_Summary and Conclusion on the New Zealand Flora._--Confining ourselves strictly to the direct relations between the plants of New Zealand and of Australia, as I have done in the preceding discussion, I think I may claim to have shown that the union between the two countries in the latter part of the Secondary epoch at a time when Eastern Australia was widely separated from Western Australia (as shown by its geological formation and by the contour of the sea-bottom) does sufficiently account for all the main features of the New Zealand flora. It shows why the basis of the flora is fundamentally Australian both as regards orders and genera, for it was due either to a direct land connection or a somewhat close approximation between the two countries. It shows also why the great ma.s.s of typical Australian forms are unrepresented, for the Australian flora is typically _western_ and _temperate_, and New Zealand received its immigrants from the _eastern_ island which had itself received only a fragment of this flora, and from the _tropical_ end of this island, and thus could only receive such forms as were not exclusively temperate in character. It shows, further, why New Zealand contains such a very large proportion of tropical forms, for we see that it derived the main portion of its flora directly from the tropics. Again, this hypothesis shows us why, though {507} the specially Australian _genera_ in New Zealand are largely tropical or sub-tropical, the specially Australian _species_ are wholly temperate or alpine; for these are comparatively recent arrivals, they must have migrated across the sea in the temperate zone, and these temperate and alpine forms are exactly such as would be best able to establish themselves in a country already stocked mainly by tropical forms and their modified descendants. This hypothesis further fulfils the conditions implied in Sir Joseph Hooker"s antic.i.p.ation that--"these great differences (of the floras) will present the least difficulties to whatever theory may explain the whole case,"--for it shows that these differences are directly due to the history and development of the Australian flora itself, while the resemblances depend upon the most certain cause of all such broad resemblances--close proximity or actual land connection.
One objection will undoubtedly be made to the above theory,--that it does not explain why some species of the prominent Australian genera Acacia, Eucalyptus, Melaleuca, Grevillea, &c., have not reached New Zealand in recent times along with the other temperate forms that have established themselves. But it is doubtful whether any detailed explanation of such a negative fact is possible, while general explanations sufficient to cover it are not wanting. Nothing is more certain than that numerous plants never run wild and establish themselves in countries where they nevertheless grow freely if cultivated; and the explanation of this fact given by Mr.
Darwin--that they are prevented doing so by the compet.i.tion of better adapted forms--is held to be sufficient. In this particular case, however, we have some very remarkable evidence of the fact of their non-adaptation.
The intercourse between New Zealand and Europe has been the means of introducing a host of common European plants,--more than 150 in number, as enumerated at the end of the second volume of the _Handbook_; yet, although the intercourse with Australia has probably been greater, only two or three Australian plants have similarly established themselves. More remarkable still, Sir Joseph Hooker states: {508} "I am informed that the late Mr.
Bidwell habitually scattered Australian seeds during his extensive travels in New Zealand." We may be pretty sure that seeds of such excessively common and characteristic groups as _Acacia_ and _Eucalyptus_ would be among those so scattered, yet we have no record of any plants of these or other peculiar Australian genera ever having been found wild, still less of their having spread and taken possession of the soil in the way that many European plants have done. We are, then, ent.i.tled to conclude that the plants above referred to have not established themselves in New Zealand (although their seeds may have reached it) because they could not successfully compete with the indigenous flora which was already well established and better adapted to the conditions of climate and of the organic environment. This explanation is so perfectly in accordance with a large body of well-known facts, including that which is known to every one--how few of our oldest and hardiest garden plants ever run wild--that the objection above stated will, I feel convinced, have no real weight with any naturalists who have paid attention to this cla.s.s of questions.
{509}
CHAPTER XXIII
ON THE ARCTIC ELEMENT IN SOUTH TEMPERATE FLORAS
European Species and Genera of Plants in the Southern Hemisphere--Aggressive Power of the Scandinavian Flora--Means by which Plants have Migrated from North to South--Newly moved Soil as Affording Temporary Stations to Migrating Plants--Elevation and Depression of the Snow-line as Aiding the Migration of Plants--Changes of Climate Favourable to Migration--The Migration from North to South has been long going on--Geological Changes as Aiding Migration--Proofs of Migration by way of the Andes--Proofs of Migration by way of the Himalayas and Southern Asia--Proofs of Migration by way of the African Highlands--Supposed Connection of South Africa and Australia--The Endemic Genera of Plants in New Zealand--The Absence of Southern Types from the Northern Hemisphere--Concluding Remarks on the New Zealand and South Temperate Floras.
We have now to deal with another portion of the New Zealand flora which presents perhaps equal difficulties--that which appears to have been derived from remote parts of the north and south temperate zones; and this will lead us to inquire into the origin of the northern or Arctic element in all the south temperate floras.
More than one-third of the entire number of New Zealand genera (115) are found also in Europe, and even fifty-eight species are identical in these remote parts of the world. Temperate South America has seventy-four genera in common with New Zealand, and there are even eleven species identical in the two countries, as well as thirty-two which are close allies or representative species. {510} A considerable number of these northern or Antarctic plants and many more which are representative species, are found also in Tasmania and in the mountains of temperate Australia; and Sir Joseph Hooker gives a list of thirty-eight species very characteristic of Europe and Northern Asia, but almost or quite unknown in the warmer regions, which yet reappear in temperate Australia. Other genera seem altogether Antarctic--that is, confined to the extreme southern lands and islands; and these often have representative species in Southern America, Tasmania, and New Zealand, while others occur only in one or two of these areas. Many north temperate genera also occur in the mountains of South Africa. On the other hand, few if any of the peculiar Australian or Antarctic types have spread northwards, except some of the former which have reached the mountains of Borneo, and a few of the latter which spread along the Andes to Mexico.
On these remarkable facts, of which I have given but the barest outline, Sir Joseph Hooker makes the following suggestive observations:--
"When I take a comprehensive view of the vegetation of the Old World, I am struck with the appearance it presents of there being a continuous current of vegetation (if I may so fancifully express myself) from Scandinavia to Tasmania; along, in short, the whole extent of that arc of the terrestrial sphere which presents the greatest continuity of land. In the first place Scandinavian genera, and even species, reappear everywhere from Lapland and Iceland to the tops of the Tasmanian Alps, in rapidly diminishing numbers it is true, but in vigorous development throughout. They abound on the Alps and Pyrenees, pa.s.s on to the Caucasus and Himalayas, thence they extend along the Khasia Mountains, and those of the peninsulas of India to those of Ceylon and the Malayan Archipelago (Java and Borneo), and after a hiatus of 30 they appear on the Alps of New South Wales, Victoria, and Tasmania, and beyond these again on those of New Zealand and the Antarctic Islands, many of the species remaining unchanged throughout! It matters not what the vegetation of the bases and flanks of these mountains may be; the northern species may be {511} a.s.sociated with alpine forms of Germanic, Siberian, Oriental, Chinese, American, Malayan, and finally Australian, and Antarctic types; but whereas these are all, more or less, local a.s.semblages, the Scandinavian a.s.serts his prerogative of ubiquity from Britain to beyond its antipodes."[137]
It is impossible to place the main facts more forcibly before the reader than in the above striking pa.s.sage. It shows clearly that this portion of the New Zealand flora is due to wide-spread causes which have acted with even greater effect in other south temperate lands, and that in order to explain its origin we must grapple with the entire problem of the transfer of the north temperate flora to the southern hemisphere. Taking, therefore, the facts as given by Sir Joseph Hooker in the works already referred to, I shall discuss the whole question broadly, and shall endeavour to point out the general laws and subordinate causes that, in my opinion, have been at work in bringing about the anomalous phenomena of distribution he has done so much to make known and to elucidate.
_Aggressive Power of the Scandinavian Flora._--The first important fact bearing upon this question is the wonderful aggressive and colonising power of the Scandinavian flora, as shown by the way in which it establishes itself in any temperate country to which it may gain access. About 150 species have thus established themselves in New Zealand, often taking possession of large tracts of country; about the same number are found in Australia, and nearly as many in the Atlantic states of America, where they form the commonest weeds. Whether or not we accept Mr. Darwin"s explanation of this power as due to development in the most extensive land area of the globe where compet.i.tion has been most severe and long-continued, the fact of the existence of this power remains, and we can see how important an agent it must be in the formation of the floras of any lands to which these aggressive plants have been able to gain access.
But not only are these plants pre-eminently capable of holding their own in any temperate country in the world, but they also have exceptional powers of migration and {512} dispersal over seas and oceans. This is especially well shown by the case of the Azores, where no less than 400 out of a total of 478 flowering plants are identical with European species. These islands are more than 800 miles from Europe, and, as we have already seen in Chapter XII., there is no reason for supposing that they have ever been more nearly connected with it than they are now, since an extension of the European coast to the 1,000-fathom line would very little reduce the distance. Now it is a most interesting and suggestive fact that more than half the European genera which occur in the Australian flora occur also in the Azores, and in several cases even the species are identical in both.[138] The importance of such a case as this cannot be exaggerated, because it affords a demonstration of the power of the very plants in question to pa.s.s over wide areas of sea, some no doubt wholly through the air, carried by storms in the same way as the European birds and insects which annually reach the Azores, others by floating on the waters, or by a combination of the two methods; while some may have been carried by aquatic birds, to whose feathers many seeds have the power of attaching themselves, and some even in the stomachs of fruit or seed eating birds. We have in such facts as these a complete disproof of the necessity for those great changes of sea and land which are continually appealed to by those who think land-connection the only efficient means of accounting for the migration of animals or plants; but at the same time we do not neglect to make the fullest use of such moderate changes as all the evidence at our command leads us to believe have actually occurred, and especially of the former existence of intermediate islands, so often indicated by shoals in the midst of the deepest oceans.
_Means by which Plants have migrated from North to South._--But if plants can thus pa.s.s in considerable numbers and variety over wide seas and oceans, it must be yet more easy for them to traverse continuous areas of land, whereever mountain-chains offer suitable stations at moderate {513} intervals on which they might temporarily establish themselves. The facilities afforded for the transmission of plants by mountains has hardly received sufficient attention. The numerous land-slips, the fresh surfaces of broken rock and precipice, the _debris_ of torrents, and the moraines deposited by glaciers, afford numerous unoccupied stations on which wind-borne seeds have a good chance of germinating. It is a well-known fact that fresh surfaces of soil or rock, such as are presented by railway cuttings and embankments, often produce plants strange to the locality, which survive for a few years, and then disappear as the normal vegetation gains strength and permanence.[139] But such a surface {514} will, in the meantime, have acted as a fresh centre of dispersal; and thus a plant might pa.s.s on step by step, by means of stations temporarily occupied, till it reached a district {515} where, the general conditions being more favourable, it was able to establish itself as a permanent member of the flora. Such, generally speaking, was probably the process by which the Scandinavian flora has made its way to the southern hemisphere; but it could hardly have done so to any important extent without the aid of those powerful causes explained in our eighth chapter--causes which acted as a constantly recurrent motive-power to produce that "continuous current of vegetation" from north to south across the whole width of the tropics referred to by Sir Joseph Hooker. Those causes were, the repeated changes {516} of climate which, during all geological time, appear to have occurred in both hemispheres, culminating at rare intervals in glacial epochs, and which have been shown to depend upon changes of excentricity of the earth"s...o...b..t and the occurrence of summer or winter in _aphelion_, in conjunction with the slower and more irregular changes of geographical conditions; these combined causes acting chiefly through the agency of heat-bearing oceanic currents, and of snow- and ice-collecting highlands. Let us now briefly consider how such changes would act in favouring the dispersal of plants.
_Elevation and Depression of the Snow Line as Aiding the Migration of Plants._--We have endeavoured to show (in an earlier portion of this volume) that wherever geographical or physical conditions were such as to produce any considerable amount of perpetual snow, this would be increased whenever a high degree of excentricity concurred with winter in _aphelion_, and diminished during the opposite phase. On all mountain ranges, therefore, which reached above the snow-line, there would be a periodical increase and decrease of snow, and when there were extensive areas of plateau at about the same level, the lowering of the snow-line might cause such an increased acc.u.mulation of snow as to produce great glaciers and ice-fields, such as we have seen occurred in South Africa during the last period of high excentricity. But along with such depression of the line of perpetual snow there would be a corresponding depression of the alpine and sub-alpine zones suitable for the growth of an arctic and temperate vegetation, and, what is perhaps more important, the depression would necessarily produce a great _extension_ of the area of these zones on all high mountains, because as we descend the average slopes become less abrupt,--thus affording a number of new stations suitable for such temperate plants as might first reach them. But just above and below the snow-line is the area of most powerful disintegration and denudation, from the alternate action of frost and sun, of ice and water; and thus the more extended area would be subject to the constant occurrence of land-slips, berg-falls, and floods, with their {517} accompanying acc.u.mulations of _debris_ and of alluvial soil, affording innumerable stations in which solitary wind-borne seeds might germinate and temporarily establish themselves.
This lowering and rising of the snow-line each 10,500 years during periods of high excentricity, would occur in the northern and southern hemispheres alternately; and where there were high mountains within the tropics the two would probably overlap each other, so that the northern depression would make itself felt in a slight degree even across the equator some way into the southern hemisphere, and _vice versa_; and even if the difference of the height of perpetual snow at the two extremes did not average more than a few hundred feet, this would be amply sufficient to supply the new and unoccupied stations needful to facilitate the migration of plants. It is well known that all great mountain ranges have undergone such fluctuations, as proved by ice-marks below the present level of snow and ice.
But the differences of temperature in the two hemispheres caused by the sun being in _perihelion_ in the winter of the one while it was in _aphelion_ during the same season in the other, would necessarily lead to increased aerial and marine currents, as already explained; and whenever geographical conditions were such as to favour the production of glaciation in any area these effects would become more powerful, and would further aid in the dispersal of the seeds of plants.
_Changes of Climate Favourable to Migration._--It is clear then, that during periods when no glacial epochs were produced in the northern hemisphere, and even when a mild climate extended over the whole polar area, alternate changes of climate favouring the dispersal of plants would occur on all high mountains, and with particular force on such as rise above the snow-line. But during that long-continued, though comparatively recent, phase of high excentricity which produced an extensive glaciation in the northern hemisphere and local glaciations in the southern, these risings and lowerings of the snow-line on all mountain ranges would have been at a maximum, and {518} would have been increased by the depression of the ocean which must have arisen from such a vast bulk of water being locked up in land-ice, and which depression would have produced the same effect as a general elevation of all the continents. At this time, too, aerial currents would have attained their maximum of force in both hemispheres; and this would greatly facilitate the dispersal of all wind-borne seeds as well as of those carried in the plumage or in the stomachs of birds, since we have seen, by the cases of the Azores and Bermuda, how vastly the migratory powers of birds are increased by a stormy atmosphere.
_Migration from North to South has been long going on._--Now, if each phase of colder and warmer mountain-climate--each alternate depression and elevation of the snow-line, only helped on the migration of a few species some stages of the long route from the north to the south temperate regions, yet, during the long course of the Tertiary period there might well have arisen that representation of the northern flora in the southern hemisphere which is now so conspicuous. For it is very important to remark that it is not the existing flora alone that is represented, such as might have been conveyed during the last glacial epoch only; but we find a whole series of northern types evidently of varying degrees of antiquity, while even some genera characteristic of the southern hemisphere appear to have been originally derived from Europe. Thus Eucalyptus and Metrosideros have been determined by Dr. Ettingshausen from their fruits in the Eocene beds of Sheppey, while Pimelea, Leptomeria and four genera of Proteaceae have been recognised by Professor Heer in the Miocene of Switzerland; and the former writer has detected fifty-five Australian forms in the Eocene plant beds of Haring (? Belgium).[140] Then we have such peculiar genera {519} as Pachychladon and Notothlaspi of New Zealand said to have affinities with Arctic plants, while Stilbocarpa--another peculiar New Zealand genus--has its nearest allies in the Himalayan and Chinese Aralias. Following these are a whole host of very distinct species of northern genera which may date back to any part of the Tertiary period, and which occur in every south temperate land. Then we have closely allied representative species of European or Arctic plants; and, lastly, a number of identical species,--and these two cla.s.ses are probably due entirely to the action of the last great glacial epoch, whose long continuance, and the repeated fluctuations of climate with which it commenced and terminated, rendered it an agent of sufficient power to have brought about this result.
Here, then, we have that constant or constantly recurrent process of dispersal acting throughout long periods with varying power--that "continuous current of vegetation" as it has been termed, which the facts demand; and the extraordinary phenomenon of the species and genera of European and even of Arctic plants being represented abundantly in South America, Australia, and New Zealand, thus adds another to the long series of phenomena which are rendered intelligible by frequent alternations of warmer and colder climates in either hemisphere, culminating, at long intervals and in favourable situations, in actual glacial epochs.
_Geological Changes as Aiding Migration._--It will be well also to notice here, that there is another aid to dispersion dependent upon the changes effected by denudation during the long periods included in the duration of the species and genera of plants. A considerable number of {520} the plants of the Miocene period of Europe were so much like existing species that although they have generally received fresh names they may well have been identical; and a large proportion of the vegetation during the whole Tertiary period consisted of genera which are still living.[141] But from what is now known of the rate of sub-aerial denudation, we are sure, that during each division of this period many mountain chains must have been considerably lowered, while we know that some of the existing ranges have been greatly elevated. Ancient volcanoes, too, have been destroyed by denudation, and new ones have been built up, so that we may be quite sure that ample means for the transmission of temperate plants across the tropics, may have existed in countries where they are now no longer to be found. The great mountain ma.s.ses of Guiana and Brazil, for example, must have been far more lofty before the sedimentary covering was denuded from their granitic bosses and metamorphic peaks, and may have aided the southern migration of plants before the final elevation of the Andes. And if Africa presents us with an example of a continent of vast antiquity, we may be sure that its great central plateaux once bore far loftier mountain ranges before they were reduced to their present condition by long ages of denudation.
_Proofs of Migration by Way of the Andes._--We are now prepared to apply the principles above laid down to the explanation of the character and affinities of the various portions of the north temperate flora in the southern hemisphere, and especially in Australia and New Zealand.
At the present time the only unbroken chain of highlands and mountains connecting the Arctic and north temperate with the Antarctic lands is to be found in the American continent, the only break of importance being the comparatively low Isthmus of Panama, where there is {521} a distance of about 300 miles occupied by rugged forest-clad hills, between the lofty peaks of Veragua and the northern extremity of the Andes of New Grenada.
Such distances are, as we have already seen, no barrier to the diffusion of plants; and we should accordingly expect that this great continuous mountain-chain has formed the most effective agent in aiding the southward migration of the Arctic and north temperate vegetation. We do find, in fact, not only that a large number of northern genera and many species are scattered all along this line of route, but that at the end of the long journey, in Southern Chile and Fuegia, they have established themselves in such numbers as to form an important part of the flora of those countries.
From the lists given in the works already referred to, it appears that there are between sixty and seventy northern genera in Fuegia and Southern Chile, while about forty of the species are absolutely identical with those of Europe and the Arctic regions. Considering how comparatively little the mountains of South Temperate America are yet known, this is a very remarkable result, and it proves that the transmission of species must have gone on up to comparatively recent times. Yet, as only a few of these species are now found along the line of migration, we see that they only occupied such stations temporarily; and we may connect their disappearance with the pa.s.sing away of the last glacial period which, by raising the snow-line, reduced the area on which alone they could exist, and exposed them to the compet.i.tion of indigenous plants from the belt of country immediately below them.
Now, just as these numerous species and genera have undoubtedly pa.s.sed along the great American range of mountains, although only now found at its two extremes, so others have doubtless pa.s.sed on further; and have found more suitable stations or less severe compet.i.tion in the Antarctic continent and islands, in New Zealand, in Tasmania, and even in Australia itself. The route by which they may have reached these countries is easily marked out. Immediately south of Cape Horn, at a distance of only 500 miles, are the South Shetland Islands and Graham"s Land, whence the Antarctic continent or a {522} group of large islands probably extends across or around the south polar area to Victoria Land and thence to Adelie Land. The outlying Young Island, 12,000 feet high, is about 750 miles south of the Macquarie Islands, which may be considered a southern outlier of the New Zealand group; and the Macquarie Islands are about the same distance from the 1,000-fathom line at a point marking the probable southern extension of Tasmania. Other islands may have existed at intermediate points; but, even as it is, these distances are not greater than we know are traversed by plants both by flotation and by aerial currents, especially in such a stormy atmosphere as that of the Antarctic regions.
Now, we may further a.s.sume, that what we know occurred within the Arctic circle also took place in the Antarctic--that is, that there have been alternations of climate during which some portion of what are now ice-clad lands became able to support a considerable amount of vegetation.[142]
During such periods there would be a steady migration of plants from all southern circ.u.mpolar countries to people the comparatively unoccupied continent, and the southern extremity of America being considerably the nearest, and also being the best stocked with those northern types which have such great powers of migration and colonisation, such plants would form the bulk of the Antarctic vegetation, and during the continuance of the milder southern climate would occupy the whole area.
When the cold returned and the land again became ice-clad, these plants would be crowded towards the outer margins of the Antarctic land and its islands, and some of them would find their way across the sea to such countries as offered on their mountain summits suitable cool stations; and as this process of alternately receiving plants from Chile and Fuegia and transmitting them in all directions from the central Antarctic land may have been {523} repeated several times during the Tertiary period, we have no difficulty in understanding the general community between the European and Antarctic plants found in all south temperate lands. Kerguelen"s Land and The Crozets are within about the same distance from the Antarctic continent as New Zealand and Tasmania, and we need not therefore be surprised at finding in each of these islands some Fuegian species which have not reached the others. Of course, there will remain difficulties of detail, as there always must remain, so long as our knowledge of the past changes of the earth"s surface and the history of the particular plants concerned is so imperfect. Sir Joseph Hooker notes, for example, the curious fact that several Compositae common to three such remote localities as the Auckland Islands, Fuegia, and Kerguelen"s Land, have no pappus or seed-down, while such as have pappus are in no case common even to two of these islands. Without knowing the exact history and distribution of the genera to which these plants belong it would be useless to offer any conjecture, except that they are ancient forms which may have survived great geographical changes, or may have some peculiar and exceptional means of dispersion.
_Proofs of Migration by way of the Himalayas and Southern Asia._--But although we may thus explain the presence of a considerable portion of the European element in the floras of New Zealand and Australia, we cannot account for the whole of it by this means, because Australia itself contains a host of European and Asiatic genera of which we find no trace in New Zealand or South America, or any other Antarctic land. We find, in fact, in Australia two distinct sets of European plants. First we have a number of species identical with those of Northern Europe or Asia (of the most characteristic of which--thirty-eight in number--Sir Joseph Hooker gives a list); and in the second place a series of European genera usually of a somewhat more southern character, mostly represented by very distinct species, and all absent from New Zealand; such as Clematis, Papaver, Cleome, Polygala, Lavatera, Ajuga, &c. Now of the first set--the North European _species_--about three-fourths occur in some parts of America, {524} and about half in South Temperate America or New Zealand; whence we may conclude that most of these, as well as some others, have reached Australia by the route already indicated. The second set of Australo-European genera, however, and many others characteristic of the South European or the Himalayan flora, have probably reached Australia by way of the mountains of Southern Asia, Borneo, the Moluccas, and New Guinea, at a somewhat remote period when loftier ranges and some intermediate peaks may have existed, sufficient to carry on the migration by the aid of the alternate climatal changes which are known to have occurred. The long belt of Secondary and Palaeozoic formations in East Australia from Tasmania to Cape York continued by the lofty ranges of New Guinea, indicates the route of this immigration, and sufficiently explains how it is that these northern types are almost wholly confined to this part of the Australian continent. Some of the earlier immigrants of this cla.s.s no doubt pa.s.sed over to New Zealand and now form a portion of the peculiar genera confined to these two countries; but most of them are of later date, and have thus remained in Australia only.
_Proofs of Migration by way of the African Highlands._--It is owing to this twofold current of vegetation flowing into Australia by widely different routes that we have in this distant land a better representation of the European flora, both as regards species and genera, than in any other part of the southern hemisphere; and, so far as I can judge of the facts, there is no general phenomenon--that is, nothing in the distribution of genera and other groups of plants as opposed to cases of individual species--that is not fairly accounted for by such an origin. It further receives support from the case of South Africa, which also contains a large and important representation of the northern flora. But here we see no indications (or very slight ones) of that southern influx which has given Australia such a community of vegetation with the Antarctic lands. There are no less than sixty _genera_ of strictly north temperate plants in South Africa, none of which occur in Australia; while very few of the _species_, so characteristic of Australia, New Zealand, and Fuegia, are found there. It {525} is clear, therefore, that South Africa has received its European plants by the direct route through the Abyssinian highlands and the lofty equatorial mountains, and mostly at a distant period when the conditions for migration were somewhat more favourable than they are now. The much greater directness of the route from Northern Europe to South Africa than to Australia; and the existence even now of lofty mountains and extensive highlands for a large portion of the distance, will explain (what Sir Joseph Hooker notes as "a very curious fact") why South Africa has more very northern European _genera_ than Australia, while Australia has more identical _species_ and a better representation on the whole of the European flora--this being clearly due to the large influx of species it has received from the Antarctic Islands, in addition to those which have entered it by way of Asia. The greater distance of South Africa even now from any of these islands, and the much deeper sea to the south of the African continent, than in the case of Tasmania and New Zealand, indicating a smaller recent extension southward, is all quite in harmony with the facts of distribution of the northern flora above referred to.
_Supposed Connection of South Africa and Australia._--There remains, however, the small amount of direct affinity between the vegetation of South Africa and that of Australia, New Zealand, and Temperate South America, consisting in all of fifteen genera, five of which are confined to Australia and South Africa, while several natural orders are better represented in these two countries than in any other part of the world.
This resemblance has been supposed to imply some former land-connection of all the great southern lands, but it appears to me that any such supposition is wholly unnecessary. The differences between the faunas and floras of these countries are too great and too radical to render it possible that any such connection should have existed except at a very remote period. But if we have to go back so far for an explanation, a much simpler one presents itself, and one more in accordance with what we have learnt of the general permanence of deep oceans and the great changes that have taken place {526} in the distribution of all forms of life. Just as we explain the presence of marsupials in Australia and America and of Centetidae in Madagascar and the Antilles, by the preservation in these localities of remnants of once wide-spread types, so we should prefer to consider the few genera common to Australia and South Africa as remnants of an ancient vegetation, once spread over the northern hemisphere, driven southward by the pressure of more specialised types, and now finding a refuge in these two widely separated southern lands. It is suggestive of such an explanation that these genera are either of very ancient groups--as Conifers and Cycads--or plants of low organisation as the Restiaceae--or of world-wide distribution, as Melanthaceae.
_The Endemic Genera of Plants in New Zealand._--Returning now to the New Zealand flora, with which we are more especially concerned, there only remains to be considered the peculiar or endemic genera which characterise it. These are thirty-two in number, and are mostly very isolated. A few have affinities with Arctic groups, others with Himalayan, or Australian genera; several are tropical forms, but the majority appear to be altogether peculiar types of world-wide groups--as Leguminosae, Saxifrageae, Compositae, Orchideae, &c. We must evidently trace back these peculiar forms to the earliest immigrants, either from the north or from the south; and the great antiquity we are obliged to give to New Zealand--an antiquity supported by every feature in its fauna and flora, no less than by its geological structure, and its extinct forms of life[143]--affords ample time for the changes in the general distribution of plants, and for those due to isolation and modification under {527} the influence of changed conditions, which are manifested by the extreme peculiarity of many of these interesting endemic forms.
_The Absence of Southern Types from the Northern Hemisphere._--We have now only to notice the singular want of reciprocity in the migrations of northern and southern types of vegetation. In return for the vast number of European plants which have reached Australia, not one single Australian plant has entered any part of the north temperate zone, and the same may be said of the typical southern vegetation in general, whether developed in the Antarctic lands, New Zealand, South America, or South Africa. The furthest northern outliers of the southern flora are a few genera of Antarctic type on the Bornean Alps; the genus Acaena which has a species in California; two representatives of the Australian flora--Casuarina and Stylidium, in the peninsula of India; while China and the Philippines have two strictly Australian genera of Orchideae--Microtis and Thelymitra, as well as a Restiaceous genus. Several distinct causes appear to have combined to produce this curious inability of the southern flora to make its way into the northern hemisphere. The primary cause is, no doubt, the totally different distribution of land in the two hemispheres, so that in the south there is the minimum of land in the colder parts of the temperate zone and in the north the maximum. This is well shown by the fact that on the parallel of Lat. 50 N. we pa.s.s over 240 of land or shallow sea, while on the same parallel of south lat.i.tude we have only 4, where we cross the southern part of Patagonia. Again the three most important south temperate land-areas--South Temperate America, South Africa, and Australia--are widely separated from each other, and have in all probability always been so; whereas the whole of the north temperate lands are practically continuous. It follows that, instead of the enormous northern area, in which highly organised and dominant groups of plants have been developed gifted with great colonising and aggressive powers, we have in the south three comparatively small and detached areas, in which rich floras have been developed with _special_ {528} adaptations to soil, climate, and organic environment, but comparatively impotent and inferior beyond their own domain.
Another circ.u.mstance which makes the contest between the northern and southern forms still more unequal, is the much greater hardiness of the former, from having been developed in a colder region, and one where alpine and arctic conditions extensively prevail; whereas the southern floras have been mainly developed in mild regions to which they have been altogether confined. While the northern plants have been driven north or south by each succeeding change of climate, the southern species have undergone comparatively slight changes of this nature, owing to the areas they occupy being unconnected with the ice-bearing Antarctic continent. It follows, that whereas the northern plants find in all these southern lands a milder and more equable climate than that to which they have been accustomed, and are thus often able to grow and flourish even more vigorously than in their native land, the southern plants would find in almost every part of Europe, North America or Northern Asia, a more severe and less equable climate, with winters that usually prove fatal to them even under cultivation. These causes, taken separately, are very powerful, but when combined they must, I think, be held to be amply sufficient to explain why examples of the typical southern vegetation are almost unknown in the north temperate zone, while a very few of them have extended so far as the northern tropic.[144]
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_Concluding Remarks on the Last Two Chapters._--Our inquiry into the external relations and probable origin of the fauna and flora of New Zealand, has thus led us on to a general theory as to the cause of the peculiar biological relations between the northern and the southern hemispheres; and no better or more typical example could be found of the wide range and great interest of the study of the geographical distribution of animals and plants.
The solution which has here been given of one of the most difficult of this cla.s.s of problems, has been rendered possible solely by the knowledge very recently obtained of the form of the sea-bottom in the southern ocean, and of the geological structure of the great Australian continent. Without this knowledge we should have nothing but a series of guesses or probabilities on which to found our hypothetical explanation, which we have now been able to build up on a solid foundation of fact. The complete separation of East from West Australia during a portion of the Cretaceous and Tertiary periods, could never have been guessed till it was established by the laborious explorations of the Australian geologists; while the hypothesis of a comparatively shallow sea, uniting New Zealand by a long route with tropical Australia, while a profoundly deep ocean always separated it from temperate Australia, would have been rejected as too improbable a supposition for the foundation of even the most enticing theory. Yet it is mainly by means of these two facts, that we are enabled to give an adequate explanation of the strange anomalies in the flora of Australia and its relation to that of New Zealand.
In the more general explanation of the relations of the various northern and southern floras, I have shown what an important aid to any such explanation is the theory of repeated changes of climate, not necessarily of great amount, given in Chapters VIII. and IX.; while the whole discussion justifies the importance attached to the theory of the general permanence of continents and oceans, as demonstrated in Chapter VI., since any rational explanation based upon facts (as opposed to mere unsupported {530} conjecture) must take such general permanence as a starting-point.
The whole inquiry into the phenomena presented by islands, which forms the main subject of the present volume has, I think, shown that this theory does afford a firm foundation for the discussion of questions of distribution and dispersal; and that by its aid, combined with a clear perception of the wonderful powers of dispersion and modification in the organic world when long periods are considered, the most difficult problems connected with this subject cease to be insoluble.
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CHAPTER XXIV
SUMMARY AND CONCLUSION
The Present Volume is the Development and Application of a Theory--Statement of the Biological and Physical Causes of Dispersal--Investigation of the Facts of Dispersal--of the Means of Dispersal--of Geographical Changes Affecting Dispersal--of Climatal Changes Affecting Dispersal--The Glacial Epoch and its Causes--Alleged Ancient Glacial Epochs--Warm Polar Climates and their Causes--Conclusions as to Geological Climates--How far Different from those of Mr. Croll--Supposed Limitations of Geological Time--Time Amply Sufficient both for Geological and Biological Development--Insular Faunas and Floras--The North Atlantic Islands--The Galapagos--St.
Helena and the Sandwich Islands--Great Britain as a Recent Continental Island--Borneo and Java--j.a.pan and Formosa--Madagascar as an Ancient Continental Island--Celebes and New Zealand as Anomalous Islands--The Flora of New Zealand and its Origin--The European Element in the South Temperate Floras--Concluding Remarks.
The present volume has gone over a very wide field both of facts and theories, and it will be well to recall these to the reader"s attention and point out their connection with each other, in a concluding chapter. I hope to be able to show that, although at first sight somewhat fragmentary and disconnected, this work is really the development of a clear and definite theory, and its application to the solution of a number of biological problems. That theory is, briefly, that the distribution of the various species and groups of living things over the earth"s surface, and their aggregation in definite a.s.semblages in certain areas, is the {532} direct result and outcome of a complex set of causes, which may be grouped as "biological" and "physical." The biological causes are mainly of two kinds--firstly, the constant tendency of all organisms to increase in numbers and to occupy a wider area, and their various powers of dispersion and migration through which, when unchecked, they are enabled to spread widely over the globe; and, secondly, those laws of evolution and extinction which determine the manner in which groups of organisms arise and grow, reach their maximum, and then dwindle away, often breaking up into separate portions which long survive in very remote regions. The physical causes are also mainly of two kinds. We have, first, the geographical changes which at one time isolate a whole fauna and flora, at another time lead to their dispersal and intermixture with adjacent faunas and floras--and it was here important to ascertain and define the exact nature and extent of these changes, and to determine the question of the general stability or instability of continents and oceans; in the second place, it was necessary to determine the exact nature, extent and frequency of the changes of climate which have occurred in various parts of the earth,--because such changes are among the most powerful agents in causing the dispersal and extinction of plants and animals. Hence the importance attached to the question of geological climates and their causes, which have been here investigated at some length with the aid of the most recent researches of geologists, physicists, and explorers. These various inquiries led on to an investigation of the mode of formation of stratified deposits, with a view to fix within some limits their probable age; and also to an estimate of the probable rate of development of the organic world; and both these processes are shown to involve, so far as we can judge, periods of time less vast than have generally been thought necessary.
The numerous facts and theories established in the First Part of the work are then applied to explain the phenomena presented by the floras and faunas of the chief islands of the globe, which are cla.s.sified, in accordance with their physical origin, in three groups or cla.s.ses, each {533} of which are shown to exhibit certain well-marked biological features.
Having thus shown that the work is a connected whole, founded on the principle of tracing out the more recondite causes of the distribution of organisms, we will briefly indicate the scope and object of the several chapters, by means of which this general conception has been carried out.