very numerous seeds; and still another, that a large proportion of them come to maturity very rapidly, and that their seeds germinate quickly. These characters are all retrogressive from one point of view-that is to say-the plants exhibiting them have tended to become less instead of more specialised in their development; but by this degradation of their reproductive organs they have really become better adapted for the peculiar conditions which are imposed upon them in their struggle with the gardener and agriculturist.

Commenting on this paper D. Petrie wrote as follows:

The spread of weeds is mainly due to useful plants-their competitors -being regularly checked and eaten down, while the weeds are mostly allowed to grow without check of any important nature. Almost all weeds found in our northern pastures owe their spread to this; e.g., several buttercups, numerous docks, pennyroyal, Holcus mollis and H. lanatus, and many weedy grasses, various spurges, mallows, mulleins, and so forth. In many cases their spread is facilitated by the ready germination of their seeds, by the long time that the seeds retain their vitality in the soil, and by the readiness with which their earliest roots strike deep down into the soil, which allows the plants to establish themselves in hot, dry weather. Black Medick (Medicago lupulina), meadow plantain (Plantago lanceolata), the docks and spurges, all start and thrive in hot, dry weather, when more superficial-rooting seedlings die off. The introduced speedwells and poor man's weather-glass (Anagallis arvensis) are much in the same case. The decline of plants that have taken possession of a district for some years is no doubt due to temporary exhaustion of some element of plant-food needful for their vigorous growth. This principle lies at the base of the theory of rotation of crops. In Central Otago, when I first knew it, Carduus lanceolatus was the prevailing weed on open downs and dry hill-slopes. Some years after C. pauciflorus1 completely replaced it, and this will, no doubt, be now giving way to something else. The doctrine that the Scandinavian plants possess extraordinary vigour, which is the cause of their aggressive character, seems to me very doubtful. In each single species particular advantages can generally be assigned that will readily explain their rapid spread. In the peninsula north of Auckland there are very large areas of land on which European weeds have but slightly established themselves, though the ground is frequently cleared of all native vegetation by fires. In these areas native plants mostly grow up with great readiness, especially species of Leptospermum and Pomaderris, Haloragis tetragyna and H. minuta, besides various cyperaceous plants. The pre-eminence in aggressive characters of North European plants is decided enough, but many non-European plants are now widely spread here and are indeed very aggressive. I may instance Modiola multifida, a North American malvaceous plant, an Australian Plantago, two species of Erigeron, and Kyllinga. The rat-tail grass, too, is no doubt introduced, and has been most aggressive, while the South African Cyperus (minimus?) is nearly as ubiquitous as sorrel.

Here, as in the South, a few native plants are spreading: Aristotelia racemosa, Fuchsia excorticata, Pomaderris phylicifolia, Erechtites, etc.; but

T. N. Z.

1 ?C. pycnocephalus.

34

the most aggressive plant of all is Pteris aquilina, which is rapidly overrunning much of the land that has been cleared of bush, and which permanently establishes itself before its roots are sufficiently decayed to admit of ploughing.

The most abundant and most wide-spread introduced weed is Hypochoris radicata.

Cheeseman writes as follows on the naturalised plants of the Auckland district:

387 species are catalogued. Of these 280 are natives of Europe, many of them also ranging into temperate Asia and North America, and some into North Africa. Ten species, not European, are from the eastern portion of N. America, and four are from the western. Total, 294 species from the north temperate zone. From Australia only 19; from Chili and the cool portions of S. America 9; from the Cape of Good Hope 21. Total 49 species from the south temperate zone. Finally 53 species subtropical and tropical.

31 are trees or shrubs, and 356 herbaceous. Of this latter number, 176 are annual, 28 biennial, 152 perennial. The large proportion of annual species is noteworthy, as in the indigenous flora nearly all the herbaceous plants are of perennial growth.

With the above facts before us, we are better able to enquire into the general subject of the naturalisation of plants in New Zealand and to attempt an answer to the question why the native vegetation should apparently be unable to hold its own against the numerous intruders streaming in on every side. In considering the subject, it appears to me most important to bear in mind constantly that the conditions of plantlife now prevailing in New Zealand are in great measure different to those that existed when European voyagers first visited its shores. When Cook landed here the whole country was covered with a dense native vegetation, hardly interfered with by man. The cultivations of the Maoris were small in areas, and as they rarely tilled the same plot of ground for many years in succession, preferring to abandon it when the soil showed signs of exhaustion and to make new clearings elsewhere, there is little chance of the establishment of a race of indigenous weeds. In fact, it can be roundly Isaid that the New Zealand Flora contained no such class. At that time there were no herbivorous animals of any kind, either wild or domesticated, to graze upon the vegetation, or to interfere with it in any way. Thus no check existed to the growth of many species which can now hardly live in a district where our introduced cattle are abundant. And the repeated burning off, year by year, of large tracts of open country, was then a circumstance almost unknown. The Maori rarely wantonly destroyed the vegetation, and if he used fire in making his new clearings, generally took precautions that it should not spread further than was absolutely required. It is hardly necessary to dwell longer on this point; for all must admit that the advent of European settlers and the colonisation of the country have brought into operation a set of conditions injurious to both the indigenous fauna and flora. The chief of these conditions may be conveniently grouped under three heads: first, the actual destruction of the vegetation by the

settlers to make room for their cultivations, or in the construction of roads, or in the cutting down of the forests for timber, etc.; second, the introduction of sheep, cattle and horses, and their spread over the greater part of the country; third, the practice, now very generally followed, of burning off the vegetation in the open districts at regular intervals.

If the above facts are duly considered there will not be so much cause for wonder in the introduction and rapid spread of so many foreign plants. For instance, it might be expected that the weeds of our corn-fields and pasture which now form such an important and conspicuous element in the naturalised Flora-would be almost wholly composed of introductions from abroad. The native Flora possessed few plants suitable for the places they have taken, and these few could hardly compete with a chance of success against species that have from time immemorial occupied the cultivations of man, and whose best adapted varieties have been rigorously selected. The introduced weeds flourish and multiply because they have an environment suited to them and to which they have been modified; the native ones fail because the conditions have become altogether different to those they had been accustomed to.

Similarly it was to be expected that foreign plants would in some degree displace the indigenous ones in districts grazed over but not actually cultivated. Many native species will not bear repeated cropping, and soon decrease in numbers when cattle and sheep are brought in. Their places will, therefore, be taken by plants that are indifferent to this, or escape by reason of being unpalatable....At the same time it must be remembered that any native plants possessing similar advantages would also increase; and in many cases this has actually taken place. The spread of such indigenous plants as Poa australis and Discaria in the river valleys in the interior of Nelson and Canterbury, of Cassinia on the shores of Cook Straits; and of some grasses (as Danthonia semiannularis and Microlana stipoides) in Auckland, are well-known examples, and it would be easy

to enumerate more.

But although we may safely credit the changed conditions of plant life with being a powerful reason for the spread of naturalised plants in New Zealand, it is impossible to consider it as the sole explanation. For we find that not a few species have penetrated into localities where cultivation and cattle are alike unknown, and where man himself is a rare visitant; where in fact, the conditions are still unchanged. This is the most interesting part of the subject, for it proves conclusively, as remarked by Mr Darwin, that the indigenous plants of any district are not necessarily those best suited for it. In most cases it is impossible to assign any obvious reason for the fact that these intruders should be able to thrust on one side the native vegetation; but it is significant that all, or nearly all, are common and widely distributed in their native countries, in short, are predominant species; and that they have followed almost everywhere the footsteps of man, being as extensively naturalised in many other countries as in New Zealand. We may, therefore, suppose that by long-continued competition with other species, in different localities and in different climates, they have gained a vigour of constitution and a faculty of adapting themselves to a great variety of conditions which enable them to readily overcome plants that have not been so advantageously modified.

This supposition will also throw some light on the curious fact that the vast majority of our plants are of northern origin. It is now generally admitted by geologists that the present continents are of immense antiquity, and that there has been no great alteration in the relative proportions of land and water during vast geological epochs. Mr Darwin therefore argues that as the northern hemisphere has probably always possessed the most extensive continuous land area, so the wonderfully aggressive and colonizing power of its plants at the present time is due to development where the competition of species has been the most severe and long continued, owing to the presence of facilities for natural migration. The plants of the comparatively isolated countries of the southern hemisphere have not been subjected to the same degree of competition, and consequently could not be so advantageously modified.

Mr Kirk, who paid special attention to the naturalisation of plants in New Zealand, in a paper on the naturalised plants of Port Nicholson,

says:

At length a turning-point is reached, the invaders lose a portion of their vigour and become less encroaching, while the indigenous plants find the struggle less severe and gradually recover a portion of their lost ground, the result being the gradual amalgamation of those kinds best adapted to hold their own in the struggle for existence with the introduced forms, and the restriction of those less favourably adapted to habitats which afford them special advantages.

Further on in the same article Mr Kirk combats the view that the majority of our native plants will become extinct, stating that the particular species for which this danger is to be feared might almost be counted upon one's fingers.

My own views on this difficult question are much nearer to Mr Kirk's than to those of Mr Travers. I can certainly find little evidence in support of the opinion that a considerable proportion of the native flora will become extinct. Even in isolated localities of limited areas, like Madeira and St Helena, where there is little variety of climate and physical conditions, and where the native plants have been subjected to far more disadvantageous influences, and to a keener competition with introduced forms than in New Zealand, the process of naturalisation has not gone so far as to stamp out the whole of the indigenous vegetation, although great and remarkable changes have been effected, and many species have become extinct. I fail to see why it is assumed that a greater effect will be produced in New Zealand, with its diversified physical features and many varieties of soil, situation, and climate. Surely its far-stretching coast-line, bold cliffs, and extensive sand-dunes, its swamps and moorlands, its lofty mountains and wide-spreading forests, will afford numerous places of refuge for its plants until sufficient time has been allowed for the gradual development of varieties better suited to the changed con

ditions. No doubt some few species will become extinct; but these will be mostly plants whose distribution was local and confined even when Europeans first arrived here; and probably all will be species that have for some time been tending towards extinction, and whose final exit has thus been hastened. I cannot call to mind a single case of a plant known to be widely distributed when settlement commenced that is at present in any danger of extinction. Species have been banished from cultivated districts, of course, but they are still abundant in other situations, and probably there will always be a sufficient area of unoccupied and uncultivated lands to afford them a secure home.

Speaking generally, I am inclined to believe that the struggle between the naturalised and the native floras will result in a limitation of the range of the native species rather than in their actual extermination. We must be prepared to see many plants once common become comparatively rare, and possibly a limited number-I should not estimate it at more than a score or two-may altogether disappear, to be only known to us in the future by the dried specimens preserved in our museums.

Perhaps the most emphatic testimony as to the staying power of the indigenous vegetation is that borne by Dr Cockayne in his Ecological Studies in Evolution (p. 32), where he says:

There have been recorded for New Zealand up to the present time some 555 species of introduced plants, but less than 180 can be considered common, while others are local, rare, or even not truly established as wild plants. Many at first sight appear better suited to the soil and climate than are the indigenous species, and over much of the land they give the characteristic stamp to the vegetation; but this is only the case where draining, cultivation, constant burning of forest, scrub, and tussock, and the grazing of a multitude of domestic animals have made absolutely new edaphic conditions which approximate to those of Europe, and where it is no wonder that the European invader can replace the aboriginal. On the other hand, although this foreign host is present in its millions, and notwithstanding abundant winds and land-birds (introduced, not native birds), the indigenous vegetation is still virgin and the introduced plants altogether absent where grazing animals have no access and where fires have never been. On certain sub-alpine herb-fields the indigenous form of the dandelion (Taraxacum officinale, Wigg.) is abundant, and yet the introduced form, with its readily wind-borne fruit, has not gained a foothold, nor even the abundant Hypocharis radicata, L., though it may be in thousands on the neighbouring tussock pasture, less than one mile away. On Auckland Island, introduced plants occur only in the neighbourhood of the depots for castaways, but on Enderby Island, where there are cattle, they are much more widely spread. Even where the rain forest has been felled or burnt, and cattle etc., are kept away, it is gradually replaced by indigenous trees and shrubs -i.e., in localities where the rain-fall is sufficient.