_Tiedemann on the brain of the foetus in vertebrated animals._--There is yet another department of anatomical discovery to which I must allude, because it has appeared to some persons to afford a distant a.n.a.logy, at least, to that progressive development by which some of the inferior species may have been gradually perfected into those of more complex organization. Tiedemann found, and his discoveries have been most fully confirmed and elucidated by M. Serres, that the brain of the foetus, in the highest cla.s.s of vertebrated animals, a.s.sumes, in succession, forms, bearing a certain degree of resemblance to those which belong to fishes, reptiles, and birds, before it acquires the additions and modifications which are peculiar to the mammiferous tribe; so that, in the pa.s.sage from the embryo to the perfect mammifer, there is a typical representation, it is said, of all those transformations which the primitive species are supposed to have undergone, during a long series of generations, between the present period and the remotest geological era.

"If you examine the brain of the mammalia," says M. Serres, "at an early stage of uterine life, you perceive the cerebral hemispheres consolidated, as in fish, in two vesicles, isolated one from the other; at a later period, you see them affect the configuration of the cerebral hemispheres of reptiles; still later again, they present you with the forms of those of birds; finally they acquire, at the era of birth, and sometimes later, the permanent forms which the adult mammalia present.

"The cerebral hemispheres, then, arrive at the state which we observe in the higher animals only by a series of successive metamorphoses. If we reduce the whole of these evolutions to four periods, we shall see, that in the first are born the cerebral lobes of fishes; and this takes place h.o.m.ogeneously in all cla.s.ses. The second period will give us the organization of reptiles; the third, the brain of birds; and the fourth, the complex hemispheres of mammalia.

"If we could develop the different parts of the brain of the inferior cla.s.ses, we should make, in succession, a reptile out of a fish, a bird out of a reptile, and a mammiferous quadruped out of a bird. If, on the contrary, we could starve this organ in the mammalia, we might reduce it successively to the condition of the brain of the three inferior cla.s.ses.

"Nature often presents us with this last phenomenon in monsters, but never exhibits the first. Among the various deformities which organized beings may experience, they never pa.s.s the limits of their own cla.s.ses to put on the forms of the cla.s.s above them. Never does a fish elevate itself so as to a.s.sume the form of the brain of a reptile; nor does the latter ever attain that of birds; nor the bird that of the mammifer. It may happen that a monster may have two heads; but the conformation of the brain always remains circ.u.mscribed narrowly within the limits of its cla.s.s."[840]

Dr. Clark of Cambridge, in a memoir on "Foetal Development" (1845), has shown that the concurrent labours of Valentin, Ratke, and Bischoff disprove the reality of the supposed anatomical a.n.a.logy between the embryo condition of certain organs in the higher orders, and the perfect structure of the same organs in animals of an inferior cla.s.s. The hearts and brains, for example, of birds and mammals do not pa.s.s through forms which are permanent in fishes and reptiles; there is only just so much resemblance as may point to a unity of plan running through the organization of the whole series of vertebrated animals; but which lends no support whatever to the notion of a gradual trans.m.u.tation of one species into another; least of all of the pa.s.sage, in the course of many generations, from an animal of a more simple to one of a more complex structure.

_Recapitulation._--For the reasons, therefore, detailed in this and the two preceding chapters, we may draw the following inferences in regard to the reality of _species_ in nature:--

1st. That there is a capacity in all species to accommodate themselves, to a certain extent, to a change of external circ.u.mstances, this extent varying greatly, according to the species.

2ndly. When the change of situation which they can endure is great, it is usually attended by some modifications of the form, colour, size, structure, or other particulars; but the mutations thus superinduced are governed by constant laws, and the capability of so varying, forms part of the permanent specific character.

3dly. Some acquired peculiarities, of form, structure, and instinct, are transmissible to the offspring; but these consist of such qualities and attributes only as are intimately related to the natural wants and propensities of the species.

4thly. The entire variation from the original type, which any given kind of change can produce, may usually be effected in a brief period of time, after which no farther deviation can be obtained by continuing to alter the circ.u.mstances, though ever so gradually; indefinite divergence, either in the way of improvement or deterioration, being prevented, and the least possible excess beyond the defined limits being fatal to the existence of the individual.

5thly. The intermixture of distinct species is guarded against by the aversion of the individuals composing them to s.e.xual union, or by the sterility of the mule offspring. It does not appear that true hybrid races have ever been perpetuated for several generations, even by the a.s.sistance of man; for the cases usually cited relate to the crossing of mules with individuals of pure species, and not to the intermixture of hybrid with hybrid.

6thly. From the above considerations, it appears that species have a real existence in nature; and that each was endowed, at the time of its creation, with the attributes and organization by which it is now distinguished.

CHAPTER x.x.xVII.

LAWS WHICH REGULATE THE GEOGRAPHICAL DISTRIBUTION OF SPECIES.

a.n.a.logy of climate not attended with ident.i.ty of species--Botanical geography--Stations--Habitations--Distinct provinces of indigenous plants--Vegetation of islands--Marine vegetation--In what manner plants become diffused--Effects of wind, rivers, marine currents--Agency of animals--Many seeds pa.s.s through the stomachs of animals and birds undigested--Agency of man in the dispersion of plants, both voluntary and involuntary--Its a.n.a.logy to that of the inferior animals.

Next to determining the question whether species have a real existence, the consideration of the laws which regulate their geographical distribution is a subject of primary importance to the geologist. It is only by studying these laws with attention, by observing the positions which groups of species occupy at present, and inquiring how these may be varied in the course of time by migrations, by changes in physical geography, and other causes, that we can hope to learn whether the duration of species be limited, or in what manner the state of the animate world is affected by the endless vicissitudes of the inanimate.

_Different regions inhabited by distinct species._--That different regions of the globe are inhabited by entirely distinct animals and plants, is a fact which has been familiar to all naturalists since Buffon first pointed out the want of _specific_ ident.i.ty between the land quadrupeds of America and those of the Old World. The same phenomenon has, in later times, been forced in a striking manner upon our attention, by the examination of New Holland, where the indigenous species of animals and plants were found to be, almost without exception, distinct from those known in other parts of the world.

But the extent of this parcelling out of the globe amongst different _nations_, as they have been termed, of plants and animals--the universality of a phenomenon so extraordinary and unexpected, may be considered as one of the most interesting facts clearly established by the advance of modern science.

Scarcely fourteen hundred species of plants appear to have been known and described by the Greeks, Romans, and Arabians. At present, more than three thousand species are enumerated, as natives of our own island.[841] In other parts of the world there have been now collected (1846) upwards of 100,000 species, specimens of which are preserved in European herbariums. It was not to be supposed, therefore, that the ancients should have acquired any correct notions respecting what may be called the geography of plants, although the influence of climate on the character of the vegetation could hardly have escaped their observation.

Antecedently to investigation, there was no reason for presuming that the vegetable productions, growing wild in the eastern hemisphere, should be unlike those of the western, in the same lat.i.tude; nor that the plants of the Cape of Good Hope should be unlike those of the south of Europe; situations where the climate is little dissimilar. The contrary supposition would have seemed more probable, and we might have antic.i.p.ated an almost perfect ident.i.ty in the animals and plants which inhabit corresponding parallels of lat.i.tude. The discovery, therefore, that each separate region of the globe, both of the land and water, is occupied by distinct groups of species, and that most of the exceptions to this general rule may be referred to disseminating causes now in operation, is eminently calculated to excite curiosity, and to stimulate us to seek some hypothesis respecting the first introduction of species which may be reconcileable with such phenomena.

_Botanical geography._--A comparison of the _plants_ of different regions of the globe affords results more to be depended upon in the present state of our knowledge than those relating to the animal kingdom, because the science of botany is more advanced, and probably comprehends a great proportion of the total number of the vegetable productions of the whole earth. Humboldt, in several eloquent pa.s.sages of his Personal Narrative, was among the first to promulgate philosophical views on this subject. Every hemisphere, says this traveller, produces plants of different species; and it is not by the diversity of climates that we can attempt to explain why equinoctial Africa has no Laurinae, and the New World no Heaths; why the Calceolariae are found only in the southern hemisphere; why the birds of the continent of India glow with colors less splendid than the birds of the hot parts of America: finally, why the tiger is peculiar to Asia, and the ornithorhynchus to New Holland.[842]

"We can conceive," he adds, "that a small number of the families of plants, for instance, the Musaceae and the Palms, cannot belong to very cold regions, on account of their internal structure and the importance of certain organs; but we cannot explain why no one of the family of Melastomas vegetates north of the parallel of thirty degrees; or why no rose-tree belongs to the southern hemisphere. a.n.a.logy of climates is often found in the two continents without ident.i.ty of productions."[843]

The luminous essay of De Candolle on "Botanical Geography" presents us with the fruits of his own researches and those of Humboldt, Brown, and other eminent botanists, so arranged, that the princ.i.p.al phenomena of the distribution of plants are exhibited in connexion with the causes to which they are chiefly referrible.[844] "It might not, perhaps, be difficult," observes this writer, "to find two points, in the United States and in Europe, or in Equinoctial America and Africa, which present all the same circ.u.mstances: as, for example, the same temperature, the same height above the sea, a similar soil, an equal dose of humidity; yet nearly all, _perhaps all_, the plants in these two similar localities shall be distinct. A certain degree of a.n.a.logy, indeed, of aspect, and even of structure, might very possibly be discoverable between the plants of the two localities in question; but the _species_ would in general be different. Circ.u.mstances, therefore, different from those which now determine the _stations_, have had an influence on the _habitations_ of plants."

_Stations and habitations of plants._--As I shall frequently have occasion to speak of the _stations_ and _habitations_ of plants in the technical sense in which the terms are used in the above pa.s.sage, I may remind the geologist that station indicates the peculiar nature of the locality where each species is accustomed to grow, and has reference to climate, soil, humidity, light, elevation above the sea, and other a.n.a.logous circ.u.mstances; whereas, by habitation is meant a general indication of the country where a plant grows wild. Thus the _station_ of a plant may be a salt-marsh, a hill-side, the bed of the sea, or a stagnant pool. Its _habitation_ may be Europe, North America, or New Holland, between the tropics. The study of stations has been styled the topography, that of habitations the geography, of botany. The terms thus defined, express each a distinct cla.s.s of ideas, which have been often confounded together, and which are equally applicable in zoology.

In farther ill.u.s.tration of the principle above alluded to, that difference of longitude, independently of any influence of temperature, is accompanied by a great, and sometimes a complete, diversity in the species of plants, De Candolle observes, that, out of 2891 species of phaenogamous plants described by Pursh, in the United States, there are only 385 which are found in northern or temperate Europe. MM. Humboldt and Bonpland, in all their travels through equinoctial America, found only twenty-four species (these being all Cyperaceae and Gramineae) common to America and any part of the Old World. They collected, it is true, chiefly on the mountains, or the proportion would have been larger; for Dr. J. Hooker informs me that many tropical plants of the New World are identical with African species. Nevertheless, the general discordance of these Floras is very striking. On comparing New Holland with Europe, Mr.

Brown ascertained that, out of 4100 species, discovered in Australia, there were only 166 common to Europe, and of this small number there were some few which may have been transported thither by man. Almost all of the 166 species were cryptogamic, and the rest consist, in nearly every case, of phaenogamous plants which also inhabit intervening regions.

But what is still more remarkable, in the more widely separated parts of the ancient continent, notwithstanding the existence of an uninterrupted land-communication, the diversity in the specific character of the respective vegetations is almost as striking. Thus there is found one a.s.semblage of species in China, another in the countries bordering the Black Sea and the Caspian, a third in those surrounding the Mediterranean, a fourth in the great platforms of Siberia and Tartary, and so forth.

The distinctness of the groups of indigenous plants, in the same parallel of lat.i.tude, is greatest where continents are disjoined by a wide expanse of ocean. In the northern hemisphere, near the pole, where the extremities of Europe, Asia, and America unite or approach near to one another, a considerable number of the same species of plants are found, common to the three continents. But it has been remarked, that these plants, which are thus so widely diffused in the arctic regions, are also found in the chain of the Aleutian islands, which stretch almost across from America to Asia, and which may probably have served as the channel of communication for the partial blending of the Floras of the adjoining regions. It has, indeed, been observed to be a general rule, that plants found at two points very remote from each other occur also in places intermediate.

Dr. J. Hooker informs me that in high lat.i.tudes in the southern ocean, in spite of the great extent of the sea, Floras of widely disconnected islands contain many species in common. Perhaps icebergs, transporting to vast distances not only stones, but soil with the seeds of plants, may explain this unusually wide diffusion of insular plants.

In islands very distant from continents the total number of plants is comparatively small; but a large proportion of the species are such as occur nowhere else. In so far as the Flora of such islands is not peculiar to them, it contains, in general, species common to the nearest main lands.[845] The islands of the great southern ocean exemplify these rules; the easternmost containing more American, and the western more Indian plants.[846] Madeira and Teneriffe contain many species, and even entire genera, peculiar to them; but they have also plants in common with Portugal, Spain, the Azores, and the north-west coast of Africa.[847]

In the Canaries, out of 533 species of phaenogamous plants, it is said that 310 are peculiar to these islands, and the rest identical with those of the African continent; but in the Flora of St. Helena, which is so far distant even from the western sh.o.r.es of Africa, there have been found, out of thirty native species of the phaenogamous cla.s.s, only _one or two_ which are to be found in any other part of the globe. On the other hand, of sixty cryptogamic plants, collected by Dr. J. Hooker in the same island, twelve only were peculiar.

The natural history of the Galapagos archipelago, described by Mr.

Darwin, affords another very instructive ill.u.s.tration of the laws governing the geographical distribution of plants and animals in islands. This group consists of ten princ.i.p.al islands, situated in the Pacific Ocean, under the equator, about 600 miles westward of the coast of South America. As they are all formed of volcanic rocks, many of the craters, of which there are about 2000 in number, having a very fresh aspect, we may regard the whole as much more modern in origin than the ma.s.s of the adjoining continent; yet neither has the Flora nor Fauna been derived from South America, but consist of species for the most part indigenous, yet stamped with a character decidedly South American.

What is still more singular, there is a difference between the species inhabiting the different islands. Of flowering plants, for example, there are 185 species at present known, and forty cryptogamic, making together 225. One hundred of the former cla.s.s are new species, probably confined to this archipelago; and of the rest, ten at least have been introduced by man. Of twenty-one species of _Compositae_, all but one are peculiar, and they belong to twelve genera, no less than ten of which genera are confined to the Galapagos. Dr. Hooker observes, that the type of this Flora has an undoubted relation to that of the western side of South America, and he detects in it no affinity with that of the numerous islands scattered over other parts of the Pacific. So in regard to the birds, reptiles, land-sh.e.l.ls, and insects, this archipelago, standing as it does in the Pacific Ocean, is zoologically part of America. Although each small island is not more than fifty or sixty miles apart, and most of them are in sight of each other, formed of precisely the same rocks, rising nearly to an equal height, and placed under a similar climate, they are tenanted each by a different set of beings, the tortoises, mocking-thrushes, finches, beetles, scarcely any of them ever ranging over the whole, and often not even common to any two of the islands.

"The archipelago," says Mr. Darwin, "is a little world within itself, or rather a satellite attached to America; whence it has derived a few stray colonists, and has received the general character of its indigenous productions. One is astonished," he adds, "at the amount of creative force displayed on so many small, barren, and rocky islands, and still more so, at its diverse, yet a.n.a.logous action on points so near each other. I have said that the Galapagos archipelago might be called a satellite attached to America, but it should rather be called a group of satellites physically similar, organically distinct, yet intimately related to each other, and all related in a marked, though much lesser degree, to the great American continent."[848]

_Number of botanical provinces._--De Candolle has enumerated twenty great botanical provinces inhabited by indigenous or aboriginal plants; and although many of these contain a variety of species which are common to several others, and sometimes to places very remote, yet the lines of demarcation are, upon the whole, astonishingly well defined.[849] Nor is it likely that the bearing of the evidence on which these general views are founded will ever be materially affected, since they are already confirmed by the examination of nearly one hundred thousand species of plants.

The entire change of opinion which the contemplation of those phenomena has brought about is worthy of remark. The first travellers were persuaded that they should find, in distant regions, the plants of their own country, and they took a pleasure in giving them the same names. It was some time before this illusion was dissipated; but so fully sensible did botanists at last become of the extreme smallness of the number of phaenogamous plants common to different continents, that the ancient Floras fell into disrepute. All grew diffident of the pretended identifications; and we now find that every naturalist is inclined to examine each supposed exception with scrupulous severity.[850] If they admit the fact, they begin to speculate on the mode whereby the seeds may have been transported from one country into the other, or enquire on which of two continents the plant was indigenous, a.s.suming that a species, like an individual, cannot have two birthplaces.

_Marine vegetation._--The marine vegetation is divisible into different systems, like those prevailing on the land; but they are much fewer, as we might have expected, the temperature of the ocean being more uniform than that of the atmosphere, and consequently the dispersion of species from one zone to another being less frequently checked by the intervention of uncongenial climates. The proportion also of land to sea throughout the globe being small, the migration of marine plants is not so often stopped by barriers of land, as is that of the terrestrial species by the ocean. The number of hydrophytes, as they are termed, is very considerable, and their stations are found to be infinitely more varied than could have been antic.i.p.ated; for while some plants are covered and uncovered daily by the tide, others live at the depth of several hundred feet. Among the known provinces of Algae, we may mention, 1st, The north circ.u.mpolar, from lat 60 N. to the pole; 2dly, The North Atlantic or the region of Fucus proper and Delesseriae, extending from lat. 40 N. to lat. 60 N.; 3dly, That of the Mediterranean, which may be regarded as a sub-region of the _fourth_ or warmer temperate zone of the Atlantic, between lat. 23 N. and lat. 40 N.; 5thly, The Tropical Atlantic, in which Sarga.s.sum, Rhodomelia, Corallinea, and Siphonia abound; 6thly, The South Atlantic, where the Fucus reappears; 7thly, The Antarctic American, comprehending from Chili to Cape Horn, the Falkland Islands, and thence round the world south of lat.i.tude 50 S.; 8thly, The Australian and New Zealand, which is very peculiar, being characterized, among other generic forms, by Cystoseiriae and Fuceae; 9thly, The Indian Ocean and Red Sea; and, 10thly, The Chinese and j.a.panese seas.[851] In addition to the above provinces, there are several others not yet well determined in the Pacific Ocean and elsewhere. There are, however, many species which range through several of these geographical regions of subaqueous vegetation, being common to very remote countries; as, for example, to the coasts of

Europe and the United States, and others, to Cape Horn and Van Diemen's Land, the same plants extending also for the most part to the New Zealand sea. Of the _species_ strictly antarctic (excluding the New Zealand and Tasmanian groups) Dr. Hooker has identified not less than a fifth part of the whole with British Algae! Yet is there a much smaller proportion of cosmopolite species among the Algae than among the terrestrial cellular plants, such as lichens, mosses, and Hepaticae.

It must always be borne in mind, that the distinctness alluded to between the provinces, whether of subaqueous or terrestrial plants, relates strictly to _species_, and not to forms. In regard to the numerical preponderance of certain forms, and many peculiarities of internal structure, there is usually a marked agreement in the vegetable productions of districts placed in corresponding lat.i.tudes, and under similar physical circ.u.mstances, however remote their position. Thus there are innumerable points of a.n.a.logy between the vegetation of the Brazils, equinoctial Africa, and India; and there are also points of difference wherein the plants of these regions are distinguishable from all extra-tropical groups. But there is a very small proportion of the entire number of species common to the three continents. The same may be said, if we compare the plants of the United States with that of the middle of Europe; the species are distinct, but the forms are often so a.n.a.logous, as to have been styled "geographical representatives." There are very few _species_ of phaenogamous plants, says Dr. J. Hooker, common to Van Diemen's Land, New Zealand, and Fuegia, but a great many _genera_, and some of them are confined to those three distant regions of the southern hemisphere, being in many instances each severally represented by a single species. The same naturalist also observes that the southern temperate as well as the antarctic regions, possess each of them representatives of some of the genera of the a.n.a.logous climates of the opposite hemisphere; but very few of the species are identical unless they be such as are equally diffused over other countries, or which inhabit the Andes, by the aid of which they have evidently effected their pa.s.sage southwards.

_Manner in which plants become diffused.--Winds._--Let us now consider what means of diffusion, independently of the agency of man, are possessed by plants, whereby, in the course of ages, they may be enabled to stray from one of the botanical provinces above mentioned to another, and to establish new colonies at a great distance from their birthplace.

The princ.i.p.al of the inanimate agents provided by nature for scattering the seeds of plants over the globe, are the movements of the atmosphere and of the ocean, and the constant flow of water from the mountains to the sea. To begin with the winds: a great number of seeds, are furnished with downy and feathery appendages, enabling them, when ripe, to float in the air, and to be wafted easily to great distances by the most gentle breeze. Other plants are fitted for dispersion by means of an attached wing, as in the case of the fir tree, so that they are caught up by the wind as they fall from the cone, and are carried to a distance. Amongst the comparatively small number of plants known to Linnaeus, no less than 138 genera are enumerated as having winged seeds.

As winds often prevail for days, weeks, or even months together, in the same direction, these means of transportation may sometimes be without limits; and even the heavier grains may be borne through considerable s.p.a.ces, in a very short time, during ordinary tempests; for strong gales, which can sweep along grains of sand, often move at the rate of about forty miles an hour, and if the storm be very violent, at the rate of fifty-six miles.[852] The hurricanes of tropical regions, which root up trees and throw down buildings, sweep along at the rate of ninety miles an hour; so that, for however short a time they prevail, they may carry even the heavier fruits and seeds over friths and seas of considerable width, and doubtless are often the means of introducing into islands the vegetation of adjoining continents. Whirlwinds are also instrumental in bearing along heavy vegetable substances to considerable distances. Slight ones may frequently be observed in our fields, in summer carrying up hayc.o.c.ks into the air, and then letting fall small tufts of hay far and wide over the country; but they are sometimes so powerful as to dry up lakes and ponds, and to break off the boughs of trees, and carry them up in a whirling column of air.

Franklin tells us, in one of his letters, that he saw, in Maryland, a whirlwind which began by taking up the dust which lay in the road, in the form of a sugar loaf with the pointed end downwards, and soon after grew to the height of forty or fifty feet, being twenty or thirty in diameter. It advanced in a direction contrary to the wind; and although the rotary motion of the column was surprisingly rapid, its onward progress was sufficiently slow to allow a man to keep pace with it on foot. Franklin followed it on horseback, accompanied by his son, for three quarters of a mile, and saw it enter a wood, where it twisted and turned round large trees with surprising force. These were carried up in a spiral line, and were seen flying in the air, together with boughs and innumerable leaves, which, from their height, appeared reduced to the apparent size of flies. As this cause operates at different intervals of time throughout a great portion of the earth's surface, it may be the means of bearing not only plants but insects, land testacea and their eggs, with many other species of animals, to points which they could never otherwise have reached, and from which they may then begin to propagate themselves again as from a new centre.

_Distribution of cryptogamous plants._--It has been found that a great numerical proportion of the exceptions to the limitation of species to certain quarters of the globe occur in the various tribes of cryptogamic plants. Linnaeus observed that, as the germs of plants of this cla.s.s, such as mosses, fungi, and lichens, consist of an impalpable powder, the particles of which are scarcely visible to the naked eye, there is no difficulty to account for their being dispersed throughout the atmosphere, and carried to every point of the globe, where there is a station fitted for them.

Lichens in particular ascend to great elevations, sometimes growing two thousand feet above the line of perpetual snow, at the utmost limits of vegetation, and where the mean temperature is nearly at the freezing point. This elevated position must contribute greatly to facilitate the dispersion of those buoyant particles of which their fructification consists.[853]

Some have inferred, from the springing up of mushrooms whenever particular soils and decomposed organic matter are mixed together, that the production of fungi is accidental, and not a.n.a.logous to that of perfect plants. But Fries, whose authority on these questions is ent.i.tled to the highest respect, has shown the fallacy of this argument in favor of the old doctrine of equivocal generation. "The sporules of fungi," says this naturalist, "are so infinite, that in a single individual of _Reticularia maxima_, I have counted above ten millions, and so subtile as to be scarcely visible, often resembling thin smoke; so light that they may be raised perhaps by evaporation into the atmosphere, and dispersed in so many ways by the attraction of the sun, by insects, wind, elasticity, adhesion, &c., that it is difficult to conceive a place from which they may be excluded."[854]

The club-moss called _Lycopodium cernuum_ affords a striking example of a cryptogamous plant universally distributed over all equinoctial countries. It scarcely ever pa.s.ses beyond the northern tropic, except in one instance, where it appears around the hot-springs in the Azores, although it is neither an inhabitant of the Canaries nor Madeira.