[Footnote 148: "Alpenblumen," by D.H. Muller. See _Nature_, vol. xxiii.
p. 333.]
[Footnote 149: This peculiarity of local distribution of colour in flowers may be compared, as regards its purpose, with the recognition colours of animals. Just as these latter colours enable the s.e.xes to recognise each other, and thus avoid sterile unions of distinct species, so the distinctive form and colour of each species of flower, as compared with those that usually grow around it, enables the fertilising insects to avoid carrying the pollen of one flower to the stigma of a distinct species.]
[Footnote 150: See H. Muller"s _Fertilisation of Flowers_, p. 18.]
[Footnote 151: The above examples are taken from Rev. G. Henslow"s paper on "Self-Fertilisation of Plants," in _Trans. Linn. Soc._ Second series, _Botany_, vol. i. pp. 317-398, with plate. Mr. H.O. Forbes has shown that the same thing occurs among tropical orchids, in his paper "On the Contrivances for insuring Self-Fertilisation in some Tropical Orchids,"
_Journ. Linn. Soc._, xxi. p. 538.]
[Footnote 152: These are the numbers given by Darwin, but I am informed by Mr. Hemsley that many additions have been since made to the list, and that cleistogamic flowers probably occur in nearly all the natural orders.]
[Footnote 153: For a full account of cleistogamic flowers, see Darwin"s _Forms of Flowers_, chap. viii.]
[Footnote 154: Henslow"s "Self-Fertilisation," _Trans. Linn. Soc._ Second series, _Botany_, vol. i. p. 391.]
[Footnote 155: The Rev. George Henslow, in his _Origin of Floral Structures_, says: "There is little doubt but that all wind-fertilised angiosperms are degradations from insect-fertilised flowers....
_Poterium sanguisorba_ is anemophilous; and _Sanguisorba officinalis_ presumably was so formerly, but has reacquired an entomophilous habit; the whole tribe Poterieae being, in fact, a degraded group which has descended from Potentilleae. Plantains retain their corolla but in a degraded form. Junceae are degraded Lilies; while Cyperaceae and Gramineae among monocotyledons may be ranked with Amentiferae among dicotyledons, as representing orders which have retrograded very far from the entomophilous forms from which they were possibly and probably descended" (p. 266).
"The genus Plantago, like _Thalictrum minus_, Poterium, and others, well ill.u.s.trate the change from an entomophilous to the anemophilous state.
_P. lanceolata_ has polymorphic flowers, and is visited by pollen-seeking insects, so that it can be fertilised either by insects or the wind. _P. media_ ill.u.s.trates transitions in point of structure, as the filaments are pink, the anthers motionless, and the pollen grains aggregated, and it is regularly visited by _Bombus terrestris_. On the other hand, the slender filaments, versatile anthers, powdery pollen, and elongated protogynous style are features of other species indicating anemophily; while the presence of a degraded corolla shows its ancestors to have been entomophilous. _P. media_, therefore, ill.u.s.trates, not a primitive entomophilous condition, but a return to it; just as is the case with _Sanguisorba officinalis_ and _Salix Caprea_; but these show no capacity of restoring the corolla, the attractive features having to be borne by the calyx, which is purplish in Sanguisorba, by the pink filaments of Plantago, and by the yellow anthers in the Sallow willow"
(p. 271).
"The interpretation, then, I would offer of inconspicuousness and all kinds of degradations is the exact opposite to that of conspicuousness and great differentiations; namely, that species with minute flowers, rarely or never visited by insects, and habitually self-fertilised, have primarily arisen through the neglect of insects, and have in consequence a.s.sumed their present floral structures" (p. 282).
In a letter just received from Mr. Henslow, he gives a few additional ill.u.s.trations of his views, of which the following are the most important: "Pa.s.sing to Incompletae, the orders known collectively as "Cyclospermeae" are related to Caryophylleae; and to my mind are degradations from it, of which Orache is anemophilous. Cupuliferae have an inferior ovary and rudimentary calyx-limb on the top. These, as far as I know, cannot be interpreted except as degradations. The whole of Monocotyledons appear to me (from anatomical reasons especially) to be degradations from Dicotyledons, and primarily through the agency of growth in water. Many subsequently became terrestrial, but retained the effects of their primitive habitat through heredity. The 3-merous [sic]
perianth of gra.s.ses, the parts of the flower being in whorls, point to a degradation from a sub-liliaceous condition."
Mr. Henslow informs me that he has long held these views, but, as far as he knows, alone. Mr. Grant Allen, however, set forth a similar theory in his _Vignettes from Nature_ (p. 15) and more fully in _The Colours of Flowers_ (chap. v.), where he develops it fully and uses similar arguments to those of Mr. Henslow.]
[Footnote 156: H. Muller gives ample proof of this in his _Fertilisation of Flowers_.]
[Footnote 157: _Cross-and Self-Fertilisation_, p. 27.]
[Footnote 158: _Animals and Plants_, vol. ii. p. 145.]
[Footnote 159: Muller"s _Fertilisation of Flowers_, pp. 448, 455. Other cases of recent degradation and readaptation to insect-fertilisation are given by Professor Henslow (see footnote, p. 324).]
[Footnote 160: _The Colour Sense_, by Grant Allen, p. 95.]
[Footnote 161: _The Colour Sense_, chap. ix.]
[Footnote 162: See _Origin of Species_, sixth edition, p. 220.]
CHAPTER XII
THE GEOGRAPHICAL DISTRIBUTION OF ORGANISMS
The facts to be explained--The conditions which have determined distribution--The permanence of oceans--Oceanic and continental areas--Madagascar and New Zealand--The teachings of the thousand-fathom line--The distribution of marsupials--The distribution of tapirs--Powers of dispersal as ill.u.s.trated by insular organisms--Birds and insects at sea--Insects at great alt.i.tudes--The dispersal of plants--Dispersal of seeds by the wind--Mineral matter carried by the wind--Objections to the theory of wind-dispersal answered--Explanation of north temperate plants in the southern hemisphere--No proof of glaciation in the tropics--Lower temperature not needed to explain the facts--Concluding remarks.
The theory which we may now take as established--that all the existing forms of life have been derived from other forms by a natural process of descent with modification, and that this same process has been in action during past geological time--should enable us to give a rational account not only of the peculiarities of form and structure presented by animals and plants, but also of their grouping together in certain areas, and their general distribution over the earth"s surface.
In the absence of any exact knowledge of the facts of distribution, a student of the theory of evolution might naturally antic.i.p.ate that all groups of allied organisms would be found in the same region, and that, as he travelled farther and farther from any given centre, the forms of life would differ more and more from those which prevailed at the starting-point, till, in the remotest regions to which he could penetrate, he would find an entirely new a.s.semblage of animals and plants, altogether unlike those with which he was familiar. He would also antic.i.p.ate that diversities of climate would always be a.s.sociated with a corresponding diversity in the forms of life.
Now these antic.i.p.ations are to a considerable extent justified.
Remoteness on the earth"s surface is usually an indication of diversity in the fauna and flora, while strongly contrasted climates are always accompanied by a considerable contrast in the forms of life. But this correspondence is by no means exact or proportionate, and the converse propositions are often quite untrue. Countries which are near to each other often differ radically in their animal and vegetable productions; while similarity of climate, together with moderate geographical proximity, are often accompanied by marked diversities in the prevailing forms of life. Again, while many groups of animals--genera, families, and sometimes even orders--are confined to limited regions, most of the families, many genera, and even some species are found in every part of the earth. An enumeration of a few of these anomalies will better ill.u.s.trate the nature of the problem we have to solve.
As examples of extreme diversity, notwithstanding geographical proximity, we may adduce Madagascar and Africa, whose animal and vegetable productions are far less alike than are those of Great Britain and j.a.pan at the remotest extremities of the great northern continent; while an equal, or perhaps even a still greater, diversity exists between Australia and New Zealand. On the other hand, Northern Africa and South Europe, though separated by the Mediterranean Sea, have faunas and floras which do not differ from each other more than do the various countries of Europe. As a proof that similarity of climate and general adaptability have had but a small part in determining the forms of life in each country, we have the fact of the enormous increase of rabbits and pigs in Australia and New Zealand, of horses and cattle in South America, and of the common sparrow in North America, though in none of these cases are the animals natives of the countries in which they thrive so well. And lastly, in ill.u.s.tration of the fact that allied forms are not always found in adjacent regions, we have the tapirs, which are found only on opposite sides of the globe, in tropical America and the Malayan Islands; the camels of the Asiatic deserts, whose nearest allies are the llamas and alpacas of the Andes; and the marsupials, only found in Australia and on the opposite side of the globe, in America. Yet, again, although mammalia may be said to be universally distributed over the globe, being found abundantly on all the continents and on a great many of the larger islands, yet they are entirely wanting in New Zealand, and in a considerable number of other islands which are, nevertheless, perfectly able to support them when introduced.
Now most of these difficulties can be solved by means of well-known geographical and geological facts. When the productions of remote countries resemble each other, there is almost always continuity of land with similarity of climate between them. When adjacent countries differ greatly in their productions, we find them separated by a sea or strait whose great depth is an indication of its antiquity or permanence. When a group of animals inhabits two countries or regions separated by wide oceans, it is found that in past geological times the same group was much more widely distributed, and may have reached the countries it inhabits from an intermediate region in which it is now extinct. We know, also, that countries now united by land were divided by arms of the sea at a not very remote epoch; while there is good reason to believe that others now entirely isolated by a broad expanse of sea were formerly united and formed a single land area. There is also another important factor to be taken account of in considering how animals and plants have acquired their present peculiarities of distribution,--changes of climate. We know that quite recently a glacial epoch extended over much of what are now the temperate regions of the northern hemisphere, and that consequently the organisms which inhabit those parts must be, comparatively speaking, recent immigrants from more southern lands. But it is a yet more important fact that, down to middle Tertiary times at all events, an equable temperate climate, with a luxuriant vegetation, extended to far within the arctic circle, over what are now barren wastes, covered for ten months of the year with snow and ice. The arctic zone has, therefore, been in past times capable of supporting almost all the forms of life of our temperate regions; and we must take account of this condition of things whenever we have to speculate on the possible migrations of organisms between the old and new continents.
_The Conditions which have determined Distribution._
When we endeavour to explain in detail the facts of the existing distribution of organic beings, we are confronted by several preliminary questions, upon the solution of which will depend our treatment of the phenomena presented to us. Upon the theory of descent which we have adopted, all the different species of a genus, as well as all the genera which compose a family or higher group, have descended from some common ancestor, and must therefore, at some remote epoch, have occupied the same area, from which their descendants have spread to the regions they now inhabit. In the numerous cases in which the same group now occupies countries separated by oceans or seas, by lofty mountain-chains, by wide deserts, or by inhospitable climates, we have to consider how the migration which must certainly have taken place has been effected. It is possible that during some portion of the time which has elapsed since the origin of the group the interposing barriers have not been in existence; or, on the other hand, the particular organisms we are dealing with may have the power of overpa.s.sing the barriers, and thus reaching their present remote dwelling-places. As this is really the fundamental question of distribution on which the solution of all its more difficult problems depends, we have to inquire, in the first place, what is the nature of, and what are the limits to, the changes of the earth"s surface, especially during the Tertiary and latter part of the Secondary periods, as it was during those periods that most of the existing types of the higher animals and plants came into existence; and, in the next place, what are the extreme limits of the powers of dispersal possessed by the chief groups of animals and plants. We will first consider the question of barriers, more especially those formed by seas and oceans.
_The Permanence of Oceans._
It was formerly a very general belief, even amongst geologists, that the great features of the earth"s surface, no less than the smaller ones, were subject to continual mutations, and that during the course of known geological time the continents and great oceans had again and again changed places with each other. Sir Charles Lyell, in the last edition of his _Principles of Geology_ (1872), said: "Continents, therefore, although permanent for whole geological epochs, shift their positions entirely in the course of ages;" and this may be said to have been the orthodox opinion down to the very recent period when, by means of deep-sea soundings, the nature of the ocean bottom was made known.
The first person to throw doubt on this view appears to have been the veteran American geologist, Professor Dana. In 1849, in the Report of Wilke"s Exploring Expedition, he adduced the argument against a former continent in the Pacific during the Tertiary period, from the absence of all native quadrupeds. In 1856, in articles in the _American Journal_, he discussed the development of the American continent, and argued for its general permanence; and in his _Manual of Geology_ in 1863 and later editions, the same views were more fully enforced and were latterly applied to all continents. Darwin, in his _Journal of Researches_, published in 1845, called attention to the fact that all the small islands far from land in the Pacific, Indian, and Atlantic Oceans are either of coralline or volcanic formation. He excepted, however, the Seych.e.l.les and St. Paul"s rocks; but the former have since been shown to be no exception, as they consist entirely of coral rock; and although Darwin himself spent a few hours on St. Paul"s rocks on his outward voyage in the _Beagle_, and believed he had found some portions of them to be of a "cherty," and others of a "felspathic" nature, this also has been shown to be erroneous, and the careful examination of the rocks by the Abbe Renard clearly proves them to be wholly of volcanic origin.[163] We have, therefore, at the present time, absolutely no exception whatever to the remarkable fact that all the oceanic islands of the globe are either of volcanic or coral formation; and there is, further, good reason to believe that those of the latter cla.s.s in every case rest upon a volcanic foundation.
In his _Origin of Species_, Darwin further showed that no true oceanic island had any native mammals or batrachia when first discovered, this fact const.i.tuting the test of the cla.s.s to which an island belongs; whence he argued that none of them had ever been connected with continents, but all had originated in mid-ocean. These considerations alone render it almost certain that the areas now occupied by the great oceans have never, during known geological time, been occupied by continents, since it is in the highest degree improbable that every fragment of those continents should have completely disappeared, and have been replaced by volcanic islands rising out of profound oceanic abysses; but recent research into the depth of the oceans and the nature of the deposits now forming on their floors, adds greatly to the evidence in this direction, and renders it almost a certainty that they represent very ancient if not primaeval features of the earth"s surface.
A very brief outline of the nature of this evidence will be now given.
The researches of the _Challenger_ expedition into the nature of the sea-bottom show, that the whole of the land debris brought down by rivers to the ocean (with the exception of pumice and other floating matter), is deposited comparatively near to the sh.o.r.es, and that the fineness of the material is an indication of the distance to which it has been carried. Everything in the nature of gravel and sand is laid down within a very few miles of land, only the finer muddy sediments being carried out for 20 or 50 miles, and the very finest of all, under the most favourable conditions, rarely extending beyond 150, or at the utmost, 300 miles from land into the deep ocean.[164] Beyond these distances, and covering the entire ocean floor, are various oozes formed wholly from the debris of marine organisms; while intermingled with these are found various volcanic products which have been either carried through the air or floated on the surface, and a small but perfectly recognisable quant.i.ty of meteoric matter. Ice-borne rocks are also found abundantly scattered over the ocean bottom within a definite distance of the arctic and antarctic circles, clearly marking out the limit of floating icebergs in recent geological times.
Now the whole series of marine stratified rocks, from the earliest Palaeozoic to the most recent Tertiary beds, consist of materials closely corresponding to the land debris now being deposited within a narrow belt round the sh.o.r.es of all continents; while no rocks have been found which can be identified with the various oozes now forming in the deep abysses of the ocean. It follows, therefore, that all the geological formations have been formed in comparatively shallow water, and always adjacent to the continental land of the period. The great thickness of some of the formations is no indication of a deep sea, but only of slow subsidence during the time that the deposition was in progress. This view is now adopted by many of the most experienced geologists, especially by Dr. Archibald Geikie, Director of the Geological Survey of Great Britain, who, in his lecture on "Geographical Evolution," says: "From all this evidence we may legitimately conclude that the present land of the globe, though consisting in great measure of marine formations, has never lain under the deep sea; but that its site must always have been near land. Even its thick marine limestones are the deposits of comparatively shallow water."[165]
But besides these geological and physical considerations, there is a mechanical difficulty in the way of repeated change of position of oceans and continents which has not yet received the attention it deserves. According to the recent careful estimate by Mr. John Murray, the land area of the globe is to the water area as 28 to 72. The mean height of the land above sea-level is 2250 feet, while the mean depth of the ocean is 14,640 feet. Hence the bulk of dry land is 23,450,000 cubic miles, and that of the waters of the ocean 323,800,000 cubic miles; and it follows that if the whole of the solid matter of the earth"s surface were reduced to one level, it would be everywhere covered by an ocean about two miles deep. The accompanying diagram will serve to render these figures more intelligible. The length of the sections of land and ocean are in the proportion of their respective areas, while the mean height of the land and the mean depth of the ocean are exhibited on a greatly increased vertical scale. If we considered the continents and their adjacent oceans separately they would differ a little, but not very materially, from this diagram; in some cases the proportion of land to ocean would be a little greater, in others a little less.
[Ill.u.s.tration: FIG. 32.]
Now, if we try to imagine a process of elevation and depression by which the sea and land shall completely change places, we shall be met by insuperable difficulties. We must, in the first place, a.s.sume a general equality between elevation and subsidence during any given period, because if the elevation over any extensive continental area were not balanced by some subsidence of approximately equal amount, an unsupported hollow would be left under the earth"s crust. Let us now suppose a continental area to sink, and an adjacent oceanic area to rise, it will be seen that the greater part of the land will disappear long before the new land has approached the surface of the ocean. This difficulty will not be removed by supposing a portion of a continent to subside, and the immediately adjacent portion of the ocean on the other side of the continent to rise, because in almost every case we find that within a comparatively short distance from the sh.o.r.es of all existing continents, the ocean floor sinks rapidly to a depth of from 2000 to 3000 fathoms, and maintains a similar depth, generally speaking, over a large portion of the oceanic areas. In order, therefore, that any area of continental extent be upraised from the great oceans, there must be a subsidence of a land area five or six times as great, unless it can be shown that an extensive elevation of the ocean floor up to and far above the surface could occur without an equivalent depression elsewhere. The fact that the waters of the ocean are sufficient to cover the whole globe to a depth of two miles, is alone sufficient to indicate that the great ocean basins are permanent features of the earth"s surface, since any process of alternation of these with the land areas would have been almost certain to result again and again in the total disappearance of large portions, if not of all, of the dry land of the globe. But the continuity of terrestrial life since the Devonian and Carboniferous periods, and the existence of very similar forms in the corresponding deposits of every continent--as well as the occurrence of sedimentary rocks, indicating the proximity of land at the time of their deposit, over a large portion of the surface of all the continents, and in every geological period--a.s.sure us that no such disappearance has ever occurred.
_Oceanic and Continental Areas._
When we speak of the permanence of oceanic and continental areas as one of the established facts of modern research, we do not mean that existing continents and oceans have always maintained the exact areas and outlines that they now present, but merely, that while all of them have been undergoing changes in outline and extent from age to age, they have yet maintained substantially the same positions, and have never actually changed places with each other. There are, moreover, certain physical and biological facts which enable us to mark out these areas with some confidence.
We have seen that there are a large number of islands which may be cla.s.sed as oceanic, because they have never formed parts of continents, but have originated in mid-ocean, and have derived their forms of life by migration across the sea. Their peculiarities are seen to be very marked in comparison with those islands which there is good reason to believe are really fragments of more extensive land areas, and are hence termed "continental." These continental islands consist in every case of a variety of stratified rocks of various ages, thus corresponding closely with the usual structure of continents; although many of the islands are small like Jersey or the Shetland Islands, or far from continental land like the Falkland Islands or New Zealand. They all contain indigenous mammalia or batrachia, and generally a much greater variety of birds, reptiles, insects, and plants, than do the oceanic islands. From these various characteristics we conclude that they have all once formed parts of continents, or at all events of much larger land areas, and have become isolated, either by subsidence of the intervening land or by the effects of long-continued marine denudation.
Now, if we trace the thousand-fathom line around all our existing continents we find that, with only two exceptions, every island which can be cla.s.sed as "continental" falls within this line, while all that lie beyond it have the undoubted characteristics of "oceanic" islands.
We, therefore, conclude that the thousand-fathom line marks out, approximately, the "continental area,"--that is, the limits within which continental development and change throughout known geological time have gone on. There may, of course, have been some extensions of land beyond this limit, while some areas within it may always have been ocean; but so far as we have any direct evidence, this line may be taken to mark out, approximately, the most probable boundary between the "continental area," which has always consisted of land and shallow sea in varying proportions, and the great oceanic basins, within the limits of which volcanic activity has been building up numerous islands, but whose profound depths have apparently undergone little change.