Chapter XXIV: Summary and Conclusion (1)

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--Japan 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 assemblages 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 classified, in accordance with their physical origin, in three groups or classes, 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.

Beginning with simple and familiar facts relating to British and European quadrupeds and birds, I have defined and shown the exact character of "areas of distribution," as applied to species, genera, and families, and have illustrated the subject by maps showing the peculiarities of distribution of some well-known groups of birds. Taking then our British mammals and land-birds, I follow them over the whole area they inhabit, and thus obtain a foundation for the establishment of "zoological regions," and a clear insight into their character as distinct from the usual geographical divisions of the globe.

The facts thus far established are then shown to be necessary results of the "law of evolution." The nature and amount of "variation" is exhibited by a number of curious examples; the origin, growth, and decay of species and genera are traced, and all the interesting phenomena of isolated groups and discontinuous generic and specific areas are shown to follow as logical consequences.

The next subject investigated is the means by which the various groups of animals are enabled to overcome the natural barriers which often seem to limit them to very restricted areas, how far those barriers are themselves liable to be altered or abolished, and what is the exact nature and amount of the changes of sea and land which our earth has undergone in past times. This latter part of the inquiry is shown to be the most important as it is the most fundamental; and as it is still a subject of controversy, and many erroneous views prevail in regard to it, it is discussed at some length. Several distinct classes of evidence are adduced to prove that the grand features of our globe--the position of the great oceans {534} and the chief land-areas--have remained, on the whole, unchanged throughout geological time. Our continents are shown to be built up mainly of "shore-deposits"; and even the chalk, which is so often said to be the exact equivalent of the "globigerina ooze" now forming in mid-Atlantic, is shown to be a comparatively shallow-water deposit formed in inland seas, or in the immediate vicinity of land. The general stability of continents has, however, been accompanied by constant changes of form, and insular conditions have prevailed over every part in succession; and the effect of such changes on the distribution of organisms is pointed out.

We then approach the consideration of another set of changes--those of climate--which have probably been agents of the first importance in modifying the specific forms as well as the distribution of animals. Here again we find ourselves in the midst of fierce controversies. The occurrence of a recent glacial epoch of great severity in the northern hemisphere is now universally admitted, but the causes which brought it on are matter of dispute. But unless we can arrive at these causes, as well as at those which produced the equally well demonstrated mild climate in the Arctic regions, we shall be quite unable to determine the nature and amount of the changes of climate which have occurred throughout past ages, and shall thus be left without a most important clue to the explanation of many of the anomalies in the distribution of animals and plants.

I have therefore devoted three chapters to a full investigation of this question. I have first given such a sketch of the most salient facts as to render the phenomena of the glacial epoch clear and intelligible. I then review the various suggested explanations, and, taking up the two which alone seem tenable, I endeavour to determine the true principles of each. While adopting generally Mr. Croll's views as to the causes of the "glacial epoch," I have introduced certain limitations and modifications. I have pointed out, I believe, more clearly than has hitherto been done, the very different effects on climate of water in the liquid and in the solid state; and I have {535} shown, by a variety of evidence, that without high land there can be no permanent snow and ice. From these facts and principles the very important conclusion is reached, that the alternate phases of precession--causing the winter of each hemisphere to be in _aphelion_ and _perihelion_ each 10,500 years--would produce a complete change of climate only where a country was _partially_ snow-clad; while, whenever a large area became almost _wholly_ buried in snow and ice--as was certainly the case with Northern Europe and America during the glacial epoch--then the glacial conditions would be continued and perhaps even intensified when the sun approached nearest to the earth in winter, instead of there being at that time, as Mr. Croll maintains, an almost perpetual spring. This important result is supported by reference to the existing differences between the climates of the northern and southern hemispheres, and by what is known to have occurred during the last glacial epoch; and it is shown to be in complete harmony with the geological evidence as to interglacial mild periods.

Discussing next the evidence for glacial epochs in earlier times, it is shown that Mr. Croll's views are opposed by a vast body of facts, and that the geological evidence leads irresistibly to the conclusion that during a large portion of the Secondary and Tertiary periods, uninterrupted warm climates prevailed in the north temperate zone, and so far ameliorated the climate of the Arctic regions as to admit of the growth of a luxuriant vegetation in the highest latitudes yet explored. The geographical condition of the northern hemisphere at these periods is then investigated, and it is shown to have been probably such as to admit the warm tropical waters freely to penetrate the land, and to reach the Arctic seas by several channels; and, adopting Mr. Croll's calculations as to the enormous quantity of heat that would thus be conveyed northwards, it is maintained that the mild Arctic climates are amply accounted for. With such favourable geographical conditions, it is shown, that changes of excentricity and of the phases of precession would have no other effect than to cause greater differences {536} of temperature between summer and winter; but, wherever there was a considerable extent of very lofty mountains the snow-line would be lowered, and the snow-collecting area being thus largely increased a considerable amount of local glaciation might result. Thus may be explained the presence of enormous ice-borne rocks in Eocene and Miocene times in Central Europe, while at the very same period all the surrounding country enjoyed a tropical or sub-tropical climate.

The general conclusion is thus reached, that geographical conditions are the essential causes of great changes of climate, and that the radically different distribution of land and sea in the northern and southern hemispheres has generally led to great diversity of climate in the Arctic and Antarctic regions. The form and arrangement of the continents is shown to be such as to favour the transfer of warm oceanic currents to the north far in excess of those which move towards the south, and whenever these currents had free passage _through_ the northern land-masses to the polar area, a mild climate must have prevailed over the whole northern hemisphere. It is only in very recent times that the great northern continents have become so completely consolidated as they now are, thus shutting out the warm water from their interiors, and rendering possible a wide-spread and intense glacial epoch. But this great climatal change was actually brought about by the high excentricity which occurred about 200,000 years ago; and it is doubtful if a similar glaciation in equally low latitudes could be produced by means of any such geographical combinations as actually occur, without the concurrence of a high excentricity.

A survey of the present condition of the earth supports this view, for though we have enormous mountain ranges in every latitude, there is no glaciated country south of Greenland in N. Lat. 61deg. But directly we go back a very short period, we find the superficial evidences of glaciation to an enormous extent over three-fourths of the globe. In the Alps and Pyrenees, in the British Isles and Scandinavia, in Spain and the Atlas, in the Caucasus {537} and the Himalayas, in Eastern North America and west of the Rocky Mountains, in the Andes of South Temperate America, in South Africa, and in New Zealand, huge moraines and other unmistakable ice-marks attest the universal descent of the snow-line for several thousand feet below its present level. If we reject the influence of high excentricity as the cause of this almost universal glaciation, we must postulate a general elevation of _all_ these mountains about the same time, geologically speaking--for the general similarity in the state of preservation of the ice-marks and the known activity of denudation as a destroying agent, forbid the idea that they belong to widely separated epochs. It has, indeed, been suggested, that denudation alone has lowered these mountains so much during the post-tertiary epoch, that they were previously of sufficient height to account for the glaciation of all of them; but this hardly needs refutation, for it is clear that denudation could not at the same time have removed some thousands of feet of rock from many hundreds of square miles of lofty snow-collecting plateaus, and yet have left moraines, and blocks, and even glacial striae, undisturbed and uneffaced on the slopes and in the valleys of these same mountains.

The theory of geological climates set forth in this volume, while founded on Mr. Croll's researches, differs from all that have yet been made public, in clearly tracing out the comparative influence of geographical and astronomical revolutions, showing that, while the former have been the chief, if not the exclusive, causes of the long-continued mild climates of the Arctic regions, the concurrence of the latter has been essential to the production of glacial epochs in the temperate zones, as well as of those local glaciations in low latitudes, of which there is such an abundance of evidence.

The next question discussed is that of geological time as bearing on the development of the organic world. The periods of time usually demanded by geologists have been very great, and it was often assumed that there was no occasion to limit them. But the theory of development demands far more; for the earliest fossiliferous rocks {538} prove the existence of many and varied forms of life which require unrecorded ages for their development--ages probably far longer than those which have elapsed from that period to the present day. The physicists, however, deny that any such indefinitely long periods are available. The sun is ever losing heat far more rapidly than it can be renewed from any known or conceivable source. The earth is a cooling body, and must once have been too hot to support life; while the friction of the tides is checking the earth's rotation, and this cannot have gone on indefinitely without making our day much longer than it is. A limit is therefore placed to the age of the habitable earth, and it has been thought that the time so allowed is not sufficient for the long processes of geological change and organic development. It is therefore important to inquire whether these processes are either of them so excessively slow as has been supposed, and I devote a chapter to the inquiry.

Geologists have measured with some accuracy the maximum thickness of all the known sedimentary rocks. The rate of denudation has also been recently measured by a method which, if not precise, at all events gives results of the right order of magnitude and which err on the side of being too slow rather than too fast. If, then, the _maximum_ thickness of the _known_ sedimentary rocks is taken to represent the _average_ thickness of _all_ the sedimentary rocks, and we also know the _amount_ of sediment carried to the sea or lakes, and the _area_ over which that sediment is spread, we have a means of calculating the _time_ required for the building up of all the sedimentary rocks of the geological system. I have here inquired how far the above suppositions are correct, or on which side they probably err; and the conclusion arrived at is, that the time required is very much less than has hitherto been supposed.

Another estimate is afforded by the date of the last glacial epoch if coincident with the last period of high excentricity, while the Alpine glaciation of the Miocene period is assumed to have been caused by the next earlier phase of very high excentricity. Taking these as data, the {539} proportionate change of the species of mollusca affords a means of arriving at the whole lapse of time represented by the fossiliferous rocks; and these two estimates agree in the _order_ of their magnitudes.

It is then argued that the changes of climate every 10,500 years during the numerous periods of high excentricity have acted as a motive power in hastening on both geological and biological change. By raising and lowering the snow-line in all mountain ranges it has caused increased denudation; while the same changes have caused much migration and disturbance in the organic world, and have thus tended to the more rapid modification of species. The present epoch being a period of very low excentricity, the earth is in a phase of _exceptional stability_ both physical and organic; and it is from this period of exceptional stability that our notions of the very slow rate of change have been derived.

The conclusion is, on the whole, that the periods allowed by physicists are not only far in excess of such as are required for geological and organic change, but that they allow ample margin for a lapse of time anterior to the deposit of the earliest fossiliferous rocks several times longer than the time which has elapsed since their deposit to the present day.

Having thus laid the foundation for a scientific interpretation of the phenomena of distribution, we proceed to the Second Part of our work--the discussion of a series of typical Insular Faunas and Floras with a view to explain the interesting phenomena they present. Taking first two North Atlantic groups--the Azores and Bermuda--it is shown how important an agent in the dispersal of most animals and plants is a stormy atmosphere. Although 900 and 700 miles respectively from the nearest continents, their productions are very largely identical with those of Europe and America; and, what is more important, fresh arrivals of birds, insects, and plants, are now taking place almost annually. These islands afford, therefore, test examples of the great dispersive powers of certain groups of organisms, and thus serve as a basis on which to found our explanations of many anomalies of distribution. Passing {540} on to the Galapagos we have a group less distant from a continent and of larger area, yet, owing to special conditions, of which the comparatively stormless equatorial atmosphere is the most important, exhibiting far more speciality in its productions than the more distant Azores. Still, however, its fauna and flora are as unmistakably derived from the American continent as those of the Azores are from the European.

We next take St. Helena and the Sandwich Islands, both wonderfully isolated in the midst of vast oceans, and no longer exhibiting in their productions an exclusive affinity to one continent. Here we have to recognise the results of immense antiquity, and of those changes of geography, of climate, and in the general distribution of organisms which we know have occurred in former geological epochs, and whose causes and consequences we have discussed in the first part of our volume. This concludes our review of the Oceanic Islands.

Coming now to Continental Islands we consider first those of most recent origin and offering the simplest phenomena; and begin with the British Isles as affording the best example of very recent and well known Continental Islands. Reviewing the interesting past history of Britain, we show why it is comparatively poor in species and why this poverty is still greater in Ireland. By a careful examination of its fauna and flora it is then shown that the British Isles are not so completely identical, biologically, with the continent as has been supposed. A considerable amount of speciality is shown to exist, and that this speciality is real and not apparent is supported by the fact, that small outlying islands, such as the Isle of Man, the Shetland Isles, Lundy Island, and the Isle of Wight, all possess certain species or varieties not found elsewhere.

Borneo and Java are next taken, as illustrations of tropical islands which may be not more ancient than Britain, but which, owing to their much larger area, greater distance from the continent, and the extreme richness of the equatorial fauna and flora, possess a large proportion of peculiar species, though these are in general very closely allied to those of the adjacent parts of Asia. The {541} preliminary studies we have made enable us to afford a simpler and more definite interpretation of the peculiar relations of Java to the continent and its differences from Borneo and Sumatra, than was given in my former work (_The Geographical Distribution of Animals_).

Japan and Formosa are next taken, as examples of islands which are decidedly somewhat more ancient than those previously considered, and which present a number of very interesting phenomena, especially in their relations to each other, and to remote rather than to adjacent parts of the Asiatic continent.

We now pass to the group of Ancient Continental Islands, of which Madagascar is the most typical example. It is surrounded by a number of smaller islands which may be termed its satellites since they partake of many of its peculiarities; though some of these--as the Comoros and Seychelles--may be considered continental, while others--as Bourbon, Mauritius, and Rodriguez--are decidedly oceanic. In order to understand the peculiarities of the Madagascar fauna we have to consider the past history of the African and Asiatic continents, which it is shown are such as to account for all the main peculiarities of the fauna of these islands without having recourse to the hypothesis of a now-submerged Lemurian continent. Considerable evidence is further adduced to show that "Lemuria" is a myth, since not only is its existence unnecessary, but it can be proved that it would not explain the actual facts of distribution. The origin of the interesting Mascarene wingless birds is discussed, and the main peculiarities of the remarkable flora of Madagascar and the Mascarene islands pointed out; while it is shown that all these phenomena are to be explained on the general principles of the permanence of the great oceans and the comparatively slight fluctuations of the land area, and by taking account of established palaeontological facts.

There remain two other islands--Celebes and New Zealand--which are classed as "anomalous," the one because it is almost impossible to place it in any of the six zoological regions, or determine whether it has ever been actually joined to a continent--the other because it {542} combines the characteristics of continental and oceanic islands.

The peculiarities of the Celebesian fauna have already been dwelt upon in several previous works, but they are so remarkable and so unique that they cannot be omitted in a treatise on "Insular Faunas"; and here, as in the case of Borneo and Java, fuller consideration and the application of the general principles laid down in our First Part, lead to a solution of the problem at once more simple and more satisfactory than any which have been previously proposed. I now look upon Celebes as an outlying portion of the great Asiatic continent of Miocene times, which either by submergence or some other cause had lost the greater portion of its animal inhabitants, and since then has remained more or less completely isolated from every other land. It has thus preserved a fragment of a very ancient fauna along with a number of later types which have reached it from surrounding islands by the ordinary means of dispersal. This sufficiently explains all the peculiar _affinities_ of its animals, though the peculiar and distinctive _characters_ of some of them remain as mysterious as ever.

New Zealand is shown to be so completely continental in its geological structure, and its numerous wingless birds so clearly imply a former connection with some other land (as do its numerous lizards and its remarkable reptile, the Hatteria), that the total absence of indigenous land-mammalia was hardly to be expected. Some attention is therefore given to the curious animal which has been seen but never captured, and this is shown to be probably identical with an animal referred to by Captain Cook. The more accurate knowledge which has recently been obtained of the sea bottom around New Zealand enables us to determine that the former connection of that island with Australia was towards the north, and this is found to agree well with many of the peculiarities of its fauna.

The flora of New Zealand and that of Australia are now both so well known, and they present so many peculiarities, and relations of so anomalous a character, {543} as to present in Sir Joseph Hooker's opinion an almost insoluble problem. Much additional information on the physical and geological history of these two countries has, however, been obtained since the appearance of Sir Joseph Hooker's works, and I therefore determined to apply to them the same method of discussion and treatment which has been usually successful with similar problems in the case of animals. The fact above noted, that New Zealand was connected with Australia in its northern and tropical portion only, of itself affords a clue to one portion of the specialities of the New Zealand flora--the presence of an unusual number of tropical families and genera, while the temperate forms consist mainly of species either identical with those found in Australia or closely allied to them. But a still more important clue is obtained in the geological structure of Australia itself, which is shown to have been for long periods divided into an eastern and a western island, in the latter of which the highly peculiar flora of temperate Australia was developed. This is found to explain with great exactness the remarkable absence from New Zealand of all the most abundant and characteristic Australian genera, both of plants and of animals, since these existed at that time only in the _western_ island, while New Zealand was in connection with the _eastern_ island alone and with the tropical portion of it. From these geological and physical facts, and the known powers of dispersal of plants, all the main features, and many of the detailed peculiarities of the New Zealand flora are shown necessarily to result.

Our last chapter is devoted to a wider, and if possible more interesting subject--the origin of the European element in the floras of New Zealand and Australia, and also in those of South America and South Africa. This is so especially a botanical question, that it was with some diffidence I entered upon it, yet it arose so naturally from the study of the New Zealand and Australian floras, and seemed to have so much light thrown upon it by our preliminary studies as to changes of climate and the causes which have favoured the distribution of plants, that I felt my work would be incomplete without a consideration of {544} it. The subject will be so fresh in the reader's mind that a complete summary of it is unnecessary. I venture to think, however, that I have shown, not only the several routes by which the northern plants have reached the various southern lands, but have pointed out the special aids to their migration, and the motive power which has urged them on.

In this discussion, if nowhere else, will be found a complete justification of that lengthy investigation of the exact nature of past changes of climate, which to some readers may have seemed unnecessary and unsuited to such a work as the present. Without the clear and definite conclusions arrived at by that discussion, and those equally important views as to the permanence of the great features of the earth's surface, and the wonderful dispersive powers of plants which have been so frequently brought before us in our studies of insular floras, I should not have ventured to attack the wide and difficult problem of the northern element in southern floras.

In concluding a work dealing with subjects which have occupied my attention for many years, I trust that the reader who has followed me throughout will be imbued with the conviction that ever presses upon myself, of the complete interdependence of organic and inorganic nature. Not only does the marvellous structure of each organised being involve the whole past history of the earth, but such apparently unimportant facts as the presence of certain types of plants or animals in one island rather than in another, are now shown to be dependent on the long series of past geological changes--on those marvellous astronomical revolutions which cause a periodic variation of terrestrial climates--on the apparently fortuitous action of storms and currents in the conveyance of germs--and on the endlessly varied actions and reactions of organised beings on each other. And although these various causes are far too complex in their combined action to enable us to follow them out in the case of any one species, yet their broad results are clearly recognisable; and we are thus encouraged to study more completely every detail and {545} every anomaly in the distribution of living things, in the firm conviction that by so doing we shall obtain a fuller and clearer insight into the course of nature, and with increased confidence that the "mighty maze" of Being we see everywhere around us is "not without a plan."

{549}

INDEX

A.
Acacia, wide range of in Australia, 185
_Acacia heterophylla_, and _Acacia koa_, 443
Acaena in California, 527
_Accipiter hawaii_, 314
Achatinellinae, average range of, 317
_Aegialitis sanctae-helenae_, 305
Africa, characteristic mammalia of, 416
former isolation of, 418
Africa and Madagascar, relations of, 418
early history of, 419
African highlands as aiding the migration of plants, 524
African reptiles absent from Madagascar, 418
Aggressive power of the Scandinavian flora, 511
Air and water, properties of, in relation to climate, 131
_Alectoraenas pulcherrimus_, 429
Allen, Mr. J. A., on variation, 58
Allied species occupy separate areas, 478
Alpine plants, their advantages as colonisers, 503
Alternations of climate in Switzerland and North America, 121
Alternations of climate, palaeontological evidence of, 119
Amazon, limitation of species by, 18
_Amblyrhynchus cristatus_, 279
American genera of reptiles in Madagascar, 417
Amphibia, dispersal of, 76
of the Seychelles, 432
introduced, of Mauritius, 435
of New Zealand, 483
Amphioxus, 63
Amphisbaenidae, 28
_Amydrus Tristramii_, restricted range of, 16
_Anas Wyvilliana_, 314
Ancient continental islands, 244, 411
Ancient glacial epochs, 169
what evidence of may be expected, 175
Ancient groups in Madagascar, 419
Andersson, N. J., on the flora of the Galapagos, 287
Andes, migration of plants along the, 520
_Angraecum sesquipedale_, 440
Animal life, effects of glacial epoch on, 117
Animal life of Formosa, 401
_Anoa depressicornis_, 456
Antarctic continent as a means of plant-dispersion, 521
Antarctic islands, with perpetual snow, 136
Antelopes, overlapping genera of, 29
Antiquity of Hawaiian fauna and flora, 328
of land-shells, 79
of New Zealand, 526
of plants as affecting their dispersal, 82
_Apera arundinacea_, 503
_Apium graveolens_ in New Zealand, 515
Apteryx, species of, 476
_Arabis hirsuta_ on railway arch, 514
Archaic forms still existing, 229
Arctic and Antarctic regions, contrasts of, 135
Arctic current, effects of a stoppage of, 150
Arctic plants in the southern hemisphere, 509
Arctic regions, mild climates of, 181
recent interglacial mild period in, 182
Arctic warm climates of Secondary and Palaeozoic times, 201
Areas of distribution, 13
separate and overlapping, 17, 28
Ascension, former climate and productions of, 303
Astronomical and geographical causes, comparative effects of, on climate,
207
Astronomical causes of change of climate, 126
of glaciation, 140
Atlantic isles, peculiar mosses of, 368
Atlantosaurus, the largest land-animal, 98
_Atriplex patula_ on a railway bank, 515
Auchenia, 27
Austen, Mr. Godwin, on littoral shells in deep water, 337
Australia, two sets of Northern plants in, 523
South European plants in, 523
Australia and South Africa, supposed connection of, 525
{550}
Australian Alps, indications of glaciation in, 163
birds absent from New Zealand, 483
flora, general features of, 491
richest in temperate zone, 491
recent and derivative in the tropics, 492
its south-eastern and south-western divisions, 493
Sir Joseph Hooker on, 494
geological explanation of, 494
its presence in New Zealand, 498
natural orders of, wanting in New Zealand, 490
orchideae in China, 527
genera of plants in India, 524
plants absent from New Zealand, 488, 490
none in north temperate zone, 527
running wild in Neilgherrie mountains, 528
region, definition of, 45
mammals and birds of, 46
seeds scattered in New Zealand, 508
Aylward, Captain, on glaciation of South Africa, 163
Azores, 247
absence from, of large-fruited trees or shrubs, 260
zoological features of, 248
birds of, 249
insects of, 253
beetles of, 253
land-shells of, 256
flora of, 256
Azores and New Zealand, identical plants in both, 512
Azorean bird-fauna, origin of, 250
fauna and flora, deductions from, 261
plants, facilities for the dispersal of, 260

B.
_Babirusa alfurus_ in Celebes, 456
Badgers, 41
Bahamas contrasted with Florida, 5
Baker, Mr., on flora of Mauritius and the Seychelles, 441
Bali and Lombok, contrasts of, 4
Banca, peculiar species of, 386
_Barbarea precox_ on railway bank, 514
Barn-owl, wide range of, 15
Baron, Rev. R., on the flora of Madagascar, 441
Barriers to dispersal, 73
Batrachia, 30
Bats in Bermuda, 269
Bears of Europe and America, 14
Beaver of Europe and America, 14
Beetles of the Azores, 253
remote affinities of some of, 255
of the Galapagos, 284
of St. Helena, 298
of the Sandwich Islands, 318
peculiar British species of, 351
Bell-birds, distribution of, 24
Bennett, Mr. Arthur, on peculiar British plants, 360
on the vegetation of railway banks, 514
Bentham, Mr., on the compositae of the Galapagos, 288
on the compositae of St. Helena, 307
on the Mascarene compositae, 445
on Sandwich Island compositae, 325
Bermuda, 262
soundings around, 263
red clay of, 265
zoology of, 266
reptiles of, 266
birds of, 266
insects of, 269
land-mollusca of, 270
flora of, 271
Bermuda and Azores, comparison of bird-faunas of, 268
_Bernicla sandvichensis_, 314
Biological causes which determine distribution, 532
Biological features of Madagascar, 416
Birds as plant-dispersers, 81
as seed-carriers, 81, 258
common to Great Britain and Japan, 396
common to India and Japan, 399
specific range of, 15
range of British, 34
range of East Asian, 38
variation in N. American, 58
dispersal of, 75
of the Azores, 249
of Bermuda, 266
of Bermuda and Azores compared, 268
of the Galapagos, 280
of the Sandwich Islands, 313
peculiar to Britain, 340
of Borneo, 377
of Java, 382
of the Philippines, 388
of Japan, 396
peculiar to Japan, 398
peculiar to Formosa, 404
common to Formosa and India or Malaya, 407
of Madagascar, and their teachings, 422
of Comoro Islands, 429
of the Seychelles, 430
of the Mascarene islands, 436
of islands east and west of Celebes, 454
of Celebes, 458
peculiar to Celebes, 459
Himalayan types of, in Celebes, 462
list of, in Celebes, 466
of New Zealand, 476, 482
wingless, of New Zealand, 476
Blackburn, Mr. T., on the beetles of the Sandwich Islands, 318
Blakiston and Pryer on birds of Japan, 396
{551}
Bland, Mr., on land-shells of Bermuda, 270
Blanford, Mr. W. T., on small effect of marine denudation, 225
Blanford, Mr. H. F., on former connection of Africa and India, 426
Blocks, travelled and perched, 109
Blue magpies, range of, 15
Borneo, geology of, 375
mammalia of, 376
birds of, 377
affinities of fauna of, 381
Borneo and Asia, resemblance of, 6
Borneo and Java, 373
Boulder-beds of the carboniferous formation, 201
Boulder clays of east of England, 118
Bovidae, 29
Brady, Mr. H. B., on habitat of globigerinae, 92
Braithwaite, Dr. R., on peculiar British mosses, 365
Britain, probable climate of, with winter in _aphelion_, 156
British birds, range of, 34-38
British Columbia, interglacial warm periods in, 121
British fauna and flora, peculiarities of, 370
British Isles, recent changes in, 332
proofs of former elevation of, 334
submerged forests of, 335
buried river channels of, 336
last union of, with continent, 337
why poor in species, 338
peculiar birds of, 339
fresh-water fishes of, 340
peculiar insects of, 344
peculiar Lepidoptera of, 347
peculiar Coleoptera of, 351
peculiar Trichoptera of, 355
peculiar land and fresh-water shells of, 356
peculiarities of the flora of, 360
peculiar mosses and Hepaticae of, 366
British mammals as indicating a zoological region, 33
Buller, Sir W. L., on the New Zealand rat, 475
Buried river-channels, 336
_Buteo solitarius_, 314
Butterflies of Celebes, peculiar shape of, 463
Butterflies, peculiar British, 347

C.
Caddis-flies peculiar to Britain, 355
Caecilia, species of, in the Seychelles, 432
wide distribution of, 432
Caeciliadae, 28
_Callithea Leprieuri_, distribution of, 18
_Callithea sapphira_, 18
Camels as destroyers of vegetation, 296
former wide distribution of, 421
Camelus, 17, 27
_Campanula vidalii_, 261
Canis, 17, 26
Carabus, numerous species of, 42
Carboniferous boulder-beds, 201
warm Arctic climate, 201
Carnivora in Madagascar, 417
Carpenter, Dr., on habitat of globigerinae, 92
Carpenter, Mr. Edward, on Mars and glacial periods, 164
_Carduus marianus_ in New Zealand, 515
_Carpodacus purpureus_ and _P. californicus_, 68
Castor, 17
Casuarina, 185
in India, 527
Cause of extinction, 63
Caves of Glamorganshire, 336
Cebibae, overlapping genera of, 29
Celebes, physical features of, 451
islands around, 452
zoology of, 455
derivation of mammals of, 457
birds of, 458
not a continental island, 461
insect peculiarities of, 462
Himalayan types in, 462
peculiarity of butterflies of, 463
list of land-birds of, 466
Centetidae, 27
Centetidae, formerly inhabited Europe, 420
Central America, mixed fauna of, 53
Ceratodus, or mud-fish, 69
Cervus, 17, 26
Chalk a supposed oceanic formation, 89
Chalk at Oahu, analysis of, 90
Chalk, analysis of, 91
Chalk mollusca indicative of shallow water, 93
Chalk sea, extent of, in Europe, 93
Chalk-formation, land-plants found in, 94
deposited in an inland sea, 93
of Faxoe an ancient coral-reef, 94
modern formation of, 95
supposed oceanic origin of, erroneous, 96
"Challenger" soundings and shore-deposits, 86
"Challenger" ridge in the Atlantic, 101
Chameleons very abundant in Madagascar, 430
Chamois, distribution of, 13
Changes of land and sea, 83
Chasmorhynchus, distribution of, 24
_C. nudicollis_, 24
_C. tricarunculatus_, 24
_C. variegatus_, 24
_C. niveus_, 24
_Chilomenus lunata_, 300
Chinchillas, 26
Chrysochloridae, 29
Cicindela, 17
Cicindelidae common to South America and Madagascar, 28
Clay, red, of Bermuda, 265
Climate, astronomical causes of changes of, 126
{552}
properties of snow and ice in relation to, 131
of Britain with winter in _aphelion_, 156
of Tertiary period in Europe and N. America, 178
temperate in Arctic regions, 181
causes of mild Arctic, 190
of Tertiary and Secondary periods, 199, 202
of the Secondary and Palaeozoic epochs, 200
change of, during Tertiary and Secondary Periods, 200
affected by arrangement of the great continents, 205
nature of changes of, caused by high excentricity, 230
exceptional stability of the present, 232
changes of, as affecting migration of plants, 517
Climatal changes, 106
change, its essential principle restated, 158
changes as modifying organisms, 229
Clouds cut off the sun's heat, 145
Coal in Sumatra, 385
Coast line of globe, extent of, 221
Cochoa, distribution of, 25
Cockerell, Mr. Th. D. A., on slugs of Bermuda, 271
on British land and fresh-water shells, 356
Cold alone does not cause glaciation, 135
how it can be stored up, 133
Coleoptera of the Azores, 253
of St. Helena, 298
of the Sandwich Islands, 318
peculiar British species of, 351
Comoro Islands, 428
mammals and birds of, 428
Compositae of the Galapagos, 288
of St. Helena, 307
of the Sandwich Islands, 325
of the Mascarene Islands, 445
species often have restricted ranges, 504
Conclusions on the New Zealand flora, 506
Contemporaneous formation of Lower Greensand and Wealden, 221
Continental conditions throughout geological time, 97-99
changes and animal distribution, 102
extensions will not explain anomalous facts of distribution, 449
Continental islands, 243
of recent origin, 331
general remarks on recent, 408
ancient, 411
Continental period, date of, 337
Continents, movements of, 88
permanence of, 97
general stability of, 101, 103
geological development of, 205
Continuity of land, 74
Continuity of now isolated groups, proof of, 70
Cook, Captain, on a native quadruped in New Zealand, 476
Cope, Professor, on the Bermuda lizard, 266
_Coracias temminckii_, in Celebes, 463
Corvus, 17
Cossonidae, in St. Helena, 299
Cretaceous deposits in North Australia, 493, 496
Cretaceous flora of Greenland, 185
of the United States, 189
Croll, Dr. James, on Antarctic icebergs, 136
on winter temperature of Britain in glacial epoch, 141
on diversion of gulf-stream during the glacial epoch, 143
on loss of heat by clouds and fogs, 145
on geographical causes as affecting climate, 148
on ancient glacial epochs, 170
on universality of glacial markings in Scotland, 174
on mild climates of Arctic regions, 189
on ocean-currents, 190, 204
on age of the earth, 213
on mean thickness of sedimentary rocks, 220
on small amount of marine denudation, 225
on buried river-channels, 336
Ctenodus, 69
Cyanopica, distribution of, 24
_Cyanopica cooki_, restricted range of, 15, 24
_Cyanopica cyanus_, 24
_Cynopithecus nigrescens_, in Celebes, 456

D.
Dacelo, 47
Dana on continental upheavals, 88
on chalk in the Sandwich Islands, 90
on elevation of land causing the glacial epoch, 152
on elevation of Western America, 194
on the development of continents, 205
on shore-deposits, 222
on life extermination by cold epochs, 230
Darwin, experiment on _Helix pomatia_, 78
on the permanence of oceans, 100
on cloudy sky of Antarctic regions, 146
on glaciers of the Southern Andes, 147
on geological time, 211
on complex relations of organisms, 226
on oceanic islands, 242
on seeds carried by birds, 257
{553}
experiments on seed-dispersal, 258
on natural history of the Keeling Islands, 286
theory of formation of atolls, 397
on cultivated plants not running wild, 507
Dawkins, Professor Boyd, on animal migrations during the glacial epoch,
120
Dawson, Mr. G. M., on alternations of climate in British Columbia, 121
Professor, on Palaeozoic boulder-beds in Nova Scotia, 201
De Candolle on dispersal of seeds, 80
Deep-sea deposits, 219
Deer in Celebes, 456
_Delphinium ajacis_, on a railway bank, 515
_Dendroeca_, 19
_D. coerulea_, 19
_D. discolor_, 19
_D. dominica_, 19
_Dendroeca coronata_, variation of, 58
Dendrophidae, 29
Denudation destroys the evidences of glaciation, 172
Denudation and deposition as a measure of time, 213
Denudation in river basins, measurement of, 215
Denudation, marine as compared with sub-aerial, 225
Deposition of sediments, how to estimate the average, 221
Deserts, cause of high temperature of, 132
Diagram of excentricity and precession, 129
Diagram of excentricity for three million years, 171
Dididae, how exterminated, 436
Didunculus, keeled sternum of, 437
Diospyros, in upper greensand of Greenland, 186
_Diplotaxis muralis_, on railway banks, 513
Dipnoi, discontinuity of, 69
Dipterus, 69
Discontinuity among North American birds, 67
Discontinuity a proof of antiquity, 69
Discontinuous generic areas, 23
Discontinuous areas, 64
why rare, 64
Dispersal of animals, 72
of land animals, how effected, 73, 76
along mountain-chains, 81
of seeds by wind, 80, 257
by birds, 81, 258
by ocean-currents, 81, 258
of Azorean plants, facilities for, 260
Distribution, changes of, shown by extinct animals, 102
how to explain anomalies of, 420
Drontheim mountains, peculiar mosses of, 368
Dobson, Mr., on bats of Japan, 394
on the affinities of _Mystacina tuberculata_, 474
Dodo, the, 436
aborted wings of, 437
Dryiophidae, 28
Dumeril, Professor, on lizards of Bourbon, 435
Duncan, Professor P. M., on ancient sea of central Australia, 496

E.
Early history of New Zealand, 484
Earth's age, 210
East Asian birds, range of, 38
East and West Australian floras, geological explanation of, 494
Echidna, 30
Echimyidae, 27
Elevation of North America during glacial period, 154
causing diversion of gulf-stream, 154
Elwes, Mr. H. J., on distribution of Asiatic birds, 380
_Emberiza schoeniclus_, discontinuity of, 66
_E. passerina_, range of, 66
_E. pyrrhulina_, 66
Endemic genera of plants in Mauritius, &c., 443
Endemic genera of plants in New Zealand, 526
English plants in St. Helena, 297
Environment, change of, as modifying organisms, 225
_Eriocaulon septangulare_, 363
Ethiopian Region, definition of, 42
birds of, 43
Ettingshausen, Baron von, on the fossil flora of New Zealand, 499
on Australian plants in England, 518
Eucalyptus, wide range of, in Australia, 185
Eucalyptus and Acacia, why not in New Zealand, 507
Eucalyptus in Eocene of Sheppey, 518
Eupetes, distribution of, 25
Europe, Asia, &c., as zoological terms, 32
European birds, range of, 16
in Bermuda, 269
European occupation, effects of, in St. Helena, 294
European plants in New Zealand, 507
in Chile and Fuegia, 521
Everett, Mr., on Bornean birds, 377
on mammalia of the Philippines, 387
on Philippine birds, 388
on raised coral-reefs in the Philippines, 389
Evolution necessitates continuity, 70
Excentricity and precession, diagram of, 129
Excentricity, variations of, during three million years, 171
Excentricity a test of rival theories of climate, 171
Excentricity, high, its effects on warm and cold climates, 198
Explanation of peculiarities of the fauna of Celebes, 460
{554}
Extinct animals showing changes of distribution, 102
Extinct birds of the Mascarene Islands, 436
of New Zealand, 476
Extinction caused by glacial epoch, 122

F.
Families, restricted areas of, 29
distribution and antiquity of, 68
Fauna and flora, peculiarities of British, 370
Fauna of Borneo, affinities of, 381
of Java, 382
of Java and Asia compared, 384
Faunas of Hainan, Formosa, and Japan compared, 407
Felis, 17, 26
Ferns, abundance of, in Mascarene flora, 445
Ficus, fossil Arctic, 186
Fire-weed, the, of Tasmania, 513
Fisher, Rev. O., on temperature of space, 131
Fishes, dispersal of, 76
peculiar British, 340
cause of great speciality in, 343
mode of migration of fresh-water, 344
fresh-water, of New Zealand, 484
Floating islands, and the dispersal of animals, 74
Flora of the Azores, 256
of Bermuda, 271
of the Galapagos, 287
of St. Helena, 305
of the Sandwich Islands, 321;
peculiar features of, 323
peculiarities of the British, 360
of Madagascar and the Mascarene Islands, 439
of Madagascar and South Africa allied, 445
of New Zealand, 487
very poor, 488
its resemblance to the Australian, 489
its differences from the Australian, 490
origin of Australian element in, 498
tropical character of, explained, 500
summary and conclusion on, 506
Floras of New Zealand and Australia, summary of conclusion as to, 542
Florida and Canada, resemblances of, 5
and Bahamas, contrasts of, 5
Fogs cut off the sun's heat in glaciated countries, 145
Forbes, Mr. D., analysis of chalk, 91
Forbes, Mr. H. O., on plants of the Keeling Islands, 286
Formosa, 400
physical features of, 401
animal life of, 401
list of mammalia of, 402
list of land-birds peculiar to, 404
Forests, submerged, 335
Fowler, Rev. Canon, on peculiar British coleoptera, 346, 351
Freezing water liberates low-grade heat, 145
Fresh-water deposits, extent of, 97
organisms absent in St. Helena, 304
snail peculiar to Ireland, 356
fishes of the Seychelles, 433
Frogs of the Seychelles, 432
of New Zealand, 483
Fuegia, European plants in, 521
_Fulica alai_, 313

G.
Galapagos Islands, 275
Galapagos, absence of mammalia and amphibia from, 278
reptiles of, 278
birds of, 280
insects of, 284
land-shells of, 285
flora of, 287
and Azores contrasted, 290
_Galbula cyaneicollis_, distribution of, 18
_rufoviridis_, 18
_viridis_, 18
Galeopithecus, 63
_Gallinula sandvichensis_, 313
Gardner, Mr. J. S., on Tertiary changes of climate, 203
Garrulus, distribution of species of, 20
_Garrulus glandarius_, 21, 23, 65
_G. cervicalis_, 21
_G. krynicki_, 21
_G. atricapillus_, 21
_G. hyrcanus_, 21
_G. brandti_, 21, 23
_G. lanceolatus___, 22
_G. bispecularis_, 22
_G. sinensis_, 22
_G. taivanus_, 22
_G. japonicus_, 22, 65
Geikie, Dr. James, on interglacial deposits, 121
Sir Archibald, on age of buried river-channels, 337
on stratified rocks being found near shores, 87
on formation of chalk in shallow water, 96
on permanence of continents, 104
on variation in rate of denudation, 173
on the rate of denudation, 215
on small amount of marine denudation, 225
Genera, extent of, 17
origin of, 61
rise and decay of, 64
Generic areas, 17
Generic and Family distribution, 25
Genus, defined and illustrated, 17
Geographical change as a cause of glaciation, 148
changes, influence of, on climate, 150, 152
{555}
changes, effect of, on Arctic climates, 195
changes of Java and Borneo, 385
changes as modifying organisms, 228
Geological climates and geographical conditions, 204
time, 210
change, probably quicker in remote times, 223
time, value of the estimate of, 224
time, measurement of, 235
changes as aiding the migration of plants, 519
climates as affecting distribution, 534
climates, summary of causes of, 536
time, summary of views on, 539
Geology of Borneo, 375
of Madagascar, 412
of Celebes, 451
of New Zealand, 472
of Australia, 494
_Geomalacus maculcosus_, 356
Glacial climate not local, 113
deposits of Scotland, 112
Glacial epoch, proofs of, 107
effects of, on animal life, 117
alternations of climate during, 118
as causing migration and extinction, 122
causes of, 125
the essentials to the production of, 136
probable date of the, 160
and the climax of continental development, 206
date of last, 233
Glacial phenomena in North America, 116
Glaciation was greatest where rainfall is now greatest, 139
action of meteorological causes on, 142
summary of chief causes of, 144
in Northern Hemisphere, the only efficient cause of, 144
of New Zealand and South Africa, 162
local, due to high excentricity, 207
widespread in recent times, 536
Gleichenia in Greenland, 186
in relation to chalk, 89
Globigerina-ooze, analysis of, 91
Globigerinae, where found, 92
Glyptostrobus, fossil, 186
Goats, destructiveness of, in St. Helena, 295
Godman, Mr., on birds reaching the Azores, 248, 250
Gray, Professor Asa, on extinction of European plants by the glacial
epoch, 123
Great Britain and Japan, birds common to, 396
Greene, Dr. J. Reay, on chameleons in Bourbon and Mauritius, 435
Greenland, loss of sun-heat by clouds in, 147
an anomaly in the Northern Hemisphere, 154
Miocene flora of, 183
Cretaceous flora of, 186
flora of ice-surrounded rocks of, 522
Grinnell Land, fossil flora of, 184
Guernsey, peculiar caddis-fly in, 355
Gulick, Rev. J. T., on Achatinellinae, 318
Guenther, Dr., on gigantic tortoises, 279
on peculiar British fishes, 341
on _Urotrichus gibsii_, 394
on lizards in the London Docks, 431
on Indian toads in Mauritius, 438
Guppy, Mr., on chalk of Solomon Islands, 91

H.
Haast, Dr., on otter-like mammal in New Zealand, 475
Habitability of globe due to disproportion of land and water, 209
_Haplothorax burchellii_, 299
Hartlaub, Dr., on "Lemuria," 423, 426
_Hatteria punctata_, 483
Haughton, Professor, on heat carried by ocean-currents, 194
comparison of Miocene and existing climates, 197
on geological time, 211, 219
on thickness of sedimentary rocks, 219
Hawaiian fauna and flora, antiquity of, 328
Heat and cold, how dispersed or stored up, 131
Heat required to melt snow, 134
evolved by frozen water, its nature and effects, 145
cut off by cloud and fogs, 145
Hector, Dr., on Triassic and Jurassic flora of New Zealand, 526
Heer, Professor, on chalk sea in Central Europe, 93
Heilprin, Professor, on insects of Bermuda, 269
on land-shells of Bermuda, 270
_Helianthemum Breweri_, 360, 363
Heliodus, an American fossil, 69
Helix, 17
Hemiptera of St. Helena, 303
Hepaticae, peculiar British, 366
non-European genera of, in Britain, 367
Hesperomys, 26
Hesperornis allied to ostriches, 481
_Hieracium iricum_, 362
High land essential to the production of a glacial epoch, 195
Hildebrand, Dr. W., on flora of the Sandwich Islands, 321
Himalayan birds and insects in Celebes, 462
Hippopotamus in Yorkshire as proving a mild climate, 119
Hochstetter on the aquatic mammal of New Zealand, 475
{556}
Hooker, Sir Joseph, on the Galapagos flora, 287
on affinities of St. Helena plants, 306
on peculiar British plants, 360, 363
on the flora of New Zealand, 488
on proportion of temperate and tropical Australian floras, 492
on current of vegetation from north to south, 510
on supposed occurrence of Australian plants in England in the Tertiary
period, 518
Horne, Mr. John, on ice-sheet covering the Isle of Man, 115
Hull, Professor, on Permian breccias in Ireland indicating ice-action,
201
Humming-birds, restricted ranges of, 16
Hutton, Captain, on struthious birds of New Zealand, 479
Huxley, Professor, on geological time, 211
on European origin of African animals, 419
Hyomoschus, 27
Hyracoidea, restricted range of, 30