very small diffusibility, and the readiness with which they are affected by local conditions.

Taking the various facts and arguments now brought forward into consideration, it appears evident that no regions (be they few or many in number) can be marked out, which will accurately represent the phenomena of the geographical distribution of all animals and plants. The distribution of the several classes, orders, and even families, will differ, because they differ in their diffusibility, their variability, and their mode of acting and reacting on each other, and on the external world. At the same time, though the details of the distribution of the different groups may differ, there will always be more or less general agreement in this respect, because the great physical features of the earth-those which have longest maintained themselves unchanged-wide oceans, lofty mountains, extensive deserts -will have forbidden the intermingling or migration of all groups alike, during long periods of time. The great primary divisions of the Earth for purposes of Natural History, should, therefore, correspond with the great permanent features of the earth's surface-those that have undergone least change in recent geological periods. Later and less important changes will have led to discrepancies in the actual distribution of the different groups, but these very discrepancies will enable us to interpret those changes, of which they are the direct effects, and very often the only evidence.

From this examination of the anomalies that occur in the distribution of different groups, and of the probable causes of such anomalies, it appears that the six regions of Dr. Sclater do approximately represent the best primary divisions of the earth for Natural History purposes. They agree well with the present distribution of mammalia, birds, reptiles, land shells, and very generally of insects also. The cases in which they do not seem correct are those of isolated groups in restricted localities. The greatest discrepancies occur in groups which have at once great capacities for diffusion, and little adaptability to change of conditions; and, in the case of plants, have probably been much increased by what may be called the adventitious aid of the glacial period and of floating ice.

Of Botanical distribution I have said little, from want of knowledge of that branch of the subject, and I can find no detailed information bearing directly upon the questions here discussed, but what I have already alluded to. It is much to be desired that some competent Botanist would point out how far these regions agree

with, and how far they contradict, the main facts of the distribution of plants. It seems evident that the various modes of glacial action have produced much more effect on the migrations of plants than on those of animals, and also that plants have, on the whole, more varied and more effectual means of dispersal. Still, if the views here advocated are true, the flora of each region should exhibit a characteristic substratum of indigenous forms, though often much modified, and sometimes nearly overwhelmed by successive streams of foreign invasion.

My object in calling attention to the subject by this very partial review of it, is to induce those Naturalists, who are working at particular groups, to give more special attention to geographical distribution than has hitherto been done. By carefully working out the distribution of allied genera and closely connected groups of species, they could give the amount of agreement or discrepancy with other groups the geography of which is best known, and furnish us with such information on the habits of the species, as might help to explain the anomalies which are found to occur. We should thus soon accumulate a sufficiency of detailed facts to enable us to determine whether these are the best primary divisions of the earth into terrestrial Zoological and Botanical regions, or whether such general divisions are altogether impracticable. Some such simple classification of regions is wanted to enable us readily to exhibit broad results, And to show at a glance the external relations of local faunas and loras. And if we go more into detail and adopt a larger number of primary divisions, we shall not only lose many of these advantages, but shall probably find insuperable difficulties in harmonizing the conflicting distribution of the different groups of organized beings.

XX.-NOTE ON THE REPLACEMENT OF SPECIES IN THE COLONIES AND ELSEWHERE. By J. D. Hooker, M.D., F.R.S., &c.

AMONG the most interesting phenomena connected with the distribution of plants, are those that concern the rapidity with which some species of one country will, when introduced into another, rapidly displace the aborigines and replace them. The Cardoon in the Argentine provinces is one conspicuous example, the Anacharis in our own

rivers is another, but neither of these cases is more remarkable than some that our Colonies present. In Australia and New Zealand, for instance, the noisy train of English emigration is not more surely doing its work, than the stealthy tide of English weeds, which are creeping over the surface of the waste, cultivated and virgin soil, in annually increasing numbers of genera, species and individuals. Apropos of this subject, a correspondent, W. T. Locke Travers, Esq., F.L.S., a most active New Zealand botanist, writing from Canterbury, says, "You would be surprised at the rapid spread of European and other foreign plants in this country. All along the sides of the main lines of road through the plains, a Polygonum (aviculare), called "Cow Grass," grows most luxuriantly, the roots sometimes two feet in depth, and the plants spreading over an area from four to five feet in diameter. The dock (Rumex obtusifolius or R. crispus) is to be found in every river bed, extending into the valleys of the mountain rivers, until these become mere torrents. The Sow-thistle* is spread all over the country, growing luxuriantly nearly up to 6000 feet. The water-cress increases in our still rivers to such an extent, as to threaten to choke them altogether: in fact, in the Avon, a still deep stream running through Christ-Church, the annual cost of keeping the river free for boat navigation and for purposes of drainage, exceeds £300. I have measured stems twelve feet long and threequarters of an inch in diameter. In some of the mountain districts, where the soil is loose, the white clover is completely displacing the native grasses, forming a close sward. Foreign trees are also very luxuriant in growth. The gum-trees of Australia, the poplars and willows, particularly, grow most rapidly. In fact, the young native vegetation appears to shrink from competition with these more vigorous intruders."

I have urged upon various Colonial correspondents the great importance of systematically collecting and recording facts upon this curious subject, but hitherto without success. The same spirit which prompts us to subscribe more for the heathen of Africa than of St. Giles, seems to animate the collectors in our Colonies, and yet this department of botany is most interesting and important. Directly, every problem of the geographical distribution of plants is interfered with by these intruders, and, indirectly, the zoology of the Colonies

* Sonchus arvensis is wild in-New Zealand, and was eaten by the aborigines; but the cultivated form is far more abundant than the aboriginal.

is even more materially affected. Hitherto, but one author has had the boldness to inquire into the rationale of this "replacement;" to grapple with the startling fact that plants are thus proved to be located by nature, not necessarily under the conditions best suited to their development; and this is Mr. Darwin. This great naturalist informs me that he believes that the facts hitherto observed favour the supposition, that in the struggle for life between the denizens of the Old continents and the New, the former are prepotent; and he attributes this to the longer period they have been engaged in strife, and consequent vigour they have acquired. European weeds have established themselves abundantly in America and Australia, but comparatively very few plants of these countries have become weeds in England. We may hence infer why it is that the indigenous vegetations of St. Helena and Madeira show no tendency to increase, whilst European and African trees, shrubs and herbs are rapidly covering those islands.

On the other hand, there are many reasons why a whole population of Old World plants should become established in the New World and in the Colonies, before any symptoms of a return emigration can be detected. The export of garden and field-crop seeds, as well as of European animals and merchandise, favours the rapid introduction of emigrant plants into the Colonies, but there is no such import of materials to Europe; it is also to be observed, that the processes of agriculture, conducted through many centuries, have resulted in a European population of sturdy weeds (the tramps of our Flora), and that there can be nothing of the kind in the Colonies, till these are as long cultivated by ourselves or others. It is therefore conceivable that, had New Zealand been cultivated for a thousand years, there might have resulted races of its native Dock, Speedwells, Polygona, &c., &c., which would, on being introduced into England, displace some of those very Docks and Polygona now establishing themselves in New Zealand; and that something of this sort is going on may be argued from the rapidity with which the Impatiens fulva, Galinsoga, Erigeron Canadense, Claytonia perfoliata and other American plants are spreading in Europe, of which some are bona-fide weeds with us, and may perhaps at a future period be re-introduced into America as more intrusive forms of the species than that continent now possesses. So too, I am given to understand, various Australian Acacia, Eucalypti and other Myrtacea are established and spreading spontaneously in the peninsula of India;

which is the case of the plants of an assumed new continent displacing those of one much older.

The rapid propagation of European animals is no less remarkable than that of plants. J. Haast, Esq., Government Geologist, Canterbury, writes as follows to Mr. Darwin, in a letter which the latter has kindly communicated to me, transferring to me the author's permission to make any use of the facts recorded :—

"The native (Maori) saying is, 'as the white man's rat has driven away the native rat, so the European fly drives away our own, and the clover kills our fern, so will the Maoris disappear before the white man himself.' It is wonderful to behold the Botanical and Zoological changes which have taken place since first Captain Cook set foot in New Zealand. Some pigs which he and other navigators left with the natives have increased and run wild in such a way that it is impossible to destroy them. There are large tracts of country where they reign supreme. The soil looks as if ploughed by their burrowing. Some station holders of 100,000 acres have had to make contracts for killing them at 6d per tail, and as many as 22,000 on a single run have been killed by adventurous parties without any diminution being discernible. Not only are they obnoxious by occupying the ground which the sheep farmer needs for his flocks, but they assiduously follow the ewes when lambing, and devour the poor lambs as soon as they make their appearance. They do not exist on the western side of the Alps, and only on the lower grounds on the eastern side where snow seldom falls, so that the explorer has not the advantage of profiting by their existence, where food is scarcest. The boars are sometimes very large, covered with long black bristles, and have enormous tusks, resembling closely the wild boar of the Ardennes, and they are equally savage and courageous.

"Another interesting fact is the appearance of the Norwegian rat; it has thoroughly extirpated the native rat, and is to be found everywhere, even in the very heart of the Alps, growing to a very large size. The European mouse follows it closely, and what is more surprising, where it makes its appearance, it drives, to a great degree, the Norway rat away. Amongst other quadrupeds, cattle, dogs, and cats are found in a wild state, but not abundantly.

"The European house-fly is another importation. When it arrives, it repels the blue-bottle of New Zealand, which seems to shun its company. But the spread of the European insect goes on very