The sub-kingdoms being thus distinguished from one another, by the presence or absence of parts devoted to fundamental functions, or else by differences in the distributions of such parts; we find, on descending to the classes, that these are distinguislied from eacli other, either by modifications in the structures of fundamental parts, or by the presence or absence of subsidiary parts, or by both. Fishes and ^w?-phihia are unlike higher vertebrates in possessing branchiae; either throughout life or early in life. And every higher vertebrate, besides having lungs, is characterized by having, during development, an amnion and an allantois. Mammals, again, are marked off from Birds and Reptiles by the presence of mammae, as well as by the form of the occipital condyles. Among Mammals, the next division is based on the presence or absence of a placenta. And divisions of the Placentalia are mainly determined by the characters of the organs of external action.
Thus, without multiplying illustrations and without descending to genera and species, we see that, speaking generally, the successively smaller groups, are distinguished from one another by traits of successively less importance, physiologically considered. The attributes possessed in common by the largest assemblages of organisms, are few in number but all-essential in kind — affect fundamentally the most vital actions. Each secondary assemblage, included in one of the primary assemblages, is characterized by further common attributes that influence the functions less profoundly. And so on with each lower grade of assemblage.
§ 103. What interpretation is to be put on these truths of classification? We find that organic forms admit of an arrangement everywhere expressive of the fact, that along with certain attributes, certain other attributes, which are not directly connected with them, always exist. How are we to account for this fact? And how are we to account for the fact that the attributes possessed in common by the largest assemblages of forms, are the most vitally-important attributes?
No one can believe that combinations of this kind may have arisen fortuitously. Or if any one believes this, it is (;lassification. 301) easy to prove to liim that the law of probabilities negatives the assumption. Ev^en supposing fortuitous combinations of attributes might result in organisms that would work, we should still be without a clue to this special mode of combination. The chances would be infinity to one against organisms which possessed in common certain fundamental attributes, having also in common numerous non-essential attributes.
No one, again, can allege that such combinations are necessary, in the sense that all other combinations are impracticable. There is not, in the nature of things, any reason why creatures covered with feathers should always have beaks: jaws holding teeth would, in many cases* have served them equally well or better. The most general characteristic of an entire sub-kingdom, equal in extent to the Vci'tehrata, might have been the possession of nictitating membranes; while the internal organizations throughout this sub-kingdom, might have been on many different plans.
If, on the other hand, this peculiar subordination of attributes which organic forms display, be ascribed to design, other difficulties suggest themselves. To suppose that a certain plan of organization was fixed on by a Creator, for each vast and varied group, the members of which were to lead many different modes of life; and that he bound himself to adhere rigidly to this plan, even in the most aberrant forms of the group, where some other plan would have been more appropriate; is to ascribe a very strange motive. When we discover that the possession of seven cervical vertebra) is a general characteristic of mammals, whether the neck be immensely long, as in the giraffe, or quite rudimentary, as in the whale; shall we say that though, for the whale's neck, one vertebra would have been equally good, and though, for the giraffe's neck, a dozen would probably have been better than seven, yet seven was the number adhered to in both cases, because seven was fixed upon for the mammalian type?
And then, when it turns out that this possession of seven cervical vertebrae is not an absolutely-universal characteristic of mammals, shall we conclude that while, in a host of cases, there is a needless adherence to a plan for the sake of consistency, there is yet, in some cases, an inconsistent abandonment of the plan? I think we may properly refuse to draw any such conclusion.
What, then, is the meaning of these peculiar relations of organic forms? The answer to this question must be postponed. Having here contemplated the problem as presented in these wide inductions which naturalists have reached; and having seen what proposed solutions of it are inadmissible; we shall see, in the next division of this work, what is the only possible solution.
CHAPTEH XII.
§ 104. There is a distribution of organisms in Space, and there is a distribution of organisms in Time. Looking first at their distribution in Space, we observe in it two different classes of facts. On the one hand, the plants and animals of each species, manifestly have their habitats limited by external conditions: they are necessarily restricted to spaces in which their vital actions can be performed. On the other hand, the existence of certain conditions does not determine the presence of organisms that are the fittest for them: there are many spaces perfectly adapted for life of a high order, in which only life of a much lower order is found. While, in this inevitable restriction of organisms to environments with which their natures correspond, we find a negative cause of distribution; there remains to be found that positive cause of distribution, whence results the presence of organisms in some of the places appropriate to them, and their absence from other places that are equally appropriate and more appropriate. Let us consider the phenomena under these categories.
§ 105. Facts which illustrate the limiting influence of surrounding conditions, are abundant, and familiar to all readers. It will be needful, however, here to cite a few typical ones of each order.
The confinement of different kinds of plants and different kinds of animals, to the media for which they are severally adapted, is the broadest fact of distribution. We have extensive groups of plants that are respectively sub- aerial and sub-aqueous; and of the sub- aqueous, some are exclusively marine, while others exist only in rivers and lakes. Among animals, we similarly find some classes confined to the air and others to the water; and of the water-breathers, some are restricted to salt water and others to fresh water. Less familiar is the fact, that within each of these strongly contrasted media, there are further wide-spread limitations. In the sea, certain organisms exist only between certain depths, while other organisms exist only between other depths — the limpet within the littoral zone, and the Glohigerina at the bottom of the Atlantic; and on the land, there are Floras and Faunas peculiar to low regions, and others peculiar to high regions. Next we have the well-known geographical limitations, made by climate. There are temperatures that restrict each kind of organism between certain isothermal lines; and hygrometric states that prevent the spread of each kind of organism beyond areas having a certain humidity or a certain dryness. Besides such general limitations, we find much more special limitations. Some minute vegetal forms occur only in snow. Hot springs have their peculiar Infusoria. The habitats of certain Fungi are mines or other dark places. And there are creatures unknown beyond the water contained in particular caves. After these limits to distribution imposed by physical conditions, come limits of a different class, imposed by the presence or absence of other organisms. Obviously, graminivorous animals are confined within tracts which produce plants fit for them to feed on. Large carnivores cannot exist out of regions where there are creatures numerous enough and large enough to serve for prey. The requirements of the sloth, limit it to certain forest-covered spaces; and there can be no insectivorous bats, where there are no night-fiying insects. To these dependences of the relatively-superior organisms on the relatively-inferior organisms which they consume, must be added certain reciprocal dependences of the inferior on the superior, Mr Darwin's inquiries have shown how generally the fertilization of plants is due to the agency of insects; and how certain plants, being fertilizable only by insects of a certain structure, are limited to regions inhabited by insects of this structure. Conversely, the spread of organisms is often bounded by the presence of particular organisms beyond the bounds — either competing organisms or organisms directly inimical. A plant that is fit for some territory adjacent to its own, fails to overrun it, because the territory is pre- occupied by some plant that is its superior, either in fertility or power of resisting destructive agencies; or else because there lives in the territory some mammal which browses on its foliage, or bird which devours nearly all its seeds. Similarly, an area in which animals of a particular species might thrive, is not colonized by them, because they are not fleet enough to escape some beast of prey inhabiting this area; or because the area is infested by some insect which destroys them, as the tsetse destroys the cattle in parts of Africa. Yet another more special series of limitations, accompanies parasitism. There are parasitic plants that flourish only on trees of some few kinds; and others that have certain animals for their habitats — as the fungus which is fatal to the silk-worm, or that which so strangely grows out of a New Zealand caterpillar. Of animal- parasitism we have various kinds: severally involving their specialities of distribution. We have that kind in which one creature uses another for purposes of locomotion; as the Chelonohia uses the turtle, and as a certain Actinia uses the shell inhabited bv a hermit-crab. We have ft/ that kind in which one creature habitually accompanies another to share its prey; like the annelid which takes up its abode in the shell occupied by a hermit-crab, and snatches from the hermit-crab, the morsels of food it is eating. We have again the commoner parasitism of the Epizoa — animals which attach themselves to the surfaces of other animals, and feed on their juices or on their secretions. And once more, we have the equally common parasitism of the Entozoa — creatures which live within other creatures. Besides being restricted in its distribution to the bodies of the organisms it infests, each species of parasite has usually still narrower limitations: in some cases the infested organisms furnish fit habitats for the parasites only in certain regions; and in other cases, only when in certain constitutional states. There are various more indirect modes in which the distributions of organisms affect each other. Plants of particular kinds are eaten by animals, only in the absence of kinds that are preferred to them; and the prosperity of such plants, hence partly depends on the presence of the preferred plants. Mr Bates has pointed out that some South American butterflies, thrive in regions where insectivorous birds would else destroy them, because they closely resemble butterflies of another genus which are disliked by those birds. And Mr Darwin gives cases of dependence still more remote and involved.
Such are the chief negative causes of distribution — the inorganic and organic agencies, that set bounds to the spaces which organisms of each species inhabit. Fully to understand their actions, we must contemplate them as working not separately, but in concert. "VVe have to regard the physical influences, varying from year to year, as now producing an extension or restriction of the habitat in this direction, and now in that; and as producing secondary extensions and restrictions, by their effects on other kinds of organisms. We have to regard the distribution of each organism, not only as affected by causes which favour multiplication of prey or of enemies within its owti area; but also hy causes which produce such results in neighbouring areas. We have to conceive the forces by which the limit is maintained, as including all meteorologic influences, united DISTIlinLTION. 315 DISTIlinLTION. 315 with the influences, direct or more or less remote, of nearly nil co-cxistin<^ organisms.
One general truth, indicated by sundry of the above illustrations, calls for special notice — the truth that organisms are ever intruding on each other's spheres of existence. Of the various modes in which this is shown, the commonest is the invasion of territory. That tendency which we see in the human races, to overrun and occupy each other's lands, as vrell as the lands inhabited by inferior creatures, is a tendency exhibited by all classes of organisms in all varieties of ways. Among them, as among mankind, there arc permanent conquests, temporary occupations, and occasional raids. Annual migrations are instances of this process in its most familiar form. Every spring an inroad is made into the area which our own fly- catchers occupy, by the swallows of the South; and every winter the fieldfares of the North, come to share the hips and haws of onr hedges with native birds — a partial possession of their territory, which entails on our native birds, some mortality. Besides these regularlyrecurring raids, there are irregular ones: as of locusts into countries not usually visited by them; or of strange birds which in small flocks from time to time visit areas adjacent to their own. Every now and then, an incursion ends in permanent settlement — perhaps in conquest over indigenous species. Within these few years, an American water- weed has taken possession of our ponds and rivers, and to some extent supplanted native water- weeds. Of animals, may be named a small kind of red ant, having habits allied to those of tropical ants, which has of late overrun many houses in London. The case of the rat, which must have taken to infesting ships within these few centuries, is a good illustration of the readiness of animals to occupy new places that are available. And the way in which vessels visiting India, are cleared of the European cockroach by the kindred BJatta orientaJis, shows us how these successful invasions last only imtil there come more powerful invaders,, Organisms encroach on one another's spheres of existence, in further ways than by trespassing on one another's areas: they adopt one another's modes of life. There are cases in which this usurpation of habits is slight and temporary; and there are cases where it is marked and permanent. Grey crows frequently join gulls and curlews in picking up food between tide-marks; and gulls and curlews may be occasionally seen many miles inland, feeding in ploughed fields and on moors.
Mr Darwin has watched a fly-catcher catching fish. He says that the greater titmouse sometimes adopts the practices of the shrike, and sometimes of the nuthatch; and that some South American woodpeckers are frugivorous, while others chase insects on the wing. Of habitual intrusions on the occupations of other creatures, one case is furnished by the sea-eagle; which, besides hunting the surface of the land for prey, like the rest of the hawk- tribe, often swoops down upon fish. And Mr Darwin names a species of petrel that has taken to diving, and has a considerable, modified organization. These last cases introduce us to a still more remarkable class of facts of kindred meaning. This intrusion of organisms on one another's modes of life, goes to the extent of intruding on one another's media. The great mass of flowering plants are terrestrial; and are required to be so by their process of fructification. But there are some which live in the water, and protrude only their flowers above the surface. Nay, there is a still more striking instance: on the sea- shore may be found an alga a hundred yards inland, and a phsenogam rooted in salt-water. Among animals, these interchanges of media are numerous. Nearly all coleopterous insects are terrestrial; but the water-beetle, which like the rest of its order is an air-breather, has aquatic habits. Water appears to be an especially unfit medium for a fly; and yet Mr Lubbock has lately discovered more than one species of fly living beneath the surface of the water, and coming up only occasionally for air.
Birds, as a class, are especially fitted for an aerial existence; DlSTinnUTION.
but certain tribes of them have taken to an aquatic existence — swimming on the surface of the water and making continual incursions beneath its surface; and there are some genera that have wholly lost the power of flight. Among mammals, too, whicli have limbs and lungs implying an organization for terrestrial life, may be named kinds that live more or less in the water, and arc more or less adapted to it. We have water-rats and otters, which unite the two kinds of life, and show but little modification; hippopotami passing the greater part of their time in the water, and somewhat more fitted to it; seals living almost exclusively in the sea, and having the mammalian form greatly obscured; whales wholly confined to the sea, and having so little the aspect of mammals as to be mistaken for fish. Conversely, sundry inhabitants of the water make more or less prolonged excursions on the land. Eels migrate at night from one pool to another. There are fish with specially-modified gills, and fin-rays serving as stilts, which, when the rivers they inhabit are partially dried-up, travel in search of better quarters. And while some kinds of crabs do not make land-excursions beyond high-water mark, other kinds pursue lives almost wholly terrestrial.
Joining together these two classes of facts, we must regard the bounds to each species' sphere of existence, as determined by the balancing of two antagonist sets of forces. The tendency which every species has to intrude on other areas, other modes of life, and other media, is restrained by the direct and indirect resistance of conditions, organic and inorganic. And these expansive and repressive energies, varying continually in their respective intensities, rhythmically equilibrate each other — maintain a limit that perpetually oscillates from side to side of a certain mean.
§ 106- As implied at the outset, the character of a region, when unfavourable to any species, sufficiently accounts for the absence of this species; and thus its absence is not incongruous with the hypothesis, that each species was originally placed in the regions most favourable to it. But the absence of a species from regions that are favourable to it, cannot be thus accounted for. Were plants and animals localized wholly with reference to the fitness of their constitutions to surrounding conditions, we might expect Floras to be similar and Faunas to be similar, where the conditions are similar; and we might expect dissimilarities among Floras and among Faunas, proportionate to the dissimilarities of their conditions. But we do not find such anticipations verified.
Mr Darwin says that '^ in the Southern hemisphere, if we compare large tracts of land in Australia, South Africa, and western South America, between latitudes 25° and 35"^, we shall find parts extremely similar in all their conditions, yet it would not be possible to point out three faunas and floras more utterly dissimilar. Or again we may compare the productions of South America south of lat. 35° with those north of 25°, which consequently inhabit a considerably different climate, and they will be found incomparably more closely related to each other, than they are to the productions of Australia or Africa under nearly the same climate." Still more striking are the contrasts which Mr Darwin points out, between closelyadjacent areas that are totally cut-off from each other. *' No two marine faunas are more distinct, with hardly a fish, shell, or crab in common, than those of the eastern and western shores of South and Central America; yet these great faunas are separated only by the narrow, but impassable, isthmus of Panama.'* On opposite sides of high mountain- chains, also, there are marked differences in the organic forms — differences not so marked as where the barriers are absolutely impassable; but much more marked than are necessitated by unlikenesses of physical conditions.
Not less suggestive is the converse fact, that wide geographical areas which offer decided geologic and meteorologic contrasts, are peopled by nearly-allied groups of organisms, if there are no barriers to migration. *' The naturalist in traveiling, for instance, irom north to south never fails to be struck by the manner in which successive groups of beings, specihcally distinct, yet clearly related, replace each other. lie hears from closely allied, yet distinct kinds of birds, notes nearly similar, and sees their nests similarly constructed, but not quite alike, with eggs coloured in nearly the same manner. The plains near the Straits of Magellan are inhabited by one species of Rhea (American Ostrich), and north- ward the plains of La Plata by another species of the same genus; and not by a true ostrich or emeu, like those found in Africa and Australia under the same latitude. On these same plains of La Plata, we see the agouti and bizcacha, animals having nearly the same habits as our hares and rabbits and belonging to the same order of Podents, but they plainly display an American type of structure. We ascend the lofty peaks of the Cordillera and we find an alpine species of bizcacha; we look to the waters, and we do not find the beaver or muskrat, but the coypu and capybara, rodents of the American type. Innumerable other instances could be given. If wo look to the islands off the American shore, however much they may diifer in geological structure, the inhabitants, though they may be all peculiar species, are essentially American.''
Y>^hat is the generalization that expresses these two groups of facts? On the one hand, we have similarly-conditioned, and sometimes nearly- adjacent, areas, occupied by quite different Faunas. On the other hand, we have areas remote from each other in latitude, and contrasted in soil as well as climate, which are occupied by closely-allied Faunas. Clearly then, as like organisms are not universally, or even generally, found in like habitats; nor very unlike organisms, in very unlike habitats; there is no manifest pre-determined adaptation of the organisms to the habitats. The organisms do not occur in such and such places, solely because they are either specially fit for these places, or more fit for them than all other organisms.
The induction under which these facts come, and which unites them with various other facts, is a totally-different one. When we see that the similar areas peopled by dissimilar forms, are those between which there are impassable barriers; while the dissimilar areas peopled by similar forms, are those between which there are no such barriers; we are at once reminded of the general truth exemplified in the last section: — the truth that each species of organism, tends ever to expand its sphere of existence — to intrude on other areas, other modes of life, other media; and through these perpetuallyrecurring attempts to thrust itself into every accessible habitat, spreads until it reaches limits that are for the time insurmountable.
§ 107. We pass now to the distribution of organic forms in Time. Geological inquiry has established the truth, that during a Past of immeasurable duration, plants and animals have existed on the Earth. In all countries their buried remains are found in greater or less abundance. From comparatively small areas, multitudinous different forms have been exhumed. Every exploration of new areas, and every closer inspection of areas already explored, brings more such forms to light. And beyond question, an exhaustive examination of all exposed strata, and of all strata now covered by the sea, would disclose forms immensely out -numbering all those at present known. Further, it is now becoming manifest to geologists, that even had we before us every kind of fossil which exists, we should still have nothing like a complete index to the past inhabitants of our globe. It has been long known that many sedimentary deposits have been so altered by the heat of adjacent molten matter, as greatly to obscure the organic remains contained in them. The extensive formations once called " transition," and now re-named " metamorphic," are acknowledged to be formations of sedimentary origin, from which all traces of such fossil as they probably included, have been obliterated by igneous action. And the conclusion forcing itself into acceptance, is, that igneous rock DISTllIRUTION. 321 DISTllIRUTION. 321 lias everywhere resulted from the complete melting-up of beds of detritus, originally deposited by water. How long the reactions of the Earth's molten nucleus on its cooled crust, have been thus destroying the records of Life which this cooled Just published, price 2s. Qd., THE CLASSIFICATION OF THE SCIENCES: TO WHICH AKE ADDED REASONS FOR DISSENTING FROM THE PHILOSOPHY OF M. COMTE.
BY HEEBEET SPENCER.
LONDON: WILLIAMS AND NOEGATE, 14, HENRIETTA STREET, COVENT GARDEN.
luscs; It IS stiU undeniable that there is a proportion between lapse of time and divergence of organic forms.
This divergence is comparatively slow and continuous, where there is continuity in the geological formations; but is sudden and comparatively wide, wherever there occurs a great break in the succession of strata. The contrasts which unites them with various other facts, is a totally-different one. When we see that the similar areas peopled by dissimilar forms, are those between which there are impassable barriers; while the dissimilar areas peopled by similar forms, are those between which there are no such barriers: we are at once retne organic remains contamea in umm. xno <:..vLoiioivtj xv^x^xxations once called '' transition,'' and now re-named " metamorphic," are acknowledged to be formations of sedimentary origin, from which all traces of such fossil as they probably included, have been obliterated by igneous action. And the conclusion forcing itself into acceptance, is, that igneous rock has everywhere resulted from the complete melting-up of beds of detritus, originally deposited by water. How long the reactions of the Earth's molten nucleus on its cooled crust, have been thus destroying the records of Life which this cooled crust entombed, it is impossible to say; but there are strong reasons for believing that the records which remain, bear but a small ratio to the records which have been destroyed. Thus we have but extremely-imperfect data for any conclusions respecting the distribution of organic forms in Time. Some few generalizations, however, maybe regarded as established.
One is, that the plants and animals now existing, mostly differ from the plants and animals which have existed. Though there are species common to our present Fauna and to past Faunas; yet the fades of our present Fauna differs, more or less, from the fades of each past Fauna. On carrying out the comparison, we find that past Faunas differ from each other; and that the differences between them are proportionate to their degrees of remoteness from each other in Time, as measured by their relative positions in the sedimentary series. So that if we take the assemblage of organic forms living now, and compare it with the successive assemblages of organic forms that have lived in successive geologic epochs, we find that the farther we go back into the past, the greater does the unlikeness become: the number of species and genera commxon to the compared assemblages, becomes smaller and smaller; and the assemblages differ more and more in their general characters. Though a species of brachiopod now extant, is almost identical with a species found in Silurian strata, and though between the Silurian Fauna and our own, there are sundry common genera of molluscs; it is still undeniable that there is a proportion between lapse of time and divergence of organic forms.
This divergence is comparatively slow and continuous, where there is continuity in the geological formations; but is sudden and comparatively wide, wherever ttiere occurs a great break in the succession of strata. The contrasts which 322 TIIE INDUCTIONS OF BIOLOGY.
thus arise gradually or all at once, in formations that are continuous or discontinuous, are of two kinds. Faunas of different eras, are distinguished partly by the absence from one of types that are present in the other; and partly by the unlikenesses between the types that are common to both. Such distinctions between Faunas as are due to the appearance or disappearance of types, are of secondary significance: they possibly, or probably, do not imply anything more than migrations or extinctions. The most significant distinctions are those between successive groups of organisms of the same type. And among such, as above said, the differences that arise are, speaking generally, small and continuous where a series of conformable strata gives proof of continued existence of the type in the locality; while they are comparatively large and abrupt, where there is evidence that between the deposit of the adjacent formations, a long period elapsed.
Another general fact, referred to by Mr Darwin as one which palaeontology has made tolerably certain, is that forms and groups of forms which have once disappeared from the Earth, do not reappear. Some few species and a good many genera, have continued throughout the whole period geologically recorded. But omitting these as exceptional, it may be said that each species after arising, spreading for an era, and continuing abundant for an era, eventually declines and becomes extinct; and that similarly, each genus during a longer period increases in the number of its species, and during a longer period dwindles and at last dies out. Having made its exit, neither species nor genus ever re-enters. And the like is true, even of those larger groups called orders. Four types of reptiles that were once abundant, have not been found in modern formations, and do not at present exist. Though nothing less than an exhaustive examination of all strata, can prove conclusively that a special or general form of organization when once lost is never reproduced; yet so many facts point to this inference, that its truth can scarcely be doubted.