameend; but the most powerful appears to be the indirect influence of theeccentricity of the orbit upon oceanic currents. According to Mr. Croll,cold periods regularly recur every ten or fifteen thousand years; and theseat long intervals are extremely severe, owing to certain contingencies, ofwhich the most important, as Sir C. Lyell has shown, is the relativeposition of the land and water. Mr. Croll believes that the last greatglacial period occurred about 240,000 years ago, and endured, with slightalterations of climate, for about 160,000 years. With respect to moreancient glacial periods, several geologists are convinced, from directevidence, that such occurred during the miocene and eocene formations, notto mention still more ancient formations. But the most important resultfor us, arrived at by Mr. Croll, is that whenever the northern hemispherepasses through a cold period the temperature of the southern hemisphere isactually raised, with the winters rendered much milder, chiefly throughchanges in the direction of the ocean currents. So conversely it will bewith the northern hemisphere, while the southern passes through a glacialperiod. This conclusion throws so much light on geographical distributionthat I am strongly inclined to trust in it; but I will first give the factswhich demand an explanation.

In South America, Dr. Hooker has shown that besides many closely alliedspecies, between forty and fifty of the flowering plants of Tierra delFuego, forming no inconsiderable part of its scanty flora, are common toNorth America and Europe, enormously remote as these areas in oppositehemispheres are from each other. On the lofty mountains of equatorialAmerica a host of peculiar species belonging to European genera occur. Onthe Organ Mountains of Brazil some few temperate European, some Antarcticand some Andean genera were found by Gardner which do not exist in the lowintervening hot countries. On the Silla of Caraccas the illustriousHumboldt long ago found species belonging to genera characteristic of theCordillera.

In Africa, several forms characteristic of Europe, and some fewrepresentatives of the flora of the Cape of Good Hope, occur on themountains of Abyssinia. At the Cape of Good Hope a very few Europeanspecies, believed not to have been introduced by man, and on the mountainsseveral representative European forms are found which have not beendiscovered in the intertropical parts of Africa. Dr. Hooker has alsolately shown that several of the plants living on the upper parts of thelofty island of Fernando Po, and on the neighbouring Cameroon Mountains, inthe Gulf of Guinea, are closely related to those on the mountains ofAbyssinia, and likewise to those of temperate Europe. It now also appears,as I hear from Dr. Hooker, that some of these same temperate plants havebeen discovered by the Rev. R.T. Lowe on the mountains of the Cape VerdeIslands. This extension of the same temperate forms, almost under theequator, across the whole continent of Africa and to the mountains of theCape Verde archipelago, is one of the most astonishing facts ever recordedin the distribution of plants.

On the Himalaya, and on the isolated mountain ranges of the peninsula ofIndia, on the heights of Ceylon, and on the volcanic cones of Java, manyplants occur either identically the same or representing each other, and atthe same time representing plants of Europe not found in the interveninghot lowlands. A list of the genera of plants collected on the loftierpeaks of Java, raises a picture of a collection made on a hillock inEurope. Still more striking is the fact that peculiar Australian forms arerepresented by certain plants growing on the summits of the mountains ofBorneo. Some of these Australian forms, as I hear from Dr. Hooker, extendalong the heights of the peninsula of Malacca, and are thinly scattered onthe one hand over India, and on the other hand as far north as Japan.

On the southern mountains of Australia, Dr. F. Muller has discoveredseveral European species; other species, not introduced by man, occur onthe lowlands; and a long list can be given, as I am informed by Dr. Hooker,of European genera, found in Australia, but not in the intermediate torridregions. In the admirable "Introduction to the Flora of New Zealand," byDr. Hooker, analogous and striking facts are given in regard to the plantsof that large island. Hence, we see that certain plants growing on themore lofty mountains of the tropics in all parts of the world, and on thetemperate plains of the north and south, are either the same species orvarieties of the same species. It should, however, be observed that theseplants are not strictly arctic forms; for, as Mr. H.C. Watson has remarked,"in receding from polar toward equatorial latitudes, the Alpine or mountainflora really become less and less Arctic." Besides these identical andclosely allied forms, many species inhabiting the same widely sunderedareas, belong to genera not now found in the intermediate tropicallowlands.

These brief remarks apply to plants alone; but some few analogous factscould be given in regard to terrestrial animals. In marine productions,similar cases likewise occur; as an example, I may quote a statement by thehighest authority, Prof. Dana, that "it is certainly a wonderful fact thatNew Zealand should have a closer resemblance in its crustacea to GreatBritain, its antipode, than to any other part of the world." Sir J.Richardson, also, speaks of the reappearance on the shores of New Zealand,Tasmania, etc., of northern forms of fish. Dr. Hooker informs me thattwenty-five species of Algae are common to New Zealand and to Europe, buthave not been found in the intermediate tropical seas.

>From the foregoing facts, namely, the presence of temperate forms on thehighlands across the whole of equatorial Africa, and along the Peninsula ofIndia, to Ceylon and the Malay Archipelago, and in a less well-markedmanner across the wide expanse of tropical South America, it appears almostcertain that at some former period, no doubt during the most severe part ofa Glacial period, the lowlands of these great continents were everywheretenanted under the equator by a considerable number of temperate forms. Atthis period the equatorial climate at the level of the sea was probablyabout the same with that now experienced at the height of from five to sixthousand feet under the same latitude, or perhaps even rather cooler. During this, the coldest period, the lowlands under the equator must havebeen clothed with a mingled tropical and temperate vegetation, like thatdescribed by Hooker as growing luxuriantly at the height of from four tofive thousand feet on the lower slopes of the Himalaya, but with perhaps astill greater preponderance of temperate forms. So again in themountainous island of Fernando Po, in the Gulf of Guinea, Mr. Mann foundtemperate European forms beginning to appear at the height of about fivethousand feet. On the mountains of Panama, at the height of only twothousand feet, Dr. Seemann found the vegetation like that of Mexico, "withforms of the torrid zone harmoniously blended with those of the temperate."

Now let us see whether Mr. Croll's conclusion that when the northernhemisphere suffered from the extreme cold of the great Glacial period, thesouthern hemisphere was actually warmer, throws any clear light on thepresent apparently inexplicable distribution of various organisms in thetemperate parts of both hemispheres, and on the mountains of the tropics.The Glacial period, as measured by years, must have been very long; andwhen we remember over what vast spaces some naturalised plants and animalshave spread within a few centuries, this period will have been ample forany amount of migration. As the cold became more and more intense, we knowthat Arctic forms invaded the temperate regions; and from the facts justgiven, there can hardly be a doubt that some of the more vigorous, dominantand widest-spreading temperate forms invaded the equatorial lowlands. Theinhabitants of these hot lowlands would at the same time have migrated tothe tropical and subtropical regions of the south, for the southernhemisphere was at this period warmer. On the decline of the Glacialperiod, as both hemispheres gradually recovered their former temperature,the northern temperate forms living on the lowlands under the equator,would have been driven to their former homes or have been destroyed, beingreplaced by the equatorial forms returning from the south. Some, however,of the northern temperate forms would almost certainly have ascended anyadjoining high land, where, if sufficiently lofty, they would have longsurvived like the Arctic forms on the mountains of Europe. They might havesurvived, even if the climate was not perfectly fitted for them, for thechange of temperature must have been very slow, and plants undoubtedlypossess a certain capacity for acclimatisation, as shown by theirtransmitting to their offspring different constitutional powers ofresisting heat and cold.

In the regular course of events the southern hemisphere would in its turnbe subjected to a severe Glacial period, with the northern hemisphererendered warmer; and then the southern temperate forms would invade theequatorial lowlands. The northern forms which had before been left on themountains would now descend and mingle with the southern forms. Theselatter, when the warmth returned, would return to their former homes,leaving some few species on the mountains, and carrying southward with themsome of the northern temperate forms which had descended from theirmountain fastnesses. Thus, we should have some few species identically thesame in the northern and southern temperate zones and on the mountains ofthe intermediate tropical regions. But the species left during a long timeon these mountains, or in opposite hemispheres, would have to compete withmany new forms and would be exposed to somewhat different physicalconditions; hence, they would be eminently liable to modification, andwould generally now exist as varieties or as representative species; andthis is the case. We must, also, bear in mind the occurrence in bothhemispheres of former Glacial periods; for these will account, inaccordance with the same principles, for the many quite distinct speciesinhabiting the same widely separated areas, and belonging to genera not nowfound in the intermediate torrid zones.

It is a remarkable fact, strongly insisted on by Hooker in regard toAmerica, and by Alph. de Candolle in regard to Australia, that many moreidentical or slightly modified species have migrated from the north to thesouth, than in a reversed direction. We see, however, a few southern formson the mountains of Borneo and Abyssinia. I suspect that this preponderantmigration from the north to the south is due to the greater extent of landin the north, and to the northern forms having existed in their own homesin greater numbers, and having consequently been advanced through naturalselection and competition to a higher stage of perfection, or dominatingpower, than the southern forms. And thus, when the two sets becamecommingled in the equatorial regions, during the alternations of theGlacial periods, the northern forms were the more powerful and were able tohold their places on the mountains, and afterwards migrate southward withthe southern forms; but not so the southern in regard to the northernforms. In the same manner, at the present day, we see that very manyEuropean productions cover the ground in La Plata, New Zealand, and to alesser degree in Australia, and have beaten the natives; whereas extremelyfew southern forms have become naturalised in any part of the northernhemisphere, though hides, wool, and other objects likely to carry seedshave been largely imported into Europe during the last two or threecenturies from La Plata and during the last forty or fifty years fromAustralia. The Neilgherrie Mountains in India, however, offer a partialexception; for here, as I hear from Dr. Hooker, Australian forms arerapidly sowing themselves and becoming naturalised. Before the last greatGlacial period, no doubt the intertropical mountains were stocked withendemic Alpine forms; but these have almost everywhere yielded to the moredominant forms generated in the larger areas and more efficient workshopsof the north. In many islands the native productions are nearly equalled,or even outnumbered, by those which have become naturalised; and this isthe first stage towards their extinction. Mountains are islands on theland; and their inhabitants have yielded to those produced within thelarger areas of the north, just in the same way as the inhabitants of realislands have everywhere yielded and are still yielding to continental formsnaturalised through man's agency.

The same principles apply to the distribution of terrestrial animals and ofmarine productions, in the northern and southern temperate zones, and onthe intertropical mountains. When, during the height of the Glacialperiod, the ocean-currents were widely different to what they now are, someof the inhabitants of the temperate seas might have reached the equator; ofthese a few would perhaps at once be able to migrate southwards, by keepingto the cooler currents, while others might remain and survive in the colderdepths until the southern hemisphere was in its turn subjected to a glacialclimate and permitted their further progress; in nearly the same manner as,according to Forbes, isolated spaces inhabited by Arctic productions existto the present day in the deeper parts of the northern temperate seas.

I am far from supposing that all the difficulties in regard to thedistribution and affinities of the identical and allied species, which nowlive so widely separated in the north and south, and sometimes on theintermediate mountain ranges, are removed on the views above given. Theexact lines of migration cannot be indicated. We cannot say why certainspecies and not others have migrated; why certain species have beenmodified and have given rise to new forms, while others have remainedunaltered. We cannot hope to explain such facts, until we can say why onespecies and not another becomes naturalised by man's agency in a foreignland; why one species ranges twice or thrice as far, and is twice or thriceas common, as another species within their own homes.

Various special difficulties also remain to be solved; for instance, theoccurrence, as shown by Dr. Hooker, of the same plants at points soenormously remote as Kerguelen Land, New Zealand, and Fuegia; but icebergs,as suggested by Lyell, may have been concerned in their dispersal. Theexistence at these and other distant points of the southern hemisphere, ofspecies, which, though distinct, belong to genera exclusively confined tothe south, is a more remarkable case. Some of these species are sodistinct, that we cannot suppose that there has been time since thecommencement of the last Glacial period for their migration and subsequentmodification to the necessary degree. The facts seem to indicate thatdistinct species belonging to the same genera have migrated in radiatinglines from a common centre; and I am inclined to look in the southern, asin the northern hemisphere, to a former and warmer period, before thecommencement of the last Glacial period, when the Antarctic lands, nowcovered with ice, supported a highly peculiar and isolated flora. It maybe suspected that before this flora was exterminated during the lastGlacial epoch, a few forms had been already widely dispersed to variouspoints of the southern hemisphere by occasional means of transport, and bythe aid, as halting-places, of now sunken islands. Thus the southernshores of America, Australia, and New Zealand may have become slightlytinted by the same peculiar forms of life.

Sir C. Lyell in a striking passage has speculated, in language almostidentical with mine, on the effects of great alternations of climatethroughout the world on geographical distribution. And we have now seenthat Mr. Croll's conclusion that successive Glacial periods in the onehemisphere coincide with warmer periods in the opposite hemisphere,together with the admission of the slow modification of species, explains amultitude of facts in the distribution of the same and of the allied formsof life in all parts of the globe. The living waters have flowed duringone period from the north and during another from the south, and in bothcases have reached the equator; but the stream of life has flowed withgreater force from the north than in the opposite direction, and hasconsequently more freely inundated the south. As the tide leaves its driftin horizontal lines, rising higher on the shores where the tide riseshighest, so have the living waters left their living drift on our mountainsummits, in a line gently rising from the Arctic lowlands to a greatlatitude under the equator. The various beings thus left stranded may becompared with savage races of man, driven up and surviving in the mountainfastnesses of almost every land, which serves as a record, full of interestto us, of the former inhabitants of the surrounding lowlands.