multiaxial development that is discontinuous, a familiar instance among plants exists in the common strawberry. This sends out over the neighbouring surface, long slender shoots, bearing at their extremities buds that presently strike roots, and become new individuals; and these by and by lose their connexions with the original axis. Other plants there are that produce certain specialized buds called bulbils, which separating themselves and falling to the ground, grow into independent plants. Among animals the fresh-water polype very clearly shows this mode of development: the young polypes, budding out from its surface, severally arrange their parts around distinct axes, and eventually detaching themselves, lead separate lives, and produce other polypes after the same fashion. By some of the lower Annulosa, this multiplication of axes from an original axis, is carried on after a different manner: the string of segments spontaneously divides; and after further growth, division recurs in one or both of the halves. And in the Aphides, we have a still fur- ther modification of this process.
Grouping together its several modes as above delineated, we see that r Unicentral / Central j or r Continuous ^ Multicentral < or Development is < or Discontinuous C Uniaxial Axial j or r Continuous ^ Multiaxial j or ^ Discontinuous 138 THE INDUCTIONS OF BIOLOGY.
Any one adequately acquainted with the facts, may readily raise objections to this arrangement. He may name forms which do not obviously come under any of these heads. He may point to plants that are for a time multicentral, but after- wards develop axially. And from lower types of animals, he may choose many in which the continuous and discontinuous modes are both displayed. But, as already hinted, an ar- rangement free from such anomalies must be impossible, if the various orders of organization have arisen by Evolution. The one above sketched out, is to be regarded as only a rough grouping of the facts, which helps us to a conception of them in their totality; and so regarded, it will be of service when we come to treat of Individuality and Reproduction.
§ 51. From these most general external aspects of organic development, let us now turn to its internal and more special aspects. When treating of Evolution as a universal process of things, a rude outline of the course of structural changes in organisms was given (First Principles, §§ 43, 66, 56). Here, however, it will be proper to describe these changes more fully.
The bud of any common plant in its earliest stage, consists of a small hemispherical or sub-conical projection. While it increases most rapidly at the apex, this presently deve- lops on one side of its base, a smaller projection of like general shape with itself. Here is the rudiment of a leaf; which pre- sently spreads more or less round the base of the central hemisphere or main axis. At the same time that the central hemisphere rises higher, this lateral prominence, also in- creasing, gives rise to subordinate prominences or lobes. These are the rudiments of stipules, where the leaves are stipulated. Meanwhile, towards the other side of the main axis, and somewhat higher up, another lateral prominence arising, marks the origin of a second leaf. By the time that the first leaf has produced another pair of lobes, and the second leaf has produced its primary pair, the central hemi- sphere, still increasing at its apex, exhibits the rudiment of a DEVELOPMENT.
DEVELOPMENT.
third leaf. Similarly throughout. While the germ of each succeeding leaf thus arises, the germs of the previous leaves, in the order of their priority, are changing their rude nodu- lated shapes into flattened-out expansions; which slowly put on those sharp outlines they show when unfolded. Thus from that extremely indefinite figure, a rounded lump, giving off from time to time lateral lumps, which severally becoming symmetrically lobed, gradually assume specific and involved forms, we pass little by little to that comparatively complex thing — a leaf-bearing shoot. Internally, a bud undergoes analogous changes. The layer of substance which forms the surface of the hemisphere, and in which these metamor- phoses commence, consists of a transparent, irregularly -aggre- gated mass of cells and centres of growth, not formed into a tissue. Especially is this the case at the apex, where the vital activity is the greatest. Here the primitive cellular mass passes without any line of demarcation into the tissues that are developing from it. While, by continued cell-multi- plication this layer increases, and doing so most rapidly at the apex thrusts outwards its lateral portions, these begin to exhibit differentiations. " Graduall}-," says Schleiden, " se- parate masses of cells, with a distinct and definite outline, appear in this chaos, and they cease to partake of the process of growth going on. At first the epidermis is separated, then the vascular bundles, later the parenchyma." Similarly with the lateral buds whence leaves arise. In the, at first, un- organized mass of cells constituting the rudimentary leaf, there are formed vascular bundles which eventually become the veins of the leaf; and gradually there appear also, though in ways that have not been specified, the parenchyma and the epithelium. I^or do we fail to find an essentially parallel set of changes, when we trace the histories of the in- dividual cells. While the tissues they compose are separ- ating, the cells are growing step by step more unlike. Some become flat, some polyhedral, some C3dindrical, some prismatic, some spindle-shaped. These develop spiral fibres 140 THE INDUCTIONS OF BIOLOGY.
ia their interiors; and those, net- works of fibres. Here a number of cells unite together to form a tube; and there they become solid by the internal deposition of woody or other matter. Through such changes, too numerous and involved to be here detailed, the originally uniform cells go on diverg- ing and re-diverging, until there are produced various forms that seem to have very little in common.
The arm of a man makes its first appearance in as simple a way as does the shoot of a plant. According to BischofF, it buds- out from the side of the embryo, as a little tongue -shaped projection, presenting no differences of parts; and it might serve for the rudiment of some one of the various other organs that also arise as buds. Continuing to lengthen, it presently becomes somewhat enlarged at its end; and is then described as a pedicle bearing a flattened, round- edged lump. This lump is the representative of the future hand; and the pedicle, of the future arm. By and by, at the edges of this flattened lump, there appear four clefts, dividing from each other the buds of the future fingers; and the hand as a whole grows a little more distinguishable from the arm. Up to this time, the pedicle has remained one continuous piece; but it now begins to show a bend at its centre, which indicates the division into arm and forearm. The distinctions thus rudely indicated, gradually increase: the fingers elongate and become jointed; and the proportions of all the parts, originally very un- like those of the complete limb, slowly approximate to them. During its bud-like stage, the rudimentary arm is nothing but a homogeneous mass of simple cells, with- out any arrangement. By the diverse changes they gradually undergo, these cells are transformed into bones, muscles, blood-vessels, and nerves. The extreme softness and delicacy of this primary cellular tissue, renders it difficult to trace the initial stages of these differentiations. In consequence of the colour of their contents, the blood-vessels are the first parts to become visible. Afterwards the cartilaginous parts, which are the bases of the future bones, become marked out by the DEVELOPMENT. 141 denser aggregation of their constituent cells, and the produc- tion between these of a liyaline substance which unites them into a translucent mass. When first perceptible, the muscles are gelatinous, pale, yellowish, transparent, and indistinguish- able from their tendons. The various otlier tissues of which the arm consists, beginning with very faintly-marked differ- ences, become day by day more definite in their outlines and appearances. In lilie manner, the units composing these tissues, severally assume increasingly-specific characters. The fibres of muscle, at first made visible in the midst of their gelatinous matrix only by immersion in alcohol, grow more numerous and distinct; and by and by they begin to exhibit transverse stripes. The bone-cells put on by degrees their curious structure of branching canals. And so in their respective ways with the units of skin and the rest.
Thus in each of the organic sub-kingdoms, we see this change from an incoherent, indefinite homogeneity, to a coherent, definite heterogeneity, illustrated in a quadruple way. The originally -like units or cells, become unlike in various ways, and in ways more numerous and marked as the development goes on. The several tissues which these several classes of cells form by aggregation, grow little by little distinct from each other; and little by little put on those structural complexities, that arise from differentiations among their component units. In the shoot, as in the limb, the external form, originally very simple, and having much in common with countless simple forms, organic and in- organic, gradually acquires an increasing complexity, and an increasing unlikeness to other forms. And meanwhile, the remaining parts of the organism to which the shoot or limb belongs, having been severally assuming structures divergent from each other and from that of this particular shoot or limb, there has arisen a greater heterogeneity in the organ- ism as a whole.
§ 52. One of the most remarkable inductions of embry- 14^ THE INDrCTIONS OF BIOLOGY.
ology comes next in order. Yon Baer found that in its earliest stage, every organism lias the greatest number of characters in common with all other organisms in their earliest stages; that at a stage somewhat later, its structure is like the structures displaj^ed at corresponding phases by a less extensive multitude of organisms; that at each sub- sequent stage, traits are acquired which successively distin- guish the developing embryo from groups of embryos that it previously resembled — thus step by step diminishing the group of embryos which it still resembles; and that thus the class of similar forms, is finally narrowed to the species of which it is a member. This abstract proposition will per- haps not be fully realized by the general reader. It will be best to re-state it in a concrete shape. The germ out of which a human being is evolved, differs in no visible respect from the germ out of w^hich every animal and plant is evolved. The first conspicuous structural change undergone by this human germ, is one characterizing the germs of animals only — ^differentiates them from the germs of plants. The next distinction established, is a distinction exhibited by all Vertehrata; but never exhibited hy Annulosa, Molluscaj or Cmlenterata. Instead of continuing to resemble, as it now does, the rudiments of all fishes, reptiles, birds, and mammals; this rudiment of a man, assumes a structure that is seen only in the rudiments of mammals. Later, the embryo undergoes changes which exclude it from the group of implacental mammals; and prove that it belongs to the group of placental mammals. Later still, it grows unlike the embryos of those placental mammals distinguished as ungulate or hoofed; and continues to resemble only the unguiculate or clawed. By and by, it ceases to be like any foetuses but those of the quad- rumana; and eventually the foetuses of only the higher quadrumana are simulated. Lastly, at birth, the infant, belonging to whichever human race it may do, is structurally very much like the infants of all other human races; and only afterwards acquires those various minor peculiarities of DEVELOPMENT. 14'] form tliat distinguish the variety of man to which it be- lono's.
Tlie generalization here expressed and illustrated, must not be confounded with an erroneous semblance of it that has obtained considerable currency. An impression has been given by those who have popularized the statements of em- bryologists, that during its development, each higher organ- ism passes through stages in which it resembles the adult forms of lower organisms — that the embryo of a man is at one time like a fish, and at another time like a reptile. This is not the fact. The fact established is, that up to a certain point, the embryos of a man and a fish continue similar, and that then differences begin to appear and increase — the one embryo approaching more and more towards the form of a fish; the other diverging from it more and more. And so with the resemblances to tUe more advanced types. Suppos- ing the germs of all kinds of organisms to be simultaneously developing, we may say that all members of the vast mul- titude take their first steps in the same direction; that at the second step one-half of this vast multitude diverges from the other half, and thereafter follows a different course of deve- lopment; that the immense assemblage contained in either of these divisions, very soon again shows a tendency to take two or more routes of development; that each of the two or more minor assemblages thus resulting, shows for a time but small divergences among its members, but presently again divides into groups which separate ever more widely as they progress; and so on, until each organism, when nearly com- plete, is accompanied in its further modifications only by organisms of the same species; and last of all, assumes the peculiarities which distinguish it as an individual — diverges to a slight extent to the organisms it is most like. The reader must also be cautioned against accepting this general- ization as exact. The likenesses thus successive^ displayed are not precise but approximate. Only leading characteris- tics are the same: not all the details. It is as though in 144 THE INDUCTIONS OF BIOLOGY.
one of the diverging groups just described, each kind of organism, though having a general direction of development like that of the others it is for a time travelling vrith, shows from the first a tendency to leave the general route — a tend- ency which presently becomes strongly marked. Making all requisite qualifications, however, these resemblances re- main conspicuous; and the fact that they follow each other in the way described, is a fact of great significance.
§ 53. This comparison between the course of development in any creature, and the course of development in all other creatures — this arrival at the conclusion that the course of development in each, at first the same as in all others, be- comes stage by stage differentiated from the courses of all others, brings us within view of an allied conclusion. If we contemplate the successive stages passed through by any higher organism, and observe the relation between it and its environment at each of these stages; we shall see that this re- lation is modified in a way analogous to that in which the relation between the organism and its environment is modi- fied, as we advance from the lowest to the highest grades. Along with the progressing differentiation of each organism from others, we find a progressing differentiation of it from its environment; like that progressing differentiation from the environment which we meet with in the ascending forms of life. Let us first glance at the way in which the ascending forms of life exhibit this progressing differentiation from the environment.
In the first place, it is illustrated in structure. Ad- vance from the homogeneous to the heterogeneous, itself in- volves an increasing distinction from the inorganic world. In the lowest Protozoa we have a simplicity approaching to that of air, water, or earth; and the ascent to organisms of greater and greater complexity of structure, is an ascent to organisms that are in that respect more strongly contrasted with the structureless environment. Inform, again, DEVELOPMENT. 145 DEVELOPMENT. 145 we see the same fact. An ordinary characteristic of inor- ganic matter is its indefinitencss of form; and this is also a characteristic of the lower organisms, as compared with the higher. Speaking generally, plants are less definite than animals, both in shape and size — admit of greater modifica- tions from variations of position and nutrition. Among ani- mals, the simplest Rhizopods are not only structureless but amorj)hous: the form is never specific, and is constantly changing. Of the organisms resulting from the aggregation of such creatures, we see that while some, as the Fo7'amim~ /era, assume a certain definiteness of form, in their shells at least; others, as the Sponges, are very irregular. The Zoo- phytes and the Polyzoa are compound organisms, most of which have a mode of growth not more determinate than that of plants. But among the higher animals, we find not only that the mature shape of each species is very definite, but that the individuals of each species difier very little in size. A parallel increase of contrast is seen in chemical composition. With but few exceptions, and those only partial ones, the lowest animal and vegetal forms are inhabit- ants of the water; and water is almost their sole constituent. Desiccated Protophyta and Protozoa shrink into mere dust; and among the Acalephes, we find but a few grains of solid matter to a pound of water. The higher aquatic plants, in common with the higher aquatic animals, possessing as they do increased tenacity of substance, also contain a greater pro- portion of the organic elements; and so are chemically more unlike their medium. And when we pass to the superior classes of organisms — land-plants and land-animals — we find that, chemically considered, they have little in common either with the earth on which they stand or the air which sur- rounds them. In specific gravity too, we may note the like truth. The very simplest forms, in common with the spores and gemmules of higher ones, are as nearly as may be of the same specific gravity as the water in which they float; and though it cannot be said that among aquatic 10 146 THE INDUCTIONS OF BIOLOGY.
creatures, superior specific gravity is a standard of general superiority, yet we may fairly say that the superior orders of them, when divested of the appliances by which their specific gravity is regulated, difier more from water in their relative weight than do the lowest. In terrestrial organisms, the contrast becomes extremely marked. Trees and plants, in common with insects, reptiles, mammals, birds, are all of a specific gravity considerably less than that of the earth and immensely greater than that of the air. Yet further, we see the law similarly fulfilled in respect of temperature. Plants generate but extremely small quantities of heat, which are to be detected only by very delicate experiments; and practically they may be considered as having the same tem- perature as their environment. The temperature of aquatic animals is very little above that of the surrounding water: that of the invertebrata being mostly less than a degree above it, and that of fishes not exceeding it by more than two or three degrees; save in the case of some large red-blooded fishes, as the tunny, which exceed it in temperature by nearly ten degrees. Among insects, the range is from two to ten degrees above that of the air: the excess varying according to their activity. The heat of reptiles is from four to fifteen degrees more than the heat of their medium. While mam- mals and birds maintain a heat which continues almost un- affected by external variations, and is often greater than that of the air by seventy, eighty, ninety, and even a hundred degrees. Once more, in greater self-mohility a progressive differentiation is traceable. The especial character- istic by which we distinguish dead matter is its inertness: some form of independent motion is our most general test of life. Passing over the indefinite border-land between the animal and vegetal kingdoms, we may roughly class plants as organisms which, while they exhibit that species of motion implied in growth, are not only devoid of locomotive power, but with some unimportant exceptions are devoid of the power of moving their parts in relation to each other; and DEVELOPMENT. 147 thus are less differentiated from the inorganic world than animals. Though in those microscopic Protoj)hyta and Pro- tozoa inhabiting the water — the spores of algcc, the gcmmules of sponges, and the infusoria generally — we see locomotion produced by ciliary action; yet this locomotion, while rapid relatively to the size of the creatures, is absolutely slow. Of the Coelcnterata, a great part are either permanently rooted or habitually stationary; and so have scarcely any self- mobility but that implied in the relative movements of parts; while the rest, of which the common jelly-fish will serve as a sam- ple, have mostly but little ability to move themselves through the water. Among the higher aquatic Invertehrata, — cuttle- fishes and lobsters, for instance, — there is a very considerable power of locomotion; and the aquatic Vertehrata are, con- sidered as a class, much more active in their movements than the other inhabitants of the water. But it is only when we come to air-breathing creatures, that we find the vital charac- teristic of self- mobility manifested in the highest degree. Flying insects, mammals, birds, travel with a velocity far exceeding that attained by any of the lower classes of ani- mals; and so are more strongly contrasted with their inert environment. Thus, on contemplating the various grades of organisms in their ascending order, we find them more and more distinguished from their inanimate media, in structure, in form, in chemical composition, in specific gravity, in temperature,' i\i self 'mobility. It is true that this general- ization does not hold with complete regularity. Organisms which are in some respects the most strongly contrasted with the environing inorganic world, are in other respects less so than inferior organisms. As a class, mammals are higher than birds; and yet they are of lower temperature, and have smaller powers of locomotion. The stationary oyster is of higher organization than the free- swimming medusa; and the cold-blooded and less heterogeneous fish, is quicker in its movements than the warm-blooded and more heterogeneous sloth. But the admission,that the several aspects under 148 THE INDUCTIONS OF BIOLOGY.
wMcli this increasing contrast shows itself, bear variable ratios to each other, does not conflict with the general truth, that as we ascend in the hierarchy of organisms, we meet with not only an increasing differentiation of parts, but also an increasing differentiation from the surrounding medium in sundry other physical attributes. It would seem that this peculiarity has some necessary connexion with superior vital manifestations. One of those lowly gelatinous forms, so transparent and colourless as to be with difficulty dis- tinguished from the water it floats in, is not more like its medium in chemical, mechanical, optical, thermal, and other properties, than it is in the passivity with which it sub- mits to all the influences and actions brought to bear upon it; while the mammal does not more widely differ from inanimate things in these properties, than it does in the ac- tivity with which it meets surrounding changes by compens- ating changes in itself. And between these two extremes, we shall observe a constant ratio between these two kinds of contrast. Whence we may say, that in proportion as an organism is physically like its environment, does it remain a passive partaker of the changes going on in its environment; while in proportion as it is endowed with powers of counteracting such changes, it exhibits greater unlikeness to its en- vironment.*