Here then we have apparently a wonderful resemblance between the New Zealand flora and that of Australia, indicated by more than two-thirds of the non-peculiar species, and more than nine-tenths of the non-peculiar genera (255) being Australian. But now let us look at the other side of the question.
There are in Australia seven great genera of plants, each containing more than 100 species, all widely spread over {490} the country, and all highly characteristic Australian forms,--Acacia, Eucalyptus, Melaleuca, Leucopogon, Stylidium, Grevillea, and Hakea. These are entirely absent from New Zealand, except one species of Leucopogon, a genus which also has representatives in the Malayan and Pacific Islands. Sixteen more Australian genera have over fifty species each, and of these eight are totally absent from New Zealand, five are represented by one or two species, and only two are fairly represented; but these two--Drosera and Helichrysum--are very widespread genera, and might have reached New Zealand from other countries than Australia.
But this by no means exhausts the differences between New Zealand and Australia. No less than seven Australian Natural Orders--Dilleniaceae, Buettneriaceae, Polygaleae, Tremandreae, Casuarineae, Haemodoraceae, and Xyrideae are entirely wanting in New Zealand, and several others which are excessively abundant and highly characteristic of the former country are very poorly represented in the latter. Thus, Leguminosae are extremely abundant in Australia, where there are over 1,000 species belonging to about 100 genera, many of them altogether peculiar to the country; yet in New Zealand this great order is most scantily represented, there being only five genera and thirteen species; and only two of these genera, Swainsonia and Clianthus, are Australian, and as the latter consists of but two species it may as well have pa.s.sed from New Zealand to Australia as the other way, or more probably from some third country to them both.[128]
Goodeniaceae with ten genera and 220 species Australian, has but two species in New Zealand--and one of these is a salt-marsh plant found also in Tasmania and in Chile; and four other large Australian orders--Rhamneae, Myoporineae, Proteaceae and Santalaceae, have very few representatives in New Zealand.
We find, then, that the great fact we have to explain and account for is, the undoubted affinity of the New {491} Zealand flora to that of Australia, but an affinity almost exclusively confined to the least predominant and least peculiar portion of that flora, leaving the most predominant, most characteristic, and most widely distributed portion absolutely unrepresented. We must however be careful not to exaggerate the amount of affinity with Australia, apparently implied by the fact that nearly six-sevenths of the New Zealand genera are also Australian, for, as we have already stated, a very large number of these are European, Antarctic, South American or Polynesian genera, whose presence in the two contiguous areas only indicates a common origin. About one-eighth, only, are absolutely confined to Australia and New Zealand (thirty-two genera), and even of these several are better represented in New Zealand than in Australia, and may therefore have pa.s.sed from the former to the latter. No less than 174 of the New Zealand genera are temperate South American, many being also Antarctic or European; while others again are especially tropical or Polynesian; yet undoubtedly a larger proportion of the Natural Orders and genera are common to Australia than to any other country, so that we may say that the basis of the flora is Australian with a large intermixture of northern and southern temperate forms and others which have remote world-wide affinities.
_General Features of the Australian Flora and its Probable Origin._--Before proceeding to point out how the peculiarities of the New Zealand flora may be best accounted for, it is necessary to consider briefly what are the main peculiarities of Australian vegetation, from which so important a part of that of New Zealand has evidently been derived.
The actual Australian flora consists of two great divisions--a temperate and a tropical, the temperate being again divisible into an eastern and a western portion. All that is most characteristic of the Australian flora belongs to the temperate division (though these often overspread the whole continent), in which are found almost all the remarkable Australian types of vegetation and the numerous genera peculiar to this part of the world.
Contrary to what occurs in most other countries, the {492} tropical appears to be less rich in species and genera than the temperate region, and what is still more remarkable it contains fewer peculiar species, and very few peculiar genera. Although the area of tropical Australia is about equal to that of the temperate portions, and it has now been pretty well explored botanically, it has probably not more than half as many species.[129]
Nearly 500 of its species are identical with Indian or Malayan plants, or are very close representatives of them; while there are more than 200 Indian genera, confined for the most part to the tropical portion of Australia. The remainder of the tropical flora consists of a few species and many genera of temperate {493} Australia which range over the whole continent, but these form only a small portion of the peculiarly Australian genera.
These remarkable facts clearly point to one conclusion--that the flora of tropical Australia is, comparatively, recent and derivative. If we imagine the greater part of North Australia to have been submerged beneath the ocean, from which it rose in the middle or latter part of the Tertiary period, offering an extensive area ready to be covered by such suitable forms of vegetation as could first reach it, something like the present condition of things would inevitably arise. From the north, widespread Indian and Malay plants would quickly enter, while from the south the most dominant forms of warm-temperate Australia, and such as were best adapted to the tropical climate and arid soil, would intermingle with them. Even if numerous islands had occupied the area of Northern Australia for long periods anterior to the final elevation, very much the same state of things would result.
The existence in North and North-east Australia of enormous areas covered with Cretaceous and other Secondary deposits, as well as extensive Tertiary formations, lends support to the view, that during very long epochs temperate Australia was cut off from all close connection with the tropical and northern lands by a wide extent of sea; and this isolation is exactly what was required, in order to bring about the wonderful amount of specialisation and the high development manifested by the typical Australian flora. Before proceeding further, however, let us examine this flora itself, so far as regards its component parts and probable past history.
_The Floras of South-eastern and South-western Australia._--The peculiarities presented by the south-eastern and south-western subdivisions of the flora of temperate Australia are most interesting and suggestive, and are, perhaps, unparalleled in any other part of the world. South-west Australia is far less extensive than the south-eastern division--less varied in soil and climate, with no lofty mountains, and much sandy desert; yet, strange to say, it contains an equally rich flora and a far greater proportion of peculiar species and genera of plants. As Sir {494} Joseph Hooker remarks:--"What differences there are in conditions would, judging from a.n.a.logy with other countries, favour the idea that South-eastern Australia, from its far greater area, many large rivers, extensive tracts of mountainous country and humid forests, would present much the most extensive flora, of which only the drier types could extend into South-western Australia. But such is not the case; for though the far greater area is much the best explored, presents more varied conditions, and is tenanted by a larger number of Natural Orders and genera, these contain fewer species by several hundreds."[130]
The fewer genera of South-western Australia are due almost wholly to the absence of the numerous European, Antarctic, and South-American types found in the south-eastern region, while in purely Australian types it is far the richer, for while it contains most of those found in the east it has a large number altogether peculiar to it; and Sir Joseph Hooker states that "there are about 180 genera, out of 600 in South-western Australia, that are either not found at all in South-eastern, or that are represented there by a very few species only, and these 180 genera include nearly 1,100 species."
_Geological Explanation of the Differences of these Two Floras._--These facts again clearly point to the conclusion that South-western Australia is the remnant of the more extensive and more isolated portion of the continent in which the peculiar Australian flora was princ.i.p.ally developed.
The existence there of a very large area of granite--800 miles in length by nearly 500 in maximum width with detached ma.s.ses 200 miles to the north and 500 miles to the east--indicates such an extension; for these {495} granitic ma.s.ses were certainly once buried under piles of stratified rock, since denuded, and then formed the nucleus of the old Western Australian continent. If we take the 1000-fathom line around the southern part of Australia to represent the probable extension of this old land we shall see that it would give a wide additional area south of the Great Australian Bight, and form a continent which, even if the greater part of tropical Australia were submerged, would be sufficient for the development of a peculiar and abundant flora. We must also remember that an elevation of 6000 feet, added to the vast amount which has been taken away by denudation, would change the whole country, including what are now the deserts of the interior, into a mountainous and well-watered region.
But while this rich and peculiar flora was in process of formation, the eastern portion of the continent must either have been widely separated from the western or had perhaps not yet risen from the ocean. The whole of this part of the country consists of Palaeozoic and Secondary formations with granite and metamorphic rocks, the Secondary deposits being largely developed on both sides of the central range, extending the whole length of the continent from Tasmania to Cape York, and const.i.tuting the greater part of the plateau of the Blue Mountains and other lofty ranges. During some portion of the Secondary and Tertiary periods therefore, this side of Australia must have been almost wholly submerged beneath the ocean; and if we suppose that during this time the western part of the continent was at nearly its maximum extent and elevation, we shall have a sufficient explanation of the great difference between the flora of Western and Eastern Australia, since the latter would only have been able to receive immigrants from the former, at a later period, and in a more or less fragmentary manner.
If we examine the geological map of Australia (given in Stanford"s Compendium of Geography and Travel, volume _Australasia_), we shall see good reason to conclude that the eastern and the western divisions of the country first existed as separate islands, and only became united at a comparatively recent epoch. This is indicated by an {496} enormous stretch of Cretaceous and Tertiary formations extending from the Gulf of Carpentaria completely across the continent to the mouth of the Murray River. During the Cretaceous period, therefore, and probably throughout a considerable portion of the Tertiary epoch,[131] there must have been a wide arm of the sea occupying this area, dividing the great ma.s.s of land on the west--the true seat and origin of the typical Australian flora--from a long but narrow belt of land on the east, indicated by the continuous ma.s.s of Secondary and Palaeozoic formations already referred to which extend uninterruptedly from Tasmania to Cape York. Whether this formed one continuous land, or was broken up into islands, cannot be positively determined; but the fact that no marine Tertiary beds occur in the whole of this area, renders it probable that it was almost, if not quite, continuous, and that it not improbably extended across to what is now New Guinea. At this epoch, then (as shown in the accompanying map), Australia may, not improbably, have consisted of a very large and fertile western island, almost or quite extratropical, and extending from the Silurian rocks of the Flinders range in South Australia, to about 150 miles west of the present west coast, and southward to about 350 miles south of the Great Australian Bight. To the east of this, at a distance of from 250 to 400 miles, extended in a north and south direction a long but comparatively narrow island, stretching from far south of Tasmania to New Guinea; while the crystalline and Secondary formations of central North Australia probably indicate the existence of one or more large islands in that direction.
{497}
The white portions represent land; the shaded parts sea.
The existing land of Australia is shown in outline.]
The eastern and the western islands--with which we are now chiefly concerned--would then differ considerably in their vegetation and animal life. The western and more ancient land already possessed, in its main features, the peculiar Australian flora, and also the ancestral forms of its strange marsupial fauna, both of which it had probably received at some earlier epoch by a temporary union with the Asiatic continent over what is now the Java sea. Eastern Australia, on the other hand, possessed only the rudiments of its existing mixed flora, derived from three distinct sources.
Some important fragments of the typical Australian vegetation had reached it across the marine {498} strait, and had spread widely owing to the soil, climate and general conditions being exactly suited to it: from the north and north-east a tropical vegetation of Polynesian type had occupied suitable areas in the north; while the extension southward of the Tasmanian peninsula, accompanied, probably, as now, with lofty mountains, favoured the immigration of south-temperate forms from whatever Antarctic lands or islands then existed. This supposition is strikingly in harmony with what is known of the ancient flora of this portion of Australia. In deposits supposed to be of Eocene age in New South Wales and Victoria fossil plants have been found showing a very different vegetation from that now existing.
Along with a few Australian types--such as Pittosporum, Knightia, and Eucalyptus, there occur birches, alders, oaks, and beeches; while in Tasmania in freshwater limestone, apparently of Miocene age, are found willows, alders, birches, oaks, and beeches,[132] all except the latter genus (f.a.gus) now quite extinct in Australia.[133] These temperate forms probably indicate a more oceanic climate, cooler and moister than at present. The union with Western Australia and the establishment of an arid interior by modifying the climate may have led to the extinction of many of these forms and their replacement by special Australian types more suited to the new conditions.
At this time the marsupial fauna had not yet reached this eastern land, which was, however, occupied in the north by some ancestral struthious birds, which had entered it by way of New Guinea through some very ancient continental extension, and of which the emu, the ca.s.sowaries, the extinct Dromornis of Queensland, and the moas and kiwis of New Zealand, are the modified descendants.
_The Origin of the Australian Element in the New Zealand Flora._--We have now brought down the history of Australia, as deduced from its geological structure and the main features of its existing and Tertiary flora, to the period {499} when New Zealand was first brought into close connection with it, by means of a great north-western extension of that country, which, as already explained in our last chapter, is so clearly indicated by the form of the sea bottom (See Map, p. 471). The condition of New Zealand previous to this event is very obscure. That it had long existed as a more or less extensive land is indicated by its ancient sedimentary rocks; while the very small areas occupied by Jura.s.sic and Cretaceous deposits, imply that much of the present land was then also above the sea-level. The country had probably at that time a scanty vegetation of mixed Antarctic and Polynesian origin; but now, for the first time, it would be open to the free immigration of such Australian types as were suitable to its climate, and which _had already reached the tropical and sub-tropical portions of the Eastern Australian island_. It is here that we obtain the clue to those strange anomalies and contradictions presented by the New Zealand flora in its relation to Australia, which have been so clearly set forth by Sir Joseph Hooker, and which have so puzzled botanists to account for. But these apparent anomalies cease to present any difficulty when we see that the Australian plants in New Zealand were acquired, not directly, but, as it were, at second hand, by union with an island which itself had as yet only received a portion of its existing flora. And then, further difficulties were placed in the way of New Zealand receiving such an adequate representation of that portion of the flora which had reached East Australia as its climate and position ent.i.tled it to, by the fact of the union being, not with the temperate, but with the tropical and sub-tropical portions of that island, so that only those groups could be acquired which were less exclusively temperate, and had already established themselves in the warmer portion of their new home.[134]
{500}
It is therefore no matter of surprise, but exactly what we should expect, that the great ma.s.s of pre-eminently temperate Australian genera should be absent from New Zealand, including the whole of such important families as, Dilleniaceae, Tremandreae, Buettneriacae, Polygaleae, Casuarineae and Haemodoraceae; while others, such as Rutaceae, Stackhousieae, Rhamneae, Myrtaceae, Proteaceae, and Santalaceae, are represented by only a few species.
Thus, too, we can explain the absence of _all_ the peculiar Australian Leguminosae; for these were still mainly confined to the great western island, along with the peculiar Acacias and Eucalypti, which at a later period spread over the whole continent. It is equally accordant with the view we are maintaining, that among the groups which Sir Joseph Hooker enumerates as "keeping up the features of extra tropical Australia in its tropical quarter," several should have reached New Zealand, such as Drosera, some Pittosporeae and Myoporineae, with a few Proteaceae, Loganiaceae, and Restiaceae; for most of these are not only found in tropical Australia, but also in the Malayan and Pacific islands.
_Tropical Character of the New Zealand Flora Explained._--In this origin of the New Zealand fauna by a north-western route from North-eastern Australia, we find also an explanation of the remarkable number of tropical groups of plants found there: for though, as Sir Joseph Hooker has {501} shown, a moist and uniform climate favours the extension of tropical forms in the temperate zone, yet some means must be afforded them for reaching a temperate island. On carefully going through the _Handbook_, and comparing its indications with those of Bentham"s _Flora Australiensis_, I find that there are in New Zealand thirty-eight thoroughly tropical genera, thirty-three of which are found in Australia--mostly in the tropical portion of it, though a few are temperate, and these may have reached it through New Zealand[135]. To these we must add thirty-two more genera, which, though chiefly developed in temperate Australia, extend into the tropical or sub-tropical portions of it, and may well have reached New Zealand by the same route.
On the other hand we find but few New Zealand genera certainly derived from Australia which are especially temperate, and it may be as well to give a list of such as {502} do occur with a few remarks. They are sixteen in number, as follows:--
1. Pennantia (1 sp.). This genus has a species in Norfolk Island, indicating perhaps its former extension to the north-west.
2. Pomaderris (3 sp.). One _species_ inhabits Victoria and New Zealand, indicating recent trans-oceanic migration.
3. Quintinia (2 sp.). This genus has winged seeds facilitating migration.
4. Olearia (20 sp.). Seeds with pappus.
5. Craspedia (2 sp.). Seeds with pappus. Alpine; identical with Australian species, and therefore of comparatively recent introduction.
6. Celmisia (25 sp.). Seeds with pappus. Only three Australian species, two of which are identical with New Zealand forms, probably therefore derived from New Zealand.
7. Ozothamnus (5 sp.). Seeds with pappus.
8. Epacris (4 sp.). Minute seeds. Some species are sub-tropical, and they are all found in the northern (warmer) island of New Zealand.
9. Archeria (2 sp.). Minute seeds. A species common to E. Australia and New Zealand.
10. Logania (3 sp.). Small seeds. Alpine plants.
11. Hedycarya (1 sp.).
12. Chiloglottis (1 sp.). Minute seeds. In Auckland Islands; alpine in Australia.
13. Prasophyllum (1 sp.). Minute seeds. Identical with Australian species, indicating recent transmission.
14. Orthoceras (1 sp.). Minute seeds. Identical with an Australian species.
15. Alepyrum (1 sp.). Alpine, moss-like. An Antarctic type.
16. Dichelachne (3 sp.). Identical with Australian species. An awned gra.s.s.
We thus see that there are special features in most of these plants that would facilitate transmission across the sea between temperate Australia and New Zealand, or to both from some Antarctic island; and the fact that in several of them the species are absolutely identical shows that such transmission has occurred in geologically recent times.
_Species Common to New Zealand and Australia Mostly Temperate Forms._--Let us now take the _species_ which are common to New Zealand and Australia, but found nowhere else, and which must therefore have pa.s.sed from one country to the other at a more recent period than the ma.s.s of _genera_ with which we have hitherto been dealing. These are ninety-six in number, and they present a striking contrast to the similarly restricted _genera_ in being wholly temperate in character, the entire list presenting only a {503} single species which is confined to sub-tropical East Australia--a gra.s.s (_Apera arundinacea_) only found in a few localities on the New Zealand coast.
Now it is clear that the larger portion, if not the whole, of these plants must have reached New Zealand from Australia (or in a few cases Australia from New Zealand), by transmission across the sea, because we know there has been no actual land connection during the Tertiary period, as proved by the absence of all the Australian mammalia, and almost all the most characteristic Australian birds, insects, and plants. The form of the sea-bed shows that the distance could not have been less than 600 miles, even during the greatest extension of Southern New Zealand and Tasmania; and we have no reason to suppose it to have been less, because in other cases an equally abundant flora of identical species has reached islands at a still greater distance--notably in the case of the Azores and Bermuda.
The character of the plants is also just what we should expect: for about two-thirds of them belong to genera of world-wide range in the temperate zones, such as Ranunculus, Drosera, Epilobium, Gnaphalium, Senecio, Convolvulus, Atriplex, Luzula, and many sedges and gra.s.ses, whose exceptionally wide distribution shows that they possess exceptional powers of dispersal and vigour of const.i.tution, enabling them not only to reach distant countries, but also to establish themselves there. Another set of plants belong to especially Antarctic or south temperate groups, such as Colobanthus, Acaena, Gaultheria, Pernettya, and Muhlenbeckia, and these may in some cases have reached both Australia and New Zealand from some now submerged Antarctic island. Again, about one-fourth of the whole are alpine plants, and these possess two advantages as colonisers. Their lofty stations place them in the best position to have their seeds carried away by winds; and they would in this case reach a country which, having derived the earlier portion of its flora from the side of the tropics, would be likely to have its higher mountains and favourable alpine stations to a great extent unoccupied, or occupied by plants unable to compete with specially adapted alpine groups. {504}
Fully one-third of the exclusively Australo-New Zealand species belong to the two great orders of the sedges and the gra.s.ses; and there can be no doubt that these have great facilities for dispersion in a variety of ways.
Their seeds, often enveloped in chaffy glumes, would be carried long distances by storms of wind, and even if finally dropped into the sea would have so much less distance to reach the land by means of surface currents; and Mr. Darwin"s experiments show that even cultivated oats germinated after 100 days" immersion in sea-water. Others have hispid awns by which they would become attached to the feathers of birds, and there is no doubt this is an effective mode of dispersal. But a still more important point is, probably, that these plants are generally, if not always, wind-fertilised, and are thus independent of any peculiar insects, which might be wanting in the new country.
_Why Easily-Dispersed Plants have often Restricted Ranges._--This last consideration throws light on a very curious point, which has been noted as a difficulty by Sir Joseph Hooker, that plants which have most clear and decided powers of dispersal by wind or other means, have _not_ generally the widest specific range; and he instances the small number of Compositae common to New Zealand and Australia. But in all these cases it will, I think, be found that although the _species_ have not a wide range the _genera_ often have. In New Zealand, for instance, the Compositae are very abundant, there being no less than 167 species, almost all belonging to Australian genera, yet only about one-sixteenth of the whole are identical in the two countries. The explanation of this is not difficult. Owing to their great powers of dispersal, the Australian Compositae reached New Zealand at a very remote epoch, and such as were adapted to the climate and the means of fertilisation established themselves; but being highly organised plants with great flexibility of organisation, they soon became modified in accordance with the new conditions, producing many special forms in different localities; and these, spreading widely, soon took possession of all suitable stations. Henceforth immigrants from Australia had to compete {505} with these indigenous and well-established plants, and only in a few cases were able to obtain a footing; whence it arises that we have many Australian types, but few Australian species, in New Zealand, and both phenomena are directly traceable to the combination of great powers of dispersal with a high degree of adaptability. Exactly the same thing occurs with the still more highly specialised Orchideae. These are not proportionally so numerous in New Zealand (thirty-eight species), and this is no doubt due to the fact that so many of them require insect-fertilisation often by a particular family or genus (whereas almost any insect will fertilise Compositae), and insects of all orders are remarkably scarce in New Zealand.[136] This would at once prevent the establishment of many of the orchids which may have reached the islands, while those which did find suitable fertilisers and other favourable conditions would soon become modified into new species. It is thus quite intelligible why only three species of orchids are identical in Australia and New Zealand, although their minute and abundant seeds must be dispersed by the wind almost as readily as the spores of ferns.
Another specialised group--the Scrophularineae--abounds in New Zealand, where there are sixty-two species; but though almost all the genera are Australian only three species are so. Here, too, the seeds are usually very small, and the powers of dispersal great, as shown by several European genera--Veronica, Euphrasia, and Limosella, being found in the southern hemisphere.
Looking at the whole series of these Australo-New Zealand plants, we find the most highly specialised groups--Compositae, Scrophularineae, Orchideae--with a small proportion of identical species (one-thirteenth to one twentieth), the less highly specialised--Ranunculaceae, Onagrariae and Ericeae--with a higher proportion (one-ninth to one-sixth), and the least specialised--Junceae, {506} Cyperaceae and Gramineae--with the high proportion in each case of one-fourth. These nine are the most important New Zealand orders which contain species common to that country and Australia and confined to them; and the marked correspondence they show between high specialisation and want of _specific_ ident.i.ty, while the _generic_ ident.i.ty is in all cases approximately equal, points to the conclusion that the means of diffusion are, in almost all plants ample, when long periods of time are concerned, and that diversities in this respect are not so important in determining the peculiar character of a derived flora, as adaptability to varied conditions, great powers of multiplication, and inherent vigour of const.i.tution. This point will have to be more fully discussed in treating of the origin of the Antarctic and north temperate members of the New Zealand flora.