lY. Doubly Symmetrical type. (Yertebrata.) Evolutio higemina. The development produces identical parts arising on both sides of an axis, growing up- wards and downwards, and shutting up along two lines, so that the inner layer of the germ is inclosed below, and the upper layer above. The embryos of these animals have a dorsal cord, dorsal plates, and ventral plates, a nervous tube and branchial fissures.
Recognizing these fundamental differences in the modes of evolution, as answering to fundamental divisions in the animal kingdom. Yon Baer shows (among the Verteh rata at least) how the minor differences that arise at successively later stages of evolution, correspond with the minor divisions.
Like the modern classification of plants, the classification of animals that has now been arrived at, is one in which the linear order is completely broken up. In his lectures at the Royal Institution, in 1857, Prof. Huxley expressed the rela- THE INDUCTIONS OF BIOLOGY.
tions existing among the several great groups of the animal kingdom, by placing these groups at the ends of four or five radii, diverging from a centre. The diagram I cannot obtain; but in the published reports of his lectures at the School of Mines the groups were arranged thus: — Yertebrata {Abranchiata) Mammalia Aves Eeptilia [Branchiaia) Amphibia Pisces MOLLUSCA Cephalopoda Hcteropoda Gasteropoda- dioecia I Pulraonata Gasteropoda- ( Ptevopoda monoccia Laraellibranchiata Annulosa Articulata Insocta Arachuida Myriapoda Crustacea Annuloida Anncllata Scoleida3 Echinodermata Treniatoda Eotifera Toeniada3 Tiirbellaria Nematoidea CffiLENTERATA Ilvdrozoa Actinozoa.
Protozoa Tiifusoria Noctilucidoi Spon^^iadfo Foramiuifera Crefravinidpe Thallassicollidce What remnant there may scorn to be of linear succession in some of these sub-groups, is merely an accident of typo- graphical convenience. Each of them is to be regarded simply as a cluster. Were Prof. Pluxley now to revise this scheme, he would probably separate more completely some of the great sub-groups, in conformity with the views expressed in his Ilunterian Lectures deli veered at the College of Sur- geons in 1863. And if he were further to develop the arrangement, by dispersing the sub-groups and sub-sub- groups on the same principle, there would result an arrange- CLASSIFICATION. o03 mcnt pcrliaps not very much unlike that shown in the annexed didgram.
\ V \ \ j lAft/ria/ioda annu'losa ^. f^colecldcL • ^ \ A/TLTLiiloida m.c7icvci-a, • • \ \ / / / / / MOLL US C A ^\ • / Slionqlda Jvfusona Hydrczca 1% •» J CCEL E N TERATA • In this diagram, the dots represent orders, the names of which it is impracticable to insert. If it be supposed that when magnified, each of these dots resolves itself into a cluster of clusters, representing genera and species, an ap- proximate idea wall be formed of the relations among the successively-subordinate groups constituting the animal king- 304 THE INDUCTIONS OF BIOLOGY.
dom. Besides the subordination of groups and their general distribution, some other facts are indicated. By the distances of the great divisions from the general centre, are rudely symbolized their respective degrees of divergence from the form of simple, undifferentiated organic matter; which we may regard as their common source. Within each group, the remoteness from the local centre represents, in a rough way, the degree of departure from the general plan of the group. And the distribution of the sub-groups within each group, is in most cases such, that those which come nearest to neighbouring groups, are those which show the nearest resemblances to them — in their analogies though not in their homologies. No diagram, however, can give a correct con- ception. Even supposing the above diagram expressed the relations of animals to one another as truly as they can be expressed on a plane surface, (which of course it does not,) it would still be inadequate. Such relations cannot be repre- sented in space of two dimensions; but only in space of three dimensions.
§ 101. While the classifications of botanists and zoologists have become more and more natural in their arrangements, there has grown up a certain artificiality in their abstract nomenclature. When aggregating the smallest groups into larger groups, and these into groups still larger, natur- alists adopted certain general terms expressive of the suc- cessively more comprehensive divisions; and the habitual use of these terms, needful for purposes of convenience, has led to the tacit assumption that they answer to actualities in Nature. It has been taken for granted that species, genera, orders, and classes, are assemblages of definite values — that every genus is the equivalent of every other genus, in respect of its degree of distinctness; and that orders are separated by lines of demarcation that are as broad in one place as another. Though this conviction is not a formulated one, yet the disputes continually arising among naturalists on the I CLASSIFICATION. 305 questions, wlicthcr such and such organisms arc specifically or generically distinct, and whether this or that peculiarity is or is not of ordinal importance, imply that the conviction is entertained even where it is not avowed. Yet that dif- ferences of opinion like these continually arise, and remain unsettled, except when they end in the establishment of sub- species, sub-genera, sub-orders, and sub-classes, sufficiently shows that no such conviction is justifiable. And this is equally shown by the impossibility of obtaining any definition of the degree of difference, which warrants each further eleva- tion in the hierarchy of classes.
It is, indeed, a wholly gratuitous assumption that organ- isms admit of being placed in groups of equivalent values; and that these may be united into larger groups that are also of equivalent values; and so on. There is no a priori reason for expecting this; and there is no a posteriori evi- dence implying it, save that which begs the question — that which asserts one distinction to be generic and another to be ordinal, because it is assumed that such distinctions must be either generic or ordinal. The endeavour to thrust plants and animals into these definite partitions, is of the same nature as the endeavour to thrust them into a linear series. Not that it does violence to the facts in anything like the same degree; but still, it does violence to the facts. Doubt- less the making of divisions and sub-divisions, is extremely useful; or rather, it is absolutely necessary. Doubtless, too, in reducing the facts to something like order, they must be partially distorted. So long as the distorted form is not mistaken for the actual form, no harm results. But it is needful for us to remember, that while our successively subordinate groups have a certain general correspondence with the realities, they inevitably give to the realities a regularity which does not exist.
§ 102. A general truth of much significance is exhibited ol 20 in these classifications. On observing the natures of the 306 THE INDUCTIONS OF BIOLOGY.
attributes wliich are common to the members of any group of the first, second, third, or fourth rank, we see that groups of the widest generality are based on characteristics of the greatest importance, physiologically considered; and that the characteristics of the successively-subordinate groups, are characteristics of successively-subordinate importance. The structural peculiarity in which all members of one sub- kingdom difier from all members of another sub-kingdom, is a peculiarity that affects the vital actions more profoundly, than does the structural peculiarity which distinguishes all members of one class from all members of another class. Let us look at a few cases.
We saw ( § 56) f that the broadest division among the functions is the division into " the accumulation of force (latent in food); the expenditure of force (latent in the tissues and certain matters absorbed by them); and the transfer of force (latent in the prepared nutriment or blood) from the parts which accumulate to the parts which expend." I^ow the lowest animals, united under the general name Protozoa, are those in which there is either no separation of the parts performing these functions or very indistinct separ- ation: in the Hhizopoda, all parts are alike accumulators of force, expenders of force, and transferrers of force; and though in the most differentiated members of the group, the Infusoria, there are something like specializations corre- sponding to these functions, yet there are no distinct tissues appropriated to them. The animals known as Cmlenterata are characterized in common by the possession of a part which accumulates force more or less marked off from the part which does not accumulate force, but only expends it; and the Ilydrozoa and Actinozoa, which are sub-divisions of the Cielentcrata, are contrasted in this, that in the one these parts are very indefinitely distinguished, but in the other definitely separated, as well as more complicated. Besides a completer differentiation of the organs respectively devoted to the accumulation of force and the expenditure of force, CLASSIFICATION. 'JOT the animals classed as Molhi^coida^ possess rude appliances for the transfer of force: the peri-visceral sac, or closed cavity between tlie intestine and the walls of the body, serves as a reservoir of absorbed nutriment, from which the surrounding tissues take up the materials they need. The more highly-organized animals, belonging to whichever sub- kingdom, all of them possess definitely-constructed channels for the transfer of force; and in all of them, the function of expenditure is divided between a directive apparatus and an executive apparatus — a nervous system and a muscular system. But these higher sub-kingdoms are clearly separated from each other by differences in the relative positions of their component sets of organs. Prof. Huxley defines the type of the Vertehrata^ as one in which the ganglionic nervous system lies on the dorsal side of the alimentary canal, while the central vascular system lies on its ventral side; and one which is yet further characterized by the possession of a second, and more conspicuous, nervous system, placed on the dorsal side of the vertebral axis — an extra endowment which is perhaps the most essentially distinctive. The types of the Annulosa and Mollusca, are together marked off from the vertebrate type, by the singleness of the nervous system, and by its occupation of the ventral side of the body: the habitual attitudes of annulose and molluscous creatures, is such that the neural centres are below the alimentary canal and the hoemal centres above. And while by these traits the annulose and molluscous types are separated from the verte- brate, they are separated from each other by this, that in the one the body is " composed of successive segments, usually pro\dded with limbs,'' but the other, the body is not segmented, " and no true articulated limbs are ever de- veloped.'^ The sub-kingdoms being thus distinguished from one an- other, by the presence or absence of parts devoted to funda- mental functions, or else by differences in the distributions of such parts; we find, on descending to the classes, that these 308 THE INDUCTIONS OF BIOLOGY.
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 de- scending to genera and species, we see that, speaking gener- ally, the successively smaller groups, are distinguished from one another by traits of successively less importance, physio- logically 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 com- bination. 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 im- practicable. 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 nicti- tating membranes; while the internal organizations through- out this sub-kingdom, might have been on many different plans.
If, on the other hand, this peculiar subordination of attri- butes 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 appro- priate; is to ascribe a very strange motive. When we dis- cover that the possession of seven cervical vertebra) is a general characteristic of mammals, whether the neck be im- mensely 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?
310 THE INDUCTIONS OF BIOLOGY.
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 post- poned. 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.
DISTRIBUTION.
§ 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 ex- ternal 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 organ- isms 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 sur- rounding conditions, are abundant, and familiar to all read- ers. It will be needful, however, here to cite a few typical ones of each order.
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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 ex- tensive 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 con- trasted 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 hu- midity 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 be- yond 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 DISTRIBUTION. 313 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 particu- lar species might thrive, is not colonized by them, because they are not fleet enough to escape some beast of prey inhab- iting this area; or because the area is infested by some in- sect 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 involv- ing 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 Ac- tinia 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 314 THE INDUCTIONS OF BIOLOGY.
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 under- stand 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 multi- plication 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 with the influences, direct or more or less remote, of nearly nil co-cxistin<^ organisms.
One general truth, indicated by sundry of the above illus- trations, 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 va- rieties 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 regularly- recurring 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