MARINE BIOLOGICAL LABORATORY

WOODS HOLE, MASSACHUSETTS

IN MEMORY OF

EDWARD GARDINER GARDINER

1854-1907

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A SYSTEM

OF

SYNTHETIC PHILOSOPHY.

VOL II.

Spencer's Synthetic Philosophy.

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I. THE UNKNOWABLE. II. LAWS OF THE KNOWABLE.

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III. THE EVOLUTION OF LIFE.

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HO.) PRINCIPLES OF MORALITY. Vol. II

D. APPLETON & CO., PUBLISHERS, NEW YOEK.

THE PRINCIPLES

BIOLOGY.

BY

HERBERT SPENCER,

OF "THE PRINCIPLES OF PSYCHOLOGY," "ILLUSTRATIONS or PROGRESS,** 41 ESSAYS : MORAL, POLITICAL, AND ESTHETIC," " FIRST PRINCIPLES,"

"SOCIAL STATICS," "EDUCATION," ETC.

VOL. L

NEW YORE: D. APPLETON AND COMPANY,

1, 3, AND 5 BOND STREET.

1886.

fiatered, according to Act of Congress, iu the year BY D. APPLETON & CO.,

En ihe Olcrk'a Office of the District Court of the United States for Southern District of New York.

31

PREFACE TO THE AMERICAN EDITION.

THE System of Philosophy now in course of publication by Mr. HERBERT SPENCEK begins with a volume of First Princi- ples, which was republished in this country a year or two since. The subject of Biology comes next in order, and is to be treated in two volumes, of which the present is the first ; Volume II. will probably appear toward the close of the year. In accordance with the author's plan, the doctrine or method of Evolution unfolded in First Principles and applied to Biol- ogy in the present work, will be carried out in the subsequent treatment of the Principles of Psychology and the Principles of Sociology.

In the preface to the English edition, Mr. Spencer remarks:

"The aim of this work is to set forth the general truths of Biology, as illustrative of, and as interpreted by, the laws of Evolution : the special truths being introduced only so far as is needful for elucidation of the general truths.

" For aid in executing it, I owe many thanks to Prof. Hux- ley and Dr. Hooker. They have supplied me with information where my own was deficient ; and in looking through the proof-sheets, have pointed out errors of detail into which I had fallen. By having kindly rendered me this valuable assist- ance, they must not, however, be held committed to any of the enunciated doctrines that are not among the recognized truths of Biology."

NEW YOKE, March, 1866.

CONTENTS OF V7OL I.

PAET I.— THE DATA OF BIOLOGY.

CHAP.

I. — ORGANIC MATTER . . . . . . . .

II. — THE ACTIONS OF FORCES ON ORGANIC MATTER . .

III. — THE RE-ACTIONS OF ORGANIC MATTER ON FORCES 42

IV. — PROXIMATE DEFINITION OF LIFE . . . . 59

Y. — THE CORRESPONDENCE BETWEEN LIFE AND ITS CIR- CUMSTANCES . . . . . . 72

VI. — THE DEGREE OF LIFE YARIES AS THE DEGREE OF

CORRESPONDENCE . . . . 82

VII. — THE SCOPE OF BIOLOGY . . . . 9-1

PAET II.— THE INDUCTIONS OF BIOLOGY.

I. — GROWTH

II. D KVELOPMENT

III. FUNCTION

IV. WASTE AND REPAIR

V. — ADAPTATION . .

. . 107

. t J.OG

. . 153

.. 169

184

VU1 CONTESTS.

CHAP.

TI. INDIVIDUALITY . . . . . . . . 201

VII. — GENESIS . . . . . . . . . . 209

VIII. HEREDITY . . . . . . . . . 239

IX. — VARIATION . . . . • . . . . . 257

X.— GENESIS, HEREDITY, AND VARIATION . , . . 273

XI. CLASSIFICATION . . . . . . . . 292

XII. DISTRIBUTION .. .. 311

PAET III.— THE EVOLUTION OF LIFE.

I. PRELIMINARY . , . . . . . . 331

II. — GENERAL ASPECTS OF THE SPECIAL-CREATION-HY-

POTHES75 . . , . . . . . 333

III. GENERAL ASPECTS OF 1HE EVOLUlIwN-H IPOTHESIS 346

IV. THE ARGUMENTS FROM CLASSIFICATION . . 356

V. THE ARGUMENTS FROM EMBRYOLOGY . . . . 365

VI. THE ARGUMENTS FROM MOEl'UOLGUl . . , . 380

VII. — THE ARGUMENTS FROM DISTRIBUTION . . . . 388

VIII. — HOAV IS ORGANIC EVOLUTION CAUSED? .. .. 402

IX. EXTERNAL FACTORS . . . . . . . . 411

X. — INTERNAL FACTORS . . . . . . . . 420

XI. — DIRECT EQUILIBRATION . . . . . . 432

XIT. INDIRECT EQUILIBRATION . . . . . . 413

1III. — THE CO-OPERATION OF THE FACTORS . . . . 4()4

. THE CONVERGENCE OF THE EVIDENCES . . . . 470

PART I.

THE DATA OF BIOLOGY

CHAPTER I.

ORGANIC MATTER.

| 1. OF the four chief elements which, in various com- binations, make up living bodies, three are gaseous. While carbon is known only as a solid, oxygen, hydrogen, and nitrogen are known only in the aeriform state. Under pressures great enough to reduce them almost to the density of liquids these elements have still defied all efforts to liquefy them. There is a certain significance in this. When we remember how those re-distributions of Matter and Motion which constitute Evolution, structural and functional, imply motions in the units that are re-distributed ; we shall see a probable meaning in the fact that organic bodies, which exhibit the phenomena of Evolution in so high a degree, are mainly composed of ultimate units having extreme mobility. The properties of substances, though destroyed to sense by combination, are not destroyed in reality : it follows from the persistence of force, that the properties of a compound are resultants of the properties of its components — resultants in which the properties of the components are severally in full action, though greatly obscured by each other. One of the leading properties of each substance is its degree of molecular mobility ; and its degree of molecular mobility more or less sensibly affects the molecular mobilities of the various compounds into which it enters. Hence we may infer some relation between the gaseous form of three out of the four

4 THE DATA OF BIOLOGY.

chief organic elements, and that comparative readiness dis- played by organic matters to undergo those changes in the arrangement of parts which we call development, and those transformations of motion which we call function.

Considering them chemically instead of physically, it is to be remarked that three out of these four main components of organic matter, have affinities which are narrow in their range and low in their intensity. Hydrogen combines with comparatively few other elements ; and such chemical energy as it does show, is scarcely at all shown within the limits of the organic temperatures. Of carbon it may similarly be said that it is totally inert at ordinary heats ; that the number of substances with which it unites is not great ; and that in most cases its tendency to unite with them is but feeble. Lastly, this chemical indifference is shown in the highest degree by nitrogen — an element which, as we shall here- after see, plays the leading part in organic changes.

Among the organic elements, including under the title not only the four chief ones, but also the less conspicuous re- mainder, that capability of assuming different states, called allotropism, is frequent. Carbon presents itself in the three unlike conditions of diamond, graphite, and charcoal. Under certain circumstances, oxygen takes on the form in which it is called ozone. Sulphur and phosphorus (both, in small proportions, essential constituents of organic matter) have allotropic modifications. Silicon, too, is allotropic ; while its oxide, silica, which is an indispensable constituent of many lower organisms, exhibits the analogue of allotropism — isomerism. And even of the iron which plays an active part in higher organisms, and a passive part in some lower ones, it may be said that though not known to be itself allo- tropic, yet isomerism characterizes those compounds of it that are found in living bodies. Allotropism being interpretable as some change of molecular arrangement, this frequency of its occurrence among the components of organic matter, is significant as implying a further kind of molecular mobility.

ORGANIC MATTER. 6

One more fact, that is here of great interest for us, must be set down. These four elements of which organisms are almost wholly composed, present us with certain extreme antitheses. "While between two of them we have an unsur- passed contrast in chemical activity ; between one of them and the other three, we have an unsurpassed contrast in molecular mobility. AYhile carbon, by successfully resisting fusion and volatilization at the highest temperatures that can ba produced, shows us a degree of atomic cohesion greater than that of any other known element, hydrogen, oxygen, and nitrogen, show the least atomic cohesion of all elements. And while oxygen displays, alike in the range and intensity of its affinities, a chemical energy exceeding that of any other substance (unless fluorine be considered an exception), nitrogen displays the greatest chemical inactivity. Now on calling to mind one of the general truths arrived at when analyzing the process of Evolution, the probable significance of this double difference will be seen. It was shown (First Principlcsy § 123) that, other things equal, unlike units are more easily separated by incident forces than like units are — that an inci- dent force falling on units that are but little dissimilar does not readily segregate them ; but that it readily segregates them if they are widely dissimilar. Thus, these two extreme contrasts, the one between physical mobilities, and the other between chemical activities, fulfil, in the highest degree, a certain further condition to facility of differentiation and in- tegration.

§ 2 Among the binary combinations of these four chief organic elements, we find a molecular mobility much less than that of these elements themselves ; at the same time that it is much greater than that of binary compounds in general. Of the two products formed by the union

of oxvgen with carbon, the first, called carbonic oxide, which

•/ o

contains one atom of carbon to one of oxygen (expressed by the symbol C 0), is an incondensible gas ; and the second

tf THE DATA OF BIOLOGY.

carbonic acid, containing ail additional atom of oxygen (C 02) assumes a liquid form only under a pressure of nearly forty atmospheres. The several compounds of oxygen with

nitrogen, present us with an instructive gradation. Protoxide of nitrogen, which contains one atom of each element (N 0), is a gas condensible only under a pressure of some fifty at- mospheres ; deutoxide of nitrogen (N 02) is a gas hitherto uncondensed (the molecular mobility remaining undiminished in consequence of the volume of the united gases remaining unchanged) ; nitrous acid (N 03) is gaseous at ordinary temperatures, bat condenses into a very volatile liquid at the zero of Fahrenheit ; peroxide of nitrogen (N 04) is gaseous at 71°, liquid between that and 16°, and becomes solid at a tem- perature below this ; while nitric acid (N 00) may be obtained in crystals which melt at 85° and boil at 113°. In this series we see, though not with complete uniformity, a de- crease of molecular mobility as the weights of the compound molecules are increased. The hydro-carbons illus-

trate the same general truth still better. One series of them will suffice. Marsh gas (C2 H^) is permanently gaseous. Olefiant gas (C4 H4) may be liquefied by pressure. Oil gas, which is identical with olefiant gas in the proportions of its constituents but has double the atomic weight, (Cs H8), becomes liquid without pressure at the zero of Fahrenheit. Amylene (Ci0H10) is a liquid which boils to 102°. And the suc- cessively higher multiples, caproylene (C12 H12), caprylena (C16 H16), elaene(C18 H18) and paramylene (C20 H20), are liquids which boil respectively at 102°, 131°, 257°, 230°, and 329°. Cetylcne (C32 II32) is a liquid which boils at 527° ; while pa- rafnne (C34 H51) and roylene (C60 H60) are solids. Only

one compound of hydrogen with nitrogen has been obtained in a free state — ammonia (H3 N) ; and this, which is gaseous, is liquefiable by pressure, or by reducing its temperature to — 40° F. In cyanogen, which is composed of nitro-

gen and carbon (N C2), we have a gas that becomes liquid at a pressure of four atmospheres and solid at — 30° F. And, in

ORGANIC MATTER. 7

paracyanogen, formed of the same proportions of these ele- ments in higher multiples (N3 C6), we have a solid which does not fuse or volatilize at ordinary temperatures. Lastly,

in the most important member of this group, water, (H 0 or else as many chemists now think H2 O2) we have a corn- pound of two incondensible gases which assumes both th3 fluid state and the solid state within ordinary ranges of temperature ; while its molecular mobility is still such that its fluid or solid masses are continually passing into the form of vapour, though not with great rapidity until the temper- ature is raised to 212°.*

Considering them chemically, it is to be remarked of these binary compounds of the four chief organic elements, that they are, on the average, less stable than binary com- pounds in general. "Water, carbonic oxide, and carbonic acid, are, it is true, difficult to decompose. But omitting these, the usual strength of union among the elements of the above-named substances is low considering the simplicity

* This immense loss of molecular mobility which oxygen and hydrogen un- dergo on uniting to form water — a loss far greater than that seen in other binary compounds of analogous composition — suggests the conclusion that the atom of •water is a multiple atom. Thinking that if this conclusion be true, some evidence of the fact must be afforded by the heat-absorbing power of aqueous vapour, I lately 'put the question to Prof. Tyndall, whether it resulted from his ex- periments that the vapour of water absorbs more heat than the supposed sim- plicity of its atom would lead him to expect. I learned from him that it has an excessive absorbent power — an absorbent power more like that of the complex- atomed vapours than like that of the simple-atomed vapours — an absorbent power that therefore harmonizes with the supposition that its atom is a multiple one. Besides this anomalous loss of molecular mobility and this anomalous heat- absorbing power, there are other facts which countenance the supposition. The unparalleled evolution of heat during the combination of oxygen and hydrogen is one. Another is that exceptional property which water possesses, of beginning to expand when its temperature is lowered below 40' ; since this exceptional property is explicable only on the assumption of some change of molecular arrangement — a change which is comprehensible if the molecules are multiple ones. And yet a further confirmatory fact is the ability of water to assume a colloid condition ; for as this implies a capacity in its atoms for aggregating into high multiples, it suggests, by analogy with known cases, that they have a capacity for aggregating iato lower multiples.

8 THE DATA OF BIOLOGY.

•

of the substances. With the exception of acetylene, ih« various hydro-carbons are not producible by directly com- bining their elements ; and the elements of most of them are readily separated by heat without the aid of any antagonistic affinity. Nitrogen and hydrogen do not unite with each other immediately ; and the ammonia which results from their mediate union, though it resists heat, yields to the electric spark. Cyanogen is stable : not being resolved into its components at a red heat, unless in iron vessels. Much less stable however are the several oxides of nitrogen. The protoxide, it is true, does not yield up its elements below a red heat ; but nitrous acid cannot exist if water be added to it ; hypo-nitric acid is decomposed both by water and by contact with the various bases ; and nitric acid not only readily parts with its oxygen to many metals, but when anhydrous, spontaneously decomposes. Here it will

be well to note, as having a bearing on what is to follow, how characteristic of most nitrogenous compounds is this special instability. In all the familiar cases of sudden and violent decomposition, the change is due to the presence of nitrogen. The explosion of gunpowder results from the readiness with which the nitrogen contained in the nitrate of potash, yields up the oxygen combined with it. The explosion of gun- cot- ton, which also contains nitric acid, is a substantially par- allel phenomenon. The various fulminating salts are all formed by the union with metals, of a certain nitrogenous acid called fulminic acid ; which is so unstable that it cannot be obtained in a separate state. Explosiveness is a property of nitro-mannite, and also of nitro- glycerin. Iodide of nitrogen detonates on the slightest touch, and often without any assign- able cause. Percussion produces detonation in sulphide of nitrogen. And the body which explodes with the most tremendous violence of any that is known, is the chloride of nitrogen. Thus these easy and rapid decompositions, due to the chemical indifference of nitrogen, are characteristic. When we come hereafter to observe the part which nitrogen

MATTER.

plays in organic actions, we shall see the significance of this extreme readiness shown by its compounds to undergo change. Returning from these facts parenthetically

introduced, we have next to note that though among these binary compounds of the four chief organic elements, there are a few active ones, yet the majority of them display a smaller degree of chemical energy than the average of binary compounds. Water is the most neutral of bodies : usually pro- ducing little chemical alteration in the substances with which it combines ; and being expelled from most of its combinations by a moderate heat. Carbonic acid is a relatively feeble acid : the carbonates being decomposed by the majority of other acids and by ignition. The various hydro-carbons are but narrow in the range of their comparatively weak affinities. The compounds formed by ammonia have not much stability : they are readily destroyed by heat, and by the other alkalies. The affinities of cyanogen are tolerably strong ; though they yield to those of the chief acids. Of the several oxides of ni- trogen it is to be remarked, that while those containing the smaller proportions of oxygen are chemically inert, that con- taining the greatest proportion of oxygen (nitric acid) though chemically active, in consequence of the readiness with which one part of it gives up its oxygen to oxidize a base with which the rest combines, is nevertheless driven from all itd combinations by a red heat.

These binary compounds, like their elements, are to a con- siderable degree characterized by the prevalence among them of allotropism ; or, as it is more usually called when displayed by compound bodies — isomerism. Professor Graham finds reason for thinking that a change in atomic arrange- ment of this nature, takes place in. water, at or near the melting point of ice. The relation between cyanogen and paracyanogen is, as we saw, an isomeric one. In the above- named series of hydro- carbons, differing from each other only in the multiples in which the elements are united, we find isomerism becoming what is distinguished as polyrnerism.

10 THE DATA OF BIOLOGY.

The like is still more conspicuous in other groups of thy hydro-carbons, as in the essential oils : sixteen to twenty of which are severally isomeric with essential oil of turpentine. Here the particular kind of molecular mobility implied by these metamorphoses, is well shown : essential oil of turpen- tine being converted into a mixture of several of these poly- merides, by simple exposure to a heat of 460°.

There is one further fact respecting these binary compounds of the four chief organic elements, which must not be over- looked. Those of them which form parts of the living tissues of plants and animals (excluding water which has a me- chanical function, and carbonic acid which is a product of decomposition) are confined to one group — the hydro-carbons. And of this group, which is on the average characterized by comparative instability and inertness, these hydro-carbons found in living tissues, are among the most unstable and inert.

§ 3. Passing now to the substances which contain three of these chief organic elements, we have first to note that along with the greater atomic weight which mostly accom- panies their increased complexity, there is, on the average, a farther marked decrease of molecular mobility. Scarcely any of them maintain a gaseous state at ordinary temperatures. One class of them only, the alcohols and their derivatives, evaporate under the usual atmospheric pressure ; but not rapidly unless heated. The fixed oils, though they show that molecular mobility implied l>y an habitually liquid state, show this in a lower degree than the alcoholic compounds ; and they cannot be reduced to the gaseous state without de- composition. In their allies, the fats, which are solid unless heated, the loss of molecular mobility is still more marked. And throughout the whole series of the fatty acids, in which to a fixed proportion of oxygen there are successively added higher equimultiples of carbon and hydrogen, we see how the molecular mobility decreases with the increasing sizes of

ORGANIC MATTER. 1 1

the atoms. In the amylaceous and saccharine group of com- pounds, solidity is the habitual state : such of them as can assume the liquid form, doing so only when heated to 300° or 400° F. ; and decomposing when further heated, rather than become gaseous. Resins and gums exhibit general physical properties of like character and meaning.

Iii chemical stability these ternary compounds, considered as a group, are in a marked degree below the binary ones. The various sugars and kindred bodies, decompose at no very high temperatures. The oils and fats are also readily carbon- ized by heat. Resinous and gummy substances are easily made to render up some of their constituents. And the alcohols with their allies, have no great power of resisting decomposition. These bodies, formed by the union of

oxygen, hydrogen and carbon, are also, as a class, chemically inactive. The formic and acetic are doubtless energetic acids ; but the higher members of the fatty-acid series are easily separated from the bases with which they combine. Saccharic acid, too, is an acid of considerable power ; and sundry of the vegetal acids possess a certain activity, though an activity far less than that of the mineral acids. But throughout the rest of the group, there is shown but a small tendency to combine with other bodies ; and such com binations as are formed have usually little permanence.

The phenomena of isomerism and polymerisin are of fre- quent occurrence in these ternary compounds. Starch and dextrine are isomeric. Fruit sugar, starch sugar, eucalyn, sorbin, and inosite, are polymeric. Sundry of the vegetal acids exhibit similar modifications. And among the resins and gums, with their derivatives, molecular re-arrangements of this kind are not uncommon.

One further fact respecting these compounds of carbon, oxygen and hydrogen, should be mentioned ; namely, that they are divisible into two classes — the one consisting of SUD- stances that result from the destructive decomposition of organic matter, and the other consisting of substances that

12 THE DATA OF BIOLOGY.

exist as such in organic matter. These two classes of sub- stances exhibit in different degrees, the properties to which we have been directing our attention. The lower alcohols, their allies and derivatives, which possess greater molecular mobility and chemical stability than the rest of these ternary compounds, are not found in animal or vegetal bodies. While the sugars and amylaceous substances, the fixed oils and fats, the gums and resins, which have all of them much less mole- cular mobility, and are, chemically considered, more unstable and inert, are components of the living tissues of plants and animals.

§ 4. Among compounds containing all the four chief organic elements, a division analogous to that just named may be made. There are some which result from the decom- position of living tissues; there are others which make parts of living tissues in their state of integrity ; and these two groups are contrasted in their properties in the same way as are the parallel groups of ternary compounds.

Of the first division, certain products found in the animal excretions are the most important, and the only ones that need be noted ; such, namely, as urea, kreatine, kreatinine. These animal bases exhibit much less molecular mobility than the average of the substances treated of in the last section : being solid at ordinary temperatures, fusing, where fusible at all, at temperatures above that of boiling water, and having no power to assume a gaseous state. Chemically considered, their stability is low, and their activity but small, in com- parison with the stabilities and activities of the simpler com- pounds.

It is, however, the nitrogenous constituents of living tis- sues, that display most markedly, those characteristics of which we have been tracing the growth. Albumen, fibrin, casein, and their allies, are bodies in wrhich that molecular mobility exhibited by three of their components in so high a degree, is reduced to a minimum. These substances are known onlv

ORGANIC MATTER. Ii3

in the solid state : that is to say, when deprived of the water usually mixed witli them, they do not admit of fusion, much less of volatilization. To which add, that they have not even that molecular mobility which solution in water implies ; since, though they form viscid mixtures with water, they do not dissolve in the same perfect way as do inorganic com- pounds. The chemical characteristics of these sub- stances, are instability and inertness carried to the extreme. How rapidly albumenoid matters decompose under ordinary conditions, is daily seen : the difficulty of every house-wife being to prevent them from decomposing. It is true that when desiccated and kept from contact with air, they may be preserved unchanged for a long period ; but the fact that they can only be thus preserved, proves their great instability. It is true, also, that these most complex nitrogenous principles are not absolutely inert ; since they enter into combinations with some bases ; but their unions are very feeble.

It should be noted, too, of these bodies, that though they exhibit in the lowest degree that kind of molecular mobility, which implies facile vibration of the atoms as wholes, they ex- hibit in a high degree that kind of molecular mobility resulting in isomerism, which implies permanent changes in the posi- tions of adjacent atoms with respect to each other. Each of them has a soluble and insoluble form. In some cases there are indications of more than two such forms. And it appears that their metamorphoses take place under very slight changes of conditions.

In these most unstable and inert organic compounds, we find that the atomic complexity reaches a maximum : not only since the four chief organic elements are here united with small proportions of sulphur and phosphorus ; but also since they are united in high multiples. The peculiarity which we found characterized even binary compounds of the organic elements, that their atoms are formed not of single equivalents of each component, but of two, three, four and more equivalents, is carried to the greatest extreme in these

11 THE DATA OF BIOLOGY.

compounds, that take the leading part in organic actions, According to Mulder, the formula of albumen is 10 (C10 H31 NJ O12) -|- S2 P. That is to say, with the sulphur and phos- phorus there are united ten equivalents of a compound atom containing forty atoms of carbon, thirty-one of hydrogen, five of nitrogen, and twelve of oxygen : the atom being thug made up of nearly nine hundred ultimate atoms.

§ 5. Did space permit, it would be useful here to consider in detail, the interpretations that may be given of the pecu- liarities we have been tracing : bringing to their solution, those general mechanical principles which are now found to hold true of molecules as of masses. But it must suffice briefly to indicate the conclusions that such an inquiry pro- mises to brin £ out.

O

Proceeding on mechanical principles, it may be argued that the molecular mobility of a substance must depend partly on the inertia of its molecules ; partly on the intensity of their mutual polarities ; partly on their mutual pressure, as deter- mined by the density of their aggregation, and (where the molecules are compound) partly on the molecular mobilities of their component molecules. Whence it is to be inferred that any three of these remaining constant, the molecular mobility will vary as the fourth. Other things equal, there- fore, the molecular mobility of atoms must decrease as their masses increase ; and so there must result that general pro- gression we have traced, from the high molecular mobility of the uncombined organic elements, to the low molecular mobility of those large-atomed substances into which they are ultimately compounded.

Applying to atoms the mechanical law which holds o! masses, that since inertia and gravity increase as the cubes of the dimensions while cohesion increases as their squares, the self-sustaining power of a body becomes relatively smaller as its bulk becomes greater ; it might be argued that these large, aggregate atoms which constitute organic sub-

ORGANIC MATTER. 15

stance, are mechanically weak — are less able than simpler atoms to bear, without alteration, the forces falling on them. That very massiveness which renders them less mobile, enables the physical forces acting on them more readily to change the relative positions of their component atoms ; and so to pro- duce what we know as re-arrangements and decompositions. Further, it seems a not improbable conclusion, that this formation of large aggregates of elementary atoms, and re- sulting diminution of self-sustaining power, must be accom- panied by a decrease of those contrasts of dimension to which polarity is ascribable. A sphere is the figure of equi- librium which any aggregate of units tends to assume, under the influence of simple mutual attraction. Where the num- ber of units is small and their mutual polarities are decided, this proclivity towards spherical grouping will be overcome by the tendency towards some more special form, determined by their mutual polarities. But it is manifest that in pro- portion as an aggregate atom becomes larger, the effects of simple mutual attraction must become relatively greater ; and so must tend to mask the effects of polar attraction. There will consequently be apt to result in highly com- pound atoms like these organic ones containing nine hun- dred elementary atoms, such approximation to the spherical form as must involve a less distinct polarity than in simpler atoms. If this inference be correct, it supplies us with an ex- planation both of the chemical inertness of these most com- plex organic substances, and of their inability to crystallize.

§ 6. Here we are naturally introduced to another aspect of our subject — an aspect of great interest. Professor Graham has recently published a series of important researches, which promise to throw much light on the constitution and changes of organic matter. He shows that solid substances exist un- der two forms of aggregation — the colloid or jelly-like, and the crystalloid or crystal-like. Examples of the last are too fa- miliar to need specifying. Of the first may be named such

THE DATA OF BIOLOC1

instances as "hydrated silicic acid, hydrated alumina, and other metallic peroxides of the aluminous class, when they exist in the soluble form ; with starch, dextrine and the gums, cara- mel, tannin, albumen, gelatine, vegetable and animal extractive matters." Describing the properties of colloids, Professor Graham says : — " Although often largely soluble in water, they are held in solution by a most feeble force. They ap- pear singularly inert in the capacity of acids and bases, and in all the ordinary chemical relations." * * * "Al- though chemically inert in the ordinary sense, colloids possess a compensating activity of their own arising out oi their physical properties. While the rigidity of the crystal- line structure shuts out external impressions, the softness oi the gelatinous colloid partakes of fluidity, and enables the colloid to become a medium of liquid diffusion, like water itself." * * * " Hence a wide sensibility on the part oi colloids to external agents. Another and eminently charac- teristic quality of colloids is their mutability." * * * " The solution of hydrated silicic acid, for instance, is easily obtain- ed in a state of purity, but it cannot be preserved. It may remain fluid for days or weeks in a sealed tube, but is sure to gelatinize and become insoluble at last. Nor does the change of this colloid appear to stop at that point ; for the mineral forms of silicic acid, deposited from water, such as flint, are often found to have passed, during the geological ages of their existence, from the vitreous or colloidal into the crystal- line condition (H. Rose). The colloid is, in fact, a dynami- cal state of matter, the crystalloidal being the statical condition. The colloid possesses energia. It may be looked upon as the primary source of the force appearing in the phenomena of vitality. To the gradual manner in which colloidal changes take place (for they always demand time as an element) may the characteristic protraction of chernico- organic changes also be referred."

The class of colloids includes not only all those most com- plex nitrogeneous compounds characteristic of organic tissue.

ORGANIC MATTER. 17

and sundry of the oxy-hydro- carbons found along with them ; but, significantly enough, it includes several of those sub- stances classed as inorganic, which enter into organized structures. Thus silica, which is a component of many plants, and constitutes the spicules of sponges as well as the shells of many fbraminifera and infusoria, has a colloid, as well as a crystalloid, condition. A solution of hydrated silicic acid, passes in the course of a few days into a solid jelly that is no longer soluble in water ; and it may be suddenly thus coagulated by a minute portion of an alkaline carbonate, as well as by gelatine, alumina, and peroxide of iron. This last- named substance, too — peroxide of iron — which is an ingre- dient in the blood of mammals and composes the shells of certain protozoa, has a colloid condition. " Water containing about one per cent, of hydrated peroxide of iron in solution, has the dark .red colour of venous blood." " The

red solution is coagulated in the cold by traces of sulphuric acid, alkalies, alkaline carbonates, sulphates, and neutral salts in general." * * * " The coagulum is a deep red-coloured jelly, resembling the clot of blood but more transparent. Indeed, the coagulum of this colloid is highly suggestive of that of blood, from the feeble agencies which suffice to effect the change in question, as well as from the appearance of the product." The jelly thus formed soon becomes, like the last, insoluble in water. Lime also, which is so important a mineral element in living bodies, animal and vegetal, enters into a compound belonging to this class. " The \vell-knownsolution of lime in sugar, forms a solid coagulum when heated. It is probably, at a high temperature, entirely colloidal."

Generalizing some of the facts which he gives, Professor Graham says — " The equivalent of a colloid appears to be always high, although the ratio between the elements of the substance may be simple. Gummic acid, for instance, may be represented by C12 II" O11 ; but, judging from the small

proportions of lime and potash which suffice to neutralize this 2

18 THE DATA OF BIOLOGY.

acid, the true numbers of its formula must be several times greater. It is difficult to avoid associating- the inertness of colloids with their high equivalents, particularly where the high number appears to be attained by the repetition of a small number. The inquiry suggests itself whether the col- loid molecule may not be constituted by the grouping together of a number of smaller crystalloid molecules, and whether the basis of colloidality may not really be this com. posite character of the molecule.*'

<j 7. A further contrast between colloids and crystalloids, is equally significant in its relations to vital phenomena. Professor Graham points out that the marked differences in volatility displayed by different bodies, are paralleled by differences in the rates of diffusion of different bodies through liquids. As alcohol and ether at ordinary temperatures, and various other substances at higher temperatures, diffuse them- selves in a gaseous form through the air ; so, a substance in aqueous solution, when placed in contact with a mass of water (in such way as to avoid mixture by circulating currents) diffuses itself through this mass of water. And just as there are various degrees of rapidity in evaporation, so there are various degrees of rapidity in diffusion : " the ra-nge also in the degree of diffusive mobility exhibited by different sub- stances appears to be as wide as the scale of vapour-tensions.'1 This parallelism is what might have been looked for ; since the tendency to assume a gaseous state, and the tendency to spread in solution through a liquid, are both consequences of molecular mobility. It also turns out, as was to be expected, that diffusibility, like volatility, has, other things equal, a re- lation to atomic weight — (other things equal, we must say, because molecular mobility must, as pointed out in § 5, be affected by other properties of atoms, besides their inertia). Thus the substance most rapidly diffused of any on which Professor Graham experimented, was hydro-chloric acid-^a '•ompouTid which is of low atomic weight, is gaseous save

ORGANIC MATTER. 19

under a pressure of forty atmospheres, and ordinarily exists as a liquid, only in combination with water. Again, " hydrate of potash may be said to possess double the velocity of diffu- sion of sulphate of potash, and sulphate of potash again double the velocity of sugar, alcohol, and sulphate of magnesia," — differences which have a general correspondence with differ- ences in the massiveness of the atoms.

But the fact of chief interest to us here, is that the rela- tively small-atomed crystalloids have immensely greater diffusive power than the relatively large-atomed colloids. Among the crystalloids themselves, there are marked differ ences of diffusibility ; and among the colloids themselves, there are parallel differences, though less marked ones. But these differences are small compared with that between the diffusibility of the crystalloids as a class, and the diffusibility of the colloids as a class. Hydro- chloric acid is seven times as diffusible as sulphate of magnesia ; but it is fifty times as diffusible as albumen, and a hundred times as diffusible as caramel.

These differences of diffusibility manifest themselves with nearly equal distinctness, when a permeable septum is placed between the solution and the water. And the result is, that when a solution contains substances of different diffusibilities, the process of dialysis, as Professor Graham calls it, becomes a means of separating the mixed substances : especially when such mixed substances are partly crystalloids and partly col- loids. The bearing of this fact on organic processes will be obvious. Still more obvious will its bearing: be, on

O '

joining it with the remarkable fact, that while crystalloids can diffuse themselves through colloids nearly as rapidly as through water, colloids can scarcelv diffuse themselves at all

*— ' v

through other colloids. From a mass of jelly containing salt, into an adjoining mass of jelly containing no salt, the salt spread more in eight days than it spread through water i i seven days ; while the spread of " caramel through the jelly appeared scarcely to have begun after eight days had

20 THE DATA OF BIOLOGY.

elapsed." So that we must regard the colloidal compounds of which organisms are built, as having by their physical nature, the ability to separate colloids from crystalloids, and to let the crystalloids pass through them with, scarcely any resistance.

One other result of these researches on the relative diffu- eibilities of different substances, has a meaning for us. Pro- fessor Graham finds, that not only does there take place by dialysis, a separation of mixed substances which are unlike in their molecular mobilities ; but also that combined substances between which the affinity is feeble, will separate on the dialyzer, if their molecular mobilities are strongly con- trasted. Speaking of the hydro-chlorate of peroxide of iron, he says, " such a compound possesses an element of instability in the extremely unequal diifusibility of its constituents ; ' and he points out that when dialyzed, the hydro-chloric acid gradually diffuses away, leaving the colloidal peroxide of iron behind. Similarly, he remarks of the peracetate of iron, that it " may be made a source of soluble peroxide, as the salt referred to is itself decomposed to a great extent by diffusion on the dialyzer." Now this tendency to separate displayed by substances that differ widely in their molecular mobilities, though usually so far antagonized by their affinities as not to produce sponta- neous decomposition, must, in all cases, induce a certain readiness to change which would not else exist. The un- equal mobilities of the combined atoms, must give disturbing forces a greater power to work transformations than they would otherwise have. Hence the probable significance of a fact named at the outset, that while three of the chief organic elements have the greatest atomic mobilities of any elements known, the fourth, carbon, has the least atomic mobility of known elements. Though, in its simple compounds, the affinities of carbon for the rest are strong enough to prevent the effects of this great difference from clearly showing them- selvea ; yet there seems reason to think, that in those com-

ORGANIC MATTER. 21

plex compounds composing organic bodies — compounds in which there are various cross affinities lead-ing to a state of chemical tension — this extreme difference in the molecular mobilities must be an important aid to molecular re-arrange- ments. In short, we are here led by concrete evidence to the conclusion which we before drew from first principles, that this great unlikeness among the combined units must facili- tate differentiations.

| 8. A portion of organic matter in a state to exhibit those phenomena which the biologist deals with, is, however, something far more complex than the separate organic mat- ters we have been studying ; since a portion of organic matter in its integrity, contains several of these.

In the first place, no one of those colloids which make up the mass of a living body, appears capable of carrying on vital changes by itself: it is always associated with other colloids. A portion of animal-tissue, however minute, almost always contains more than one form of protein-substance : different chemical modifications of albumen and gelatine are present together, as well as, probably, a soluble and insoluble modification of each ; and there is usually more or less of fatty matter. In a single vegetal cell, the minute quantity of nitrogenous colloid present, is imbedded in colloids of the non-nitrogenous class. The microscope makes it at once manifest, that even the smallest and simplest organic forms are not absolutely homogeneous.

Further, we have to contemplate organic tissue, formed of mingled colloids in both soluble and insoluble states, as permeated throughout by crystalloids. Some of these crys- Ulloids, as oxygen,* water, and perhaps certain salts, are agents of decomposition ; some, as the saccharine and fatty

* It will perhaps seem strange to class oxygen as a crystalloid. But inasmuch as the crystalloids are distinguished from the colloids by their atomic simplicity, and inasmuch as sundry gases are reducible to a crystalline state, we are justified in so classing it.

22 THE DATA OF BIOLOGY.

matters, are probably materials for decomposition ; and some, as carbonic acid, water, urea, kreatine, and kreatinine, are products of decomposition. Into the mass of mingled colloids, mostly insoluble and where soluble of very low molecular mobility or diffusive power, we have constantly passing, crys- talloids of high molecular mobility or diffusive power, that are capable of decomposing these complex colloids ; and from these complex colloids, so decomposed, there result other crystalloids (the two chief ones extremely simple and mobile, and the rest comparatively so) which diffuse away as rapidly as they are formed.

And now we may clearly see the necessity for that pecu- liar composition which we find in organic matter. On the one hand, were it not for the extreme molecular mobility possessed by three of its chief elements out of the four ; and were it not for the consequently high molecular mobility of their simpler compounds ; there could not be this quick escape of the waste products of organic action ; and there could not be that continuously active change of matter which vitality implies. On the other hand, were it not for the union of these extremely mobile elements into immensely complex compounds, having relatively vast atoms that are made com- paratively immobile by their inertia, there could not result that mechanical fixity which prevents the components of liv- ing tissue from diffusing away along with the effete matters produced by the decomposition of tissue.

§ 9. Thus in the substances of which organisms are composed, the conditions necessary to that re- distribution of Matter and Motion which constitutes Evolution, are fulfilled in a far higher degree than at first appears.

The mutual affinities of the chief organic elements are not active within the limits of those temperatures at which organic actions take place ; and one of these elements ia especially characterized by its chemical indifference. The compounds formed by these elements in ascending grades of

ORGANIC MATTER.

complexity, become progressively less stable. And those most complex compounds into which all these four elements enter, together y>rith small proportions of two other elements that very readily oxidize, have an instability so great that decomposition ensues under ordinary atmospheric conditions. Among these elements out of which living bodies are built, there is an unusual tendency to unite in multiples ; and so to form groups of products which have the same chemical com- ponents, but, being different in their modes of aggregation, possess different properties. This prevalence among them of isomerism and polymerism, shows, in another way, the special fitness of organic substances for undergoing re-distributions.

In those most complex compounds that are instrumental to vital actions, there exists a kind and degree of molecular mobility which constitutes the plastic quality fitting them for organization. Instead of the extreme molecular mobility possessed by three out of the four organic elements in their separate states — instead of the diminished, but still great, molecular mobility possessed by their simpler combinations, the gaseous and liquid characters of which unfit them for showing to any extent the process of Evolution — instead of the properties of their less simple combinations, which, when not made unduly mobile by heat, assume the unduly rigid form of crystals ; we have in these colloids, of which organisms are mainly composed, just the required com- promise between fluidity and solidity. They cannot be re- duced to the unduly mobile conditions of liquid and gas ; and yet they do not assume the unduly fixed condition usually cha- racterizing solids. The absence of power to unite together in polar arrangement, leaves their atoms with a certain freedom of relative movement which makes them, sensitive to small forces, and produces plasticity in the aggregates composed of them.

While the relatively great inertia of these large and com- plex organic atoms, renders them comparatively incapable of being set in motion by the ethereal undulations, and so re-

24 THE DATA OF BIOLOGY.

duced to less coherent forms of aggregation : there is reason

o o o *

to think that this same inertia facilitates changes of arrange- ment among their constituent atoms ; since, in proportion as an incident force impresses but little motion on a mass, it is the better able to impress motion on the parts of the mass in relation to each other. And it is further probable that the extreme contrasts in molecular mobilities among the compo- nents of these nighly complex atoms, aid in producing modi- fiability of arrangement among them.

Lastly, the great difference in diffusibility between colloids and crystalloids, makes possible in the tissues of organisms, a specially rapid re-distribution of matter and motion ; both because colloids, being easily permeable by crystalloids, can be chemically acted on throughout their whole mass, in- stead of only on thair surfaces ; and because the products of decomposition, being also crystalloids, can escape as fast as they are produced, leaving room for farther like transforma- tions. So that while the composite atoms of which organic tissues are built up, possess that low molecular mobility fit- ting them for plastic purposes, it results from the extreme molecular mobilities of their ultimate constituents, that the waste products of vital activity escape as fast as they are formed.

To all which add, that the state of warmth, or increased molecular vibration, in which all the higher organisms are

* O o

kept, increases these various facilities for re-distribution : not only as aiding chemical changes, but as accelerating the dif- fusion of crystalloid substances.

CHAPTER II.

THE ACTIONS OF FORCES ON ORGANIC MATTER.

•

P 10. To some extent, the parts of every body are changed in their arrangement by any incident mechanical force. But in organic bodies, the changes of arrangement produced by mechanical forces are usually conspicuous. It is a dis- tinctive mark of colloids, that they yield with great readiness to pressures and tensions ; and that they yet recover, more or less completely, their original shapes, when the pres- sures or tensions cease. It is clear that without this pliability and elasticity, most organic actions would be im- possible. Not only temporary but permanent alter- ations of form are facilitated \>y this colloid character of organic matter. Continued pressure on living tissue, by modifying the processes going on in it, (perhaps retarding the absorption of new material to replace the old that has decomposed and diffused away,) gradually diminishes an«i finally destroys its power of resuming the outline it had at first. Thus the matter of which organisms are built up, is modi liable by arrested momentum or by continuous strain, in a far greater degree than is ordinary matter.

§ 11. Sensitiveness to certain forces that are quasi- mechanical, if not mechanical in the usual sense, is seen in two closely-related peculiarities displayed by organic matter

26 THE DATA OF BIOLOGY.

as well as other mutter that acsumes the same state of mole» cular a Correlation.

oo o

Colloids take up by a power that has been called " capillary affinity," a large quantity of water : undergoing at the same time great increase of bulk with change of form. Conversely, with like readiness, they give up this water by evaporation : resuming more or less completely their original states. Whether resulting from capillarity, or from the relatively great diffusibility of water, or from both ; these changes are to be here noted as showing another mode in which the arrangement of parts in organic bodies, is affected by mechanical forces.

In what is called osmose, we have a further mode of allied kind. When on opposite sides of a permeable septum, and especially a septum of colloidal substance, are placed miscible solutions of different densities, a double transfer takes place : a large quantity of the less dense solution finds its way through the septum into the more dense solution ; and a small quan- tity of the more dense finds its way into the less dense — one result being a considerable increase in the bulk of the more dense at the expense of 'the less dense. This process, which appears to depend on several conditions, is not yet fully un- derstood. But be the explanation what it may, the process is one that tends continually to work alterations in organic bodies. Through the surfaces of plants and animals, transfers of this kind are ever taking place. Very many of the con- spicuous changes of form undergone by organic germs, are due mainly to the permeation of their limiting membranes by the surrounding liquids.

It should be added that besides the direct alterations which *,he imbibition and transmission of water and watery solutions by colloids produce in organic matter, they produce indirect alterations. Being instrumental in conveying into the tissues the agents of chemical change, and conveying out of them the products of chemical change, they aid in carrying oa other re- distributions.

THE ACTIONS OF FORCES ON ORGANIC MATTER. 27

§ 12. As elsewhere shown (First Principles, § 103) Heat, or a raised state of molecular vibration, enables incident forces more easily to produce changes of molecular arrangement in organic matter. But besides this, it conduces to certain vital changes in so direct a way as to become their chief cause.

The power of the organic colloids to imbibe water, and to bring along with it into their substance the materials which work transformations, would not be continuously operative if the water imbibed were to remain. It is because it escapes, and is replaced by more containing more materials, that the succession of changes is maintained. Among the higher

o o o

animals and higher plants its escape is facilitated by evapor- ation. And the rate of evaporation is, other things equal, determined by heat. Though the current of sap in

a tree is mainly caused by some action, probably osmotic, that is at work in the roots ; yet the loss of water from the surfaces of the leaves, and the consequent absorption of more sap into the leaves by capillary attraction, must largely aid the circulation. The drooping of a plant when exposed to the sunshine while the earth round its roots is dry, shows us how evaporation empties the sap-vessels ; and the quickness with which a withered slip revives on being placed in water, shows us the part which capillary action plays. In so far then, as the evaporation from a plant's sur- face helps to produce currents of sap through the plant, we must regard the heat which produces this evaporation as a part-cause of those re-distributions of matter which these currents effect. In terrestrial animals, heat

similarly aids the changes that are going on. The exha- lation of vapour from the lungs and the surface of the skin, forming the chief escape of the water that is swallowed, conduces to the maintenance of those currents through the tissues, without which the functions would cease. For though the vascular system distributes nutritive fluids in

*/

ramified channels through the body ; yet the absorption of these fluids into tissues, partly depends on the escape of fluids

28 THE DATA OF BIOLOGY.

which the tissues already contain. Hence, to the extent that such escape is facilitated by evaporation, and this evaporation facilitated by heat, heat becomes an agent of re-distribution in the animal organism.

§ 13. Light, which is now known to modify many inor- ganic compounds — which works those chemical changes utilized in photography, causes the combinations of certain gases, alters the molecular arrangements of many crystals, and leaves traces of its action even on substances that are ex- tremely stable, — may be expected to produce marked effects on substances so complex and unstable as those which make up organic bodies. It does produce such marked effects ; and some of them are among the most important that organic matter undergoes.

The molecular changes wrought bv li°;ht in animals, are

o o «/

but of secondary moment. There is the darkening of the skin that follows exposure to the sun's rays. There are those alterations in the retina which cause in us sensations of colours. And on certain eyeless creatures that are semi- transparent, the light permeating their substance works some effect evinced by movement. But speaking generally, the opacity of animals limits the action of light to their surfaces ; and so renders its direct physiological influence but small.* On plants, however, the solar rays that

produce in us the impression of yellow, are the immediate agents of those molecular changes through which are hourly accumulated the materials for further growth. Experiments have shown that when the sun shines on living leaves, they begin to exhale oxygen and to accumulate carbon and hydrogen — results which are traced to the decomposition by the solar rays, of the carbonic acid and water absorbed. It is now an accepted conclusion that, by the help of certain

* The increase of respiration found to result from the presence of light, is prohably an indirect effect. It is most likely due to the reception of more vivid impressions through the eyes, and to the consequent nervous stimulation.

HE ACTIONS OF FORCES ON ORGANIC MATTER. 29

classes of the ethereal undulations penetrating their leaves, plants are enabled to separate from the associated oxygen, those t\vo elements of which their tissues are chiefly built up.

This transformation of ethereal undulations into certain molecular re- arrangements of an unstable kind, on the over- throw of which the stored-up forces are liberated in new forms, is a process that underlies all organic phenomena. It will therefore be well, if we pause a moment to consider whe- ther any proximate interpretation of it is possible. Certain recent researches in molecular physics, give us some clue to its nature.

The elements of the problem are these : — The atoms of several ponderable matters exist in combination : those that are combined having strong affinities, but having also affin- ities less strong for some of the surrounding atoms that are otherwise combined. The atoms thus united, and thus mixed among others with which they are capable of uniting, are exposed to the undulations of a medium that is relatively so rare as to seem imponderable. These undulations are of numerous kinds : they differ greatly in their lengths, or in the frequency with which they recur at any given point. And under the influence of undulations of a certain frequency, some of these atoms are transferred from atoms for which they have a stronger affinity, to atoms for which they have a weaker affinity. That is to say, particular orders of waves of a rela- tively imponderable matter, remove particular atoms of pon- derable matter from their attachments, and carry them within reach of other attachments: Now the discoveries of

Bunsen and Kirchoff respecting the absorption of particular luminiferous undulations by the vapours of particular sub- stances, joined with Prof. TyndalPs discoveries respecting the absorption of heat by gases, show very clearly that the atoms of each substance have a rate of vibration in harmony with ethereal waves of a certain length, or rapidity of recur- rence. Every special kind of atom can be made to oscillate

30 THE DATA OF BIOLOGY.

by a special order of ethereal waves, which are absorbed in producing its oscillations ; and can by its oscillations generate this same order of ethereal waves. Whence it appears that immense as is the difference in density between ether and ponderable matter, the waves of the one can set the atoms of the other in motion, when the successive impacts of the waves are so timed as to correspond with the oscillations of the atoms. The effects of the waves are, in such case, cumula- tive ; and each atom gradually acquires a momentum made up of countless infinitesimal momenta. Note further,

that unless the members of a chemically- compound atom are so bound up as to be incapable of any relative movements (a supposition at variance with the conceptions of modern science) we must conceive them as severally able to vibrate in unison or harmony with those same classes of ethereal waves that affect them in their uncombined states. While the compound atom as a whole, will have some new rate of oscillation de- termined by its attributes as a whole ; its components will retain their original rates of oscillation, subject only to modifi- cations by mutual influence. Such being the cir- cumstances of the case, we may partially understand how the sun's rays can effect chemical decompositions. If the members of a binary atom stand so related to the undulations falling on them, that one is thrown into a state of increased oscillation and the other not ; it is manifest that there must arise a tendency towards the dislocation of the two — a tendency which may or may not take effect, according to the weakness or strength of their union, and according to the presence or absence of collateral affinities. This inference is in harmony with several significant facts. Dr Draper remarks that " among metallic substances (compounds) those first detected to be changed by light, such as silver, gold, mercury, lead, have all high atomic weights ; and such as sodium and potassium, the atomic weights of which are low, appeared to be less changeable." As here interpreted, the fact specified amounts to this ; that the compounds most

THE ACTIONS OF FORCES OX ORGANIC MATTER. 8\

readily decomposed by light, are those in which thero is a marked contrast between the atomic weights of the constituents, and probably therefore a marked contrast between the rapidities of their vibrations. The circumstance, too, that different chemical compounds are decomposed or modified in different parts of the spectrum, implies that there is a relation between special orders of undulations and special orders of composite atoms — doubtless a correspondence between the rates of these undulations and the rates of oscillation which some of the components of such atoms will assume. Strong confirmation of this view may

be drawn from the decomposing actions of those longer ethereal waves which we perceive as heat. On contemplating the whole series of binary compounds, we see that the ele- ments which are most remote in their atomic weights, as hydrogen and the noble metals, will not combine at all : their

•/ c_?

vibrations are so unlike that they cannot keep together under any conditions of temperature. If again we look at a smaller group, as the metallic oxides, we see that whereas those metals that have atoms nearest in weight to the atoms

o

of oxygen, cannot be separated from oxygen by heat; even when it is joined by a powerful collateral affinity ; those metals which differ more widely from oxygen in their atomic weights, can be de-oxidized by carbon at high temperatures ; and those which differ from it most widely, combine with it very reluctantly, and yield it up if exposed to thermal undu- lations of moderate intensity. And here indeed, remem- bering the relations among the atomic weights in the two cases, may we not suspect a close analogy between the de- oxidation of a metallic oxide by carbon under the influence of the longer ethereal waves, and the de-carbonization of carbonic acid by hydrogen under the influence of the shorter ethereal waves ?

These conceptions help us to some dim notion of the mode in which changes are wrought by light in the leaves of plants. Among the several elements concerned, there are wide differ-

THE DATA OF T5IOLOGY.

ences in molecular mobility, and probably in the rates of molecular vibration. Each is combined with one of the others ; but is capable of forming various combinations with the rest. And they are severally in presence of a complex compound into which they all enter, and which is ready to assimilate with itself the new compound atoms that they form. Certain of the ethereal waves falling on them when thus arranged.

O *

there results a detachment of some of the combined atoms and a union of the rest. And the conclusion suggested is, that the induced vibrations among the various atoms as at first arranged, are so incongruous as to produce instability ; and to give collateral affinities the power to work a re- arrangement, which, though less stable under other conditions, is more stable in the presence of these particular undula- tions. There seems, indeed, no choice but to conceive the matter thus. An atom united with one for which it has a strong affinity, has to be transferred to another for which it has a weaker affinity. This transfer implies motion. The motion is given by the waves of a medium that is relatively imponderable. K"o one wave of this imponderable medium can give the requisite motion to this atom of ponderable matter : especially as the atom is held by a positive force besides its inertia. The motion required can hence be given only by successive waves ; and that these may not destroy each other's effects, it is needful that each shall strike the atom just when it has completed that recoil produced by the impact of previous ones. That is, the ethereal undulations must coincide in rate with the oscillations of the atom, determined by its inertia and the forces acting on it. It is also requisite that the rate of oscillation of the atom to be detached, shall differ from that of the atom with which it is united ; since if the two oscillated in unison, the ethereal waves would not tend to separate them. And, finally, the successive impacts of the ethereal waves must be accumulated, until the resulting oscillations have become so wide in their sweep as greatly to weaken the cohesion of the united atoms, at the same time

THE ACTIONS OF FORCES ON ORGANIC MATTER. 33

^

thai they brino: one of them within reach of other atoms with

*

which it will combine. In this way only does it seem possible for such a force to produce such a transfer. More-

over, while we are thus enabled to conceive how light may work these molecular changes ; we also gain an insight into the method by which the insensible motions propagated to us from the sun, are treasured up in such way as afterwards to generate sensible motions. By the accumulation of in- finitesimal impacts, atoms of ponderable matter are made to oscillate. The quantity of motion which each of them eventually acquires, effects its transfer to a position of un- stable equilibrium, from which it can afterwards be readily dislodged. And when so dislodged, along with other atoms similarly and simultaneously affected, there is suddenly given out all the motion which had been before impressed on it.

Speculation aside, however, that which it concerns us to notice, is the broad fact that light is an all-important agent of molecular changes in organic substances. It is not here necessary for us to ascertain how light produces these compo- sitions and decompositions : it is necessary only for us to observe that it docs produce them. That the characteristic matter called chlorophyll, which gives the green colour to leaves, makes its appearance whenever the blanched shoots of plants are exposed to the sun ; that the petals of flowers, uncoloured while in the bud, acquire their bright tints as they unfold ; and that on the outer surfaces of animals, analogous changes are induced ; are wide inductions which are enough for our present purpose.

14. We come next to the agency of chief importance among those that work changes in organic matter ; namely, chemical affinity. How readily vegetal and animal substances are modified by other substances put in contact with them, we see daily illustrated. Besides the many compounds which cause the death of an organism into which they are put, wo have the much greater number of compounds which work

THE DATA OF BIOLOGY.

those milder effects termed medicinal — effects implying, liko the others, molecular re-arrangements. Indeed, nearly all soluble chemical compounds, natural and artificial, produce, when taken into the body, alterations that are more or less conspicuous in their results.

After what was shown in the last chapter, it will be mani- fest that this extreme modinability of organic matter by chemical agencies, is the chief cause of that active molecular re-arrangement which organisms, and especially animal or- ganisms, display. In the two fundamental functions of nutrition and respiration, we have the means by which the supply of materials for this active molecular re-arrangement is maintained.

Thus the process of animal nutrition consists in the absorp- tion, partly of those complex substances that are thus highly capable of being chemically altered, and partly in the absorp tion of simpler substances capable of chemically altering them. The tissues always contain small quantities of alka- line and earthy salts, which enter the system in one form

»' v

and are excreted in another. Though we do not know spe- cifically the parts which these salts play, yet from their universal presence, and from the transformations which they undergo in the body, it may be safely inferred that their chemical affinities are instrumental in working some of the metamorphoses ever going on.

The inorganic substance, however, on which mainly depend these metamorphoses in organic matter, is not swallowed along with the solid and liquid food, but is absorbed from the surrounding medium — air or water, as the case may be.

• '

Whether «the oxygen taken in, either, as by the lowest animals, through the general surface, or, as by the higher animals, through respiratory organs, is the immediate cause of those molecular changes that are ever going on through- out the living tissues ; or whether the oxygen, playing the part of scavenger, merely aids these changes by carrying away the products of decompositions otherwise caused ; if

THE ACTIONS OF FORCES ON ORGANIC MATTER.

equally remains true, that these changes are maintained by its instrumentality. Whether the oxygen absorbed and diffused through the system, effects a direct oxidation of the organic colloids which it permeates ; or whether it first leads to the formation of simpler and more oxidized compound?, that are afterwards further oxidized and reduced to still simpler forms ; matters not, in so far as the general result is concerned. In any case it holds good, that the substances of which the animal body is built up, enter it in a but slightly oxidized and highly unstable state ; while the great mass of them leave it in a fully oxidized and stable state. It follows, therefore, that whatever the special changes gone through, the general process is a falling from a state of un- stable chemical equilibrium, to a state of stable chemical equilibrium. Whether this process be direct or indirect, the total molecular re-arrangement and the total motion given out in effecting it, must be the same.

§ 15. There is another species of re-distribution among the component units of organisms, which is not immediately effected by th'e affinities of the units concerned, but is me-

f

diately effected by other affinities ; and there is reason to think that the re-distribution thus caused, is important in amount, if not indeed the most important. In ordinary cases of chemical action, the two or more substances concerned, themselves undergo changes of molecular arrangement ; and the changes are confined to the substances themselves. But there are other cases in which the chemical action going on, does not end with the substances at first concerned ; but seta going chemical actions, or changes of molecular arrangement, among surrounding substances that would else remain qui- escent. And there are yet further cases in which merj contact with a substance that is itself quiescent, will cause other substances to undergo rapid metamorphoses. In

what we call fermentation, the first species of this communi- cated chemical action is exemplified. One part of yeast,

38

THE DATA OF BIOLOGY.

while itself undergoing molecular changes, will convert 100 parts of sugar into alcohol and carbonic acid ; and during its own decomposition, one part of diastase " is able to effect the transformation of more than 1000 times its weight of starch into sugar." As illustrations of the second species,

may be mentioned those changes which are suddenly produced in many colloids by minute portions of various substances added to them — substances that are not undergoing any manifest transformation, and suffer no appreciable effect from the contact. The nature of the first of these two

kinds of communicated molecular change, which here chiefly concerns us, may be rudely represented by certain visible changes that are communicated from mass to mass, when a series of masses has been arranged in a special way. The simplest example is that furnished by the child's play of setting bricks on end in a row, in such positions that when the first is overthrown it overthrows the second ; the second, the third ; the third, the fourth ; and so on to the end of tho row. Here we have a number of units severally placed in unstable equilibrium, and in such relative positions that each, while falling into a state of stable equilibrium, gives an im- pulse to the next, sufficient to make the next, also, fall from unstable to stable equilibrium. Now since among mingled compound atoms, no one can undergo change in the arrange- ment of its parts without a molecular motion that must cause some disturbance all around ; and since an adjacent atom disturbed by this communicated motion, may have the arrange- ment of its constituent molecules altered, if it is not a stable arrangement ; and since we know, both that the atoms which are changed by this so-called catalysis are unstable, and that the atoms resulting from their change are more stable ; it seems probable that the transformation is really analogous, in principle, to the familiar one named. Whether thug interpretable or not, however, there is great reason for think- ing that to this kind of action, is due a large amount of vitid

THE ACTIONS OF FORCES ON ORGANIC MATTER. 37

metamorphosis. Let us contemplate the several groups of facts which point to this conclusion.

In the last chapter (§ 2) we incidentally noted the extreme instability of nitrogenous compounds in general. AYe saw that sundry of them are liable to explode on the slightest incentive — sometimes without any apparent cause ; and that of the rest, the great majority are very easily decomposed by heat, and by other substances. We shall perceive much significance in this general characteristic, when we join it with the fact, that the substances capable of initiating extensive

' A. O

molecular changes in the manner above described, are all nitrogenous ones. Yeast consists of vegetal cells containing

O O *~s

nitrogen, — cells that grow by assimilating the nitrogenous matter contained in wort. Similarly, the " vinegar-plant," which so greatly facilitates the formation of acetic acid from alcohol, is a fungoid growth, that is doubtless, like others of its class, rich in nitrogenous compounds. Diastase, by which the transformation of starch into sugar is effected, during the process of malting, is also a nitrogenous body. So too is a substance called synaptase — an albumenous principle contained in almonds, that has the power of working several metamorphoses in the matters associated with it. These nitrogenized compounds, like the rest of their family, are remarkable for the rapidity with which they decompose ; and the extensive changes produced by them in the accompanying oxy-hydro-carbons, are found to vary in their kinds accord- ing as the decompositions of the ferments vary in their stages. We have next to note, as having here a

meaning for as, the chemical contrasts between those organ- isms which carry on their functions by the help of external forces, and those which carry on their functions by forces evolved from within. If we compare animals and plants, we see that whereas plants, characterized as a class by containing but little nitrogen, are dependent on the solar rays for their vital activities ; animals, the vital activities of which are not

38 THE DATA OF BIOLOGY.

thus dependent, mainly consist of nitrogenous substances, There is one marked exception to this broad distinction, how- ever ; and this exception is specially instructive. Among plants, there is a considerable group — the Fungi — many mem- bers of "which, if not all, can live and grow in the dark ; and it is their peculiarity that they are very much more nitro- genous than other plants. Yet a third class of facts of like significance, is disclosed when we compare different portions of the same organisms. The seed of a plant contains nitrogenous substance in a far higher ratio than the rest of

o o

the plant ; and the seed differs from the rest of the plant in its ability to initiate, in the absence of light, extensive vital changes — the changes constituting germination. Similarly in the bodies of animals, those parts which carry on active functions are nitrogenous ; while parts that are non-nitro- genous— as the deposits of fat — carry on no active functions. And we even find that the appearance of non-nitrogenous matter, throughout tissues normally composed almost wholly of nitrogenous matter, is accompanied by loss of activity : what is called fatty degeneration, being the concomitant of failing vitality. One more fact which serves to make

Btill clearer the meaning of the foregoing ones, still remains—- the fact, namely, that in no part of any organism where vital changes are going on, is nitrogenous matter wholly absent. It is common to speak of plants — or at least all parts of plants but the seeds — as non-nitrogenous. But they are only relatively so ; not absolutely. The quantity of albumenoid substance contained in the tissues of plants, is extremely small compared with the quantity contained in the tissues of ani- mals ; but all plant-tissues which are discharging active functions, contain some albumenoid substance. In everv

V

living vegetal cell there is a certain part that contains nitro- gen. This part initiates those changes which constitute the development of the cell. And if it cannot be said that the primordial utricle, as this nitrogenous part is called, is the worker of all subsequent changes undergone by the cell, it

THE ACTIONS OF FORCES ON ORGANIC MATTER. 39

nevertheless continues to be the part in which the independeni activity is most marked.

Looking at the evidence thus brought together, do we not get an insight into the part played by nitrogenous matter in organic changes ? We see that nitrogenous com- i > >imcls in general, are extremely prone to decompose : their decomposition often involving a sudden and great evolution of force. We see that the substances classed as ferments, which, during their own molecular changes, set up molecular changes in the accompanying oxy-hydro- carbons, are all nitrogenous. We see that among classes of organisms, and among the parts of each organism, there is a relation between the amount of nitrogenous matter present and the amount of independent activity. And we see that even in organisms and parts of organisms where the activity is least, such changes as do take place are initiated by a substance contain- ing nitrogen. Does it not seem probable, then, that these extremely unstable compounds, have everywhere the effect of communicating to the less unstable compounds associated with them, molecular movements towards a stable state, like those they are themselves undergoing ? The changes which we thus suppose nitrogenous matter to produce in a body, are clearly analogous to those which we see it produce out of the body. Out of the body, certain oxy-h}'dro-carbons in con- tinued contact with nitrogenous matter, are transformed into carbonic acid and alcohol, and unless prevented the alcohol is transformed into acetic acid : the substances formed being thus more highly oxidized and more stable than the substances destroyed. In the body, these same oxy-hydro-carbons together with some hydro-carbons, in continued contact with nitrogenous matter, are transformed into carbonic acid and water : substances which are also more highly oxidized and more stable than those from which they result. And since acetic acid is itself resolved by further oxidation into carbonio ncid and water ; we see that the chief difference between the t,"o cases, is, that the process is more completely effected in tka

40 THE DATA OF BIOLOGY.

body, than it is out of the body.* Thus, to carry further the simile used above, the atoms of hydro-carbons and oxy-hydro- carbons contained in the tissues, are, like bricks on end, not in the stablest equilibrium, but still in an equilibrium so stable, that they cannot be overthrown by the chemical and thermal forces which the body brings to bear on them. On the other hand, being like similarly-placed bricks that have very nar- row ends, the nitrogenous atoms contained in the tissues are in so unstable an equilibrium that they cannot withstand these forces. And when these delicately-poised nitrogenous atoms fall into stable arrangements, they give impulses to the more firmly-poised non-nitrogenous atoms, which cause them also to fall into stable arrangements. It is a

curious and significant fact, that in the arts, we not only utilize this same principle of initiating extensive changes among comparatively stable compounds, by the help of com- pounds much less stable ; but we employ for the purpose compounds of the same general class. Our modern method of firing a gun, is to place in close proximity with the gun- powder which we wish to decompose or explode, a small por- . tion of fulminating powder, which is decomposed or exploded with extreme facility ; and which, on decomposing, communi- cates the consequent molecular disturbance to the less-easily decomposed gunpowder. When we ask what this fulminating powder is composed of, we find that it is a nitrogenous salt.

Thus various evidences point to the conclusion, that besides the molecular re-arrangements produced in organic matter by direct chemical action, there are others of kindred importance produced by indirect chemical action. Indeed, the inference

* May it not be well to inquire whether alcohol is not, in a greater or less measure, transformed in the body into acetic acid ? If, when in contact with changing nitrogenous matter, in presence of oxygen, alcohol undergoes this transformation out of the body, it seems not improbable that it does so in the body — especially as the raised temperature which aids the change in the one case exists HI the other. It would be out of place here to set down the sundry facts which countenance this hypothesis. I may say, however, that it apparently removta some of the difficulties which at present perplex the question.

THE ACTIONS OF FORCES ON ORGANIC MATTER. 41

that some of the leading transformations occurring in the animal organism, are due to this so-called catalysis, appears necessitated by the general aspect of the facts ; apart from any such detailed interpretations as the foregoing. We know that various amylaceous and saccharine matters taken as food, are decomposed in their course through the body. We know that these matters do not become components of the tissues, but only of the fluids circulating through them ; and that thus their metamorphosis is not an immediate result of the organic activities. We know that their stability is such that the thermal and chemical forces to which they are exposed in the body, cannot alone decompose them. The only explan- ation open to us, therefore, is that the transformation of these oxy-hydro- carbons, into carbonic acid and water, is due to communicated chemical action.

§ 16. This chapter will havn served its purpose if it has given a conception of the extreme modifiability of organic matter by surrounding agencies. Even did space permit, it would be needless to describe in detail the immensely varied and complicated changes which the forces from mo- ment to moment acting on them, work in living bodies. Dealing with biology in its general principles, it concerns us only to notice how specially sensitive are the substances of which organisms are built up, to the varied influences that act upon organisms. And their special sensitiveness has been made sufficiently manifest, in the several foregoing ap

CKAPTEE III.

THE RE-ACTIONS OF ORGANIC MATTER ON FORCES.

^ 17. HE-DISTRIBUTIONS of Matter, imply concomitant re- distributions of Motion. That which under one of its aspects we contemplate as an alteration of arrangement among the parts of a body, is, under a correlative aspect, an alteration of arrangement among certain momenta whereby these parts are impelled to their new positions. At the same time that a force, acting differently on the different units of an aggre- gate, changes their relations to each other; these units, re- acting differently on the different parts of the force, work equivalent changes in the relations of these to one another. Inseparably connected as they are, these two orders of phe- nomena are liable to be confounded together. It is very needi'al, however, to distinguish between them. In the last chapter, we took a rapid survey of the re-distributions which forces produce in organic matter ; and here we must take a like survey of the simultaneous re- distributions undergone by '•he forces.

At the outset we are met by a difficulty. The parts of an inorganic mass undergoing re-arrangement by an incident force, are, in most cases, passive — do not complicate those necessary re-actions that result from their inertia, by other forces which they originate, But in organic matter, the re-arranged parts do not re-act in virtue of their inertia only : they are so constituted that ths incident force usually sets up

THE RE-ACTIONS OF ORGANIC MATTER ON FORCES. 43

in them, other actions which are much more important. Indeed, what we may call the indirect re-actions thus caused, are so great in their amounts compared with the direct re- actions, that they quite obscure them.

In strictness, these two kinds of re-action should not be dealt with together. The impossibility of separating them, however, compels us to disregard the distinction between •them. Under the above general title, we must include both the immediate re-actions and those re-actions mediately produced, which are among the most conspicuous of vital phenomena.

§ 18. From organic matter, as from all other matter, incident forces call forth that re- action which we know as heat. ]\Iore or less of molecular vibration almost necessarily results, when, to the forces at work among the molecules of any aggregate, other forces are added. Experiment abundantly demonstrates this in the case of inorganic masses ; and it must equally hold in the caso of organic masses. In both cases the force which, more mark-

edly than any other, produces this thermal re-action, is that which causes the union of different substances with each, other. Though inanimate bodies admit of being greatly heated by pressure and by the electric current, yet the evolutions of heat thus induced, are neither so common, nor in most cases so conspicuous, as those resulting from chemical combination. And though in animate bodies, there are doubtless certain amounts of heat generated by other actions ; yet these are all secondary to the heat generated by the action of oxygen on the substances composing the tissues and the substances contained in them. Here, however,

we see one of the characteristic distinctions between inani- mate and animate bodies. Among the first, there are but few which ordinarily exist in a condition to evolve the heat caused by chemical combination ; and such as are in this condition soon cease to be so, when chemical combination

THE DATA OF BIOLOGY.

and genesis of heat once begin in them. Whereas among the second, there universally exists the ability, more or less decided, thus to evolve heat ; and the evolution of heat, in some cases very slight and in no cases very great, continues as long as they remain animate bodies.

The relation between active change of matter and re-active genesis of atomic vibration, is clearly shown by the contrasts between different organisms, and between different states and parts of the same organism. In plants, the genesis of heat is extremely small, in correspondence with their extremely small production of carbonic acid : those portions only, as flowers and germinating seeds, in which considerable oxidation is going on, having a decidedly raised temperature. Among animals, we see that the hot-blooded are those which expend much force and respire actively. We see that though such creatures as insects are scarcely at all warmer than the surround- ing air when they are still, they rise several degrees above it when they exert themselves ; and that in creatures like our- selves, which habitually maintain a heat much greater than that of their medium, exercise is accompanied by an ad- ditional production of heat, often to an inconvenient extent.

This molecular agitation accompanying the molecular re-arrangements that are caused by oxygen taken into the animal organism, must result both from the union of oxygen •with those nitrogenous matters of which the tissues are composed, and from its union with those non-nitrogenous matters which are diffused through the tissues. Just as much heat as would be caused by the oxidation of such matters out of the body, must be caused by their oxidation in the body. In the one case as in the other, the heat must be re- garded as a concomitant. Whether the distinction made by Liebig between nitrogenous substances as tissue- food, and non-nitrogenous substances as heat-food, be true or not in a narrower sense, it cannot be accepted in the sense that tissue-food is not also heat-food. Indeed he does not himself assert it in this sense. The ability of carnivorous

THE RE- ACTIONS OF ORGANIC MATTER ON FORCES. 45

animals to live and generate heat while consuming matter that is almost exclusively nitrogenous, to say nothing of the con- stant relation above shown between functional activity and the evolution of heat, suffices to prove that the nitrogenous com- pounds forming the tissues are heat-producers, as well as the non-nitrogenous compounds circulating among and through the tissues. But it is possible that this antithesis is not

true even in the more restricted sense. It seems quite an admissible hypothesis that the hydro-carbons and oxy-hydro- carbons which, in traversing the system, are transformed by communicated chemical action, evolve during their transform- ation, not heat alone, but also other kinds of force. It may be that as the nitrogenous matter, while falling into more stable molecular arrangements, generates both that molecular agi- tation called heat, and such other molecular movements as are resolved into forces expended by the organism ; so, too, does the non-nitrogenous matter. Or perhaps the concomitants of this metamorphosis of non-nitrogenous matter, vary with the conditions. Heat alone may result when it is transformed while in the circulating fluids , but partly heat, and partly another force, when it is transformed in some active tissue that has absorbed it : just as coal, though producing little else but heat as ordinarily burnt, has its heat partially transformed into mechanical motion if burnt in a steam-engine furnace. In such case, the antithesis of Liebig would be reduced to this ; • — that whereas nitrogenous substance is tissue-food both as material for building-up tissue and as material for its function; non-nitrogenous substance is tissue-food only as material fo? function.

There can be no doubt that this thermal re-action which chemical action from moment to moment produces in the body, is from moment to moment an aid to further chemical action. We before saw (First Principles, § 103) that a state of raised molecular vibration, is favourable to those re-dis- tributions of matter and motion which constitute Evolution. We saw that in organisms distinguished by the amount and

46 THE DATA OF BIOLOGY.

rapidity of such, re- distributions, this raised state of molecula? vibration is conspicuous. And we here see that this raised state of molecular vibration, is itself a continuous consequence of the continuous molecular re-distributions it facilitates. The heat generated by each increment of chemical change, makes possible the succeeding increment of chemical change. In. the body this connexion of phenomena is the same as we see it to be out of the body. Just as in a burning piece of wood, the heat given out by the portion actually combining with oxygen, raises the adjacent portion to a temperature at which it also can combine with oxygen ; so, in a living animal, the heat produced by oxidation of each portion of tissue, maintains the temperature at which the unoxidized portions can be readily oxidized.

§ 19. Among the forces called forth from organisms by re-action against the actions to which they are subject, is Light. Phosphorescence is in some few cases displayed by plants — especially by certain fungi. Among animals it is comparatively common. All know that there are several kinds of luminous insects ; and many are familiar with the fact that luminosity is a characteristic of various marine creatures.

Most of the evidence 'goes to show that this evohirion of light, as well as the evolution of heat, is consequent on oxi- dation of the tissues. Light, like heat, is the expression of a raised state of molecular vibration : the difference between them being a difference in the rates of vibration. Hence by chemical action on substances contained in the organism, heat or light may be produced, according to the character of the resulting molecular vibrations. The inference -that

oxidation is the cause of this luminosity, does not, however, rest only on a priori grounds. It is supported by experi- mental evidence. In phosphorescent insects, the continuance of the light is found to depend on the continuance of respira- tion ; and any exertion which renders respiration more active,

THE RE-ACTIOXS OF ORGANIC MATTER ON FORCES. 4?

increases the brilliancy of the light. Moreover, by separating the luminous matter, Prof. Matteucci has shown that its emission of light is accompanied by absorption of oxygen and escape of carbonic acid. The phosphorescence

of marine animals has been referred to other causes than oxidation. In some cases, however, it is, I think, explicable without assuming any more special agency. Considering that in creatures of the genus Noctiluca^ for example, to which the phosphorescence most commonly seen on our own coasts is due, there is no means of keeping up a constant circulation, we may infer that the movements of aerated fluids through their tissues, must be greatly affected by impulses received from without. Hence it may be that the sparkles visible at night when the waves break gently on the beach, or when an oar is dipped into the water, are called forth from these creatures by the concussion, not because of any unknown influence it excites, but because, being propagated through their delicate tissues, it produces a sudden movement of the fluids and a sudden increase of chemical action. Neverthe- less, in other phosphorescent animals inhabiting the sea, as in the Pyrosoma and in certain Annelida, light seems to be really produced, not by direct re-action on the action of oxygen, but by some indirect re- action involving a trans- formation of force.

§ 20. The re-distributions of matter in general, are accom- panied by electrical disturbances ; and there is abundant evidence that electricity is generated during those re-distri- butions that are ever taking place in organisms. Experi- ments have shown " that the skin and most of the internal membranes are in opposite electrical states ; ' and also that between different internal organs, as the liver and the stomach, there are electrical contrasts — such contrasts being greatest where the processes going on in the compared parts are most unlike. It has been proved by M. du Bois-Keymond that when any point in the longitudinal section of a muscle ia

THE DATA OF BIOLOGY.

connected by a conductor with any point in its transverse section, an electric current is established ; and further, that like results occur when nerves are substituted for muscles. The special causes of these phenomena have not yet been determined. Considering that the electric contrasts are most marked where active secretions are going on — considering, too, that while they do not exist between external parts which are similarly related to the vascular currents, they do exist between external parts which are dissimilarly related to the vascular currents — and considering also that they are extremely difficult to detect where there are no appre- ciable movements of fluids ; it may be that they are due simply to the friction of heterogeneous substances, which is universally a cause of electric disturbance. But whatever be the interpretation, the fact remains the same, that there is hroughout the living organism, an unceasing production of differences between the electric states of different parts ; and consequently an unceasing restoration of electric equilibrium by the establishment of currents among these parts.

Besides these general, and not conspicuous, electrical phe- nomena which appear to be common to all organisms, vegetal as well as animal, there are certain special and strongly marked ones. I refer, of course, to those which have made the Torpedo and the Gymnotiis objects of so much interest. In these creatures we have a genesis of electricity that is not incidental on the performance of their different functions by the different organs ; but one which is itself a function, having an organ appropriate to it. The character of this organ in both these fishes, and its largely- developed con- nexions with the nervous centres, have raised the suspicion, which various experiments have thus far justified, that in it there takes place a transformation of what we call nerve- force into the force known as electricity : this conclusion being more especially supported by the fact, that substances, such as morphia and strychnia, which are known to be powerful

THE RE- ACTIONS OF ORGANIC MATTER ON FORCES. 49

nervous stimulants, greatly increase the violence and rapidity of the electric discharges.

But whether general or special, and in whatever manner produced, these evolutions of electricity are among the re-actions of organic matter, called forth by the actions to which it is subject. Though these re-actions are not direct, bat seem rather to be remote consequences of those changes wrought by external agencies on the organism, they are yet incidents in that general re-distribution of motion, which these external agencies initiate ; and as such must here be noticed.

§ 21. To these known modes of motion, has next to bo added an unknown one. Heat, Light, and Electricity are emitted by inorganic matter when undergoing changes, as well as by organic matter. But there is a kind of force mani- fested in some classes of living bodies, which we cannot identify with any of the forces manifested by bodies that are not alive, — a force which is thus unknown, in the sense that it cannot be assimilated with any otherwise-recognized class. I allude to what is called nerve-force.

This is habitually generated in all animals, save the lowest, by incident forces of every kind. The gentle and violent mechanical contacts, which in ourselves produce sensations of touch and pressure — the additions and abstractions of mole- cular vibration, which in ourselves produce sensations of heat and cold; produce in all creatures that have nervous systems, certain nervous disturbances — disturbances which, as in ourselves, are either communicated to the chief nervous centre, and there constitute consciousness, or else result in merely ph}^sical processes that are set going elsewhere in the organism. In special parts distinguished as organs of sense, other external actions bring about other nervous re-actions ; that show themselves either as special sensations, or as ex- citements which, without the intermediation of consciousness,

50 THE DATA OF BIOLOGY.

beget actions in muscles or other organs. BcsMeo

neural discharges that follow the direct incidence of external forces, there are others ever being caused .by the incidence of forces which, though originally external, have become internal by absorption into the organism of the agents exerting them. For thus may be classed those neural discharges that from moment to moment result from modifications of the tissues, wrought by substances carried to them in the blood. That the unceasing: change of matter which oxygen and other

o * o

agents produce throughout the system, is accompanied by a genesis of nerve-force, is shown by various facts ; — by the fact that nerve-force is no longer generated, if oxygen be with- held, or the blood prevented from circulating ; by the fact that when the chemical transformation is diminished, as during sleep with its slow respiration and circulation, there is a diminution in the quantity of nerve-force ; in the fact that an excessive expenditure of nerve-force, involves excessive re- spiration and circulation, and excessive waste of tissue. To these proofs that nerve-force is evolved in greater or less quan- tity, according as the conditions to rapid molecular change throughout the body, are well or ill fulfilled ; may be added proofs that certain special molecular actions, are the causes of these special re-actions. The effects of alcohol, ether, chloroform, and the vegeto-alkalies, put beyond doubt the inference, that the overthrow of molecular equilibrium by chemical affinity, when it occurs at certain places in the body, results in the overthrow of equilibrium in the nerves pro- ceeding from, these places — results, that is, in the propagation through these nerves, of the change called a nervous dis- charge. Indeed, looked at from this point of view, the two classes of nervous changes — the one initiated from without and the other from within — are seen to merge into one class. Both of them may be traced to metamorphosis of tissue. Thwe can be little doubt that the sensations of touch and pressure, are consequent on accelerated changes of matter, produced by mechanical disturbance of the mingled

THE HE-ACTIONS OF ORGANIC MATTER ON FORCES. 51

duids and solids composing the parts affected. There is abundant evidence that the sensation of taste, is due to the chemical actions set up by particles which find their way through the membrane covering the nerves of taste ; for, as Prof. Graham points out, sapid substances all belong to the class of crystalloids, which are able rapidly to permeate animal tissue, while colloids, which cannot pass through animal tissue, are all insipid. Similarly with the sense of smell. Substances which excite this sense, are necessarily more or less volatile : and their volatility being the result of their molecular mobility, implies that they have in a high degree, the power of getting at the olfactory nerves by pene- trating their mucous investment. Asrain, the facts which

O O '

photography has familiarized us with, make it clear that those nervous impressions called colours, are primarily due to certain changes wrought by light in the substance of the retina. And though, in the case of hearing, we cannot so clearly trace the connexion of cause and effect ; yet as we see that the auditory apparatus is one fitted to intensify those vibrations constituting sound, and to convey them to a recep- tacle containing fluid in which nerves are immersed ; it can scarcely be doubted that the sensation of sound proximately results from atomic re-arrange rnents caused in these nerves by the vibrations of the fluid : knowing, as we do, that the re-arrangement of atoms is in all cases aided by agita- tion. Perhaps, however, the best proof that nerve- force, whether peripheral or central in its origin, results from chemical transformation, lies in the fact that most of the chemical agents which powerfully affect the nervous system, affect it whether applied at the centre or the periphery. Vari- ous acids, mineral and vegetal, are tonics — the stronger ones being usually the stronger tonics ; and this which we call their acidity, implies a power in them of acting on the nerves of taste, while the tingling or pain that follows their absorp- tion through the skin, implies that the nerves of touch are acted on by them. Similarly with certain vog-to-alkaHes

THE DATA OF BIOLOGY.

which are peculiarly bitter. These by their bitterness, show that they affect the extremities of the nerves ; -while by their tonic properties, they show that they affect the nervous centres — the most intensely bitter among them, strychnia, being the most powerful nervous stimulant. However true it may be that this relation is not a regular one, since opium, hashish, and some other drugs, which work marked effects on the brain, are not remarkably sapid — however true it may be that there are relations between particular substances and particular parts of the nervous system ; yet such instances •do but qualify, without negativing, the general proposition. The truth of this proposition can scarcely be doubted when, to the evidence above given, is added the fact that various condiments and aromatic drugs are given as nervous stimu- lants ; and the fact that anaesthetics, besides the general effects they produce when inhaled or swallowed, produce local effects of like kind when absorbed through the skin ; and the fact that ammonia, which inconsequence of its extreme molecular mobility, so quickly and so violently -excites the nerves be- neath the skin, as well as those of the tongue and the nose, is a rapidly-acting stimulant when taken internally.

Whether we shall ever know anything more of this nerve-

f

force, than that it is some species of molecular disturbance that is propagated from end to end of a nerve, it is impossi- ble to say. Whether a nerve is merely a conductor, which delivers at one of its extremities an impulse received at the other ; or whether, as some now think, it is itself a generator of force which is initiated at one extremity and accumulates in its course to the other extremity ; are also questions which cannot yet be answered. All we know is, that forces capable of working molecular changes in nerves, are capable of calling forth from them manifestations of activity — dis- charges of some force, which, though probably allied to elcc* tricity, is not identical with it. And our evidence that nerve- force is thus originated, consists not only of such facts as the above, but also of more conclusive facts established by direct

THE RE- ACTIONS OF ORGANIC MATTER ON FORCES. 53

experiments on nerves — experiments which show that nerve- force is generated when the cut end of a nerve is either me- chanically irritated, or acted on by some chemical agent, or subject to the galvanic current — experiments which thus prove that nerve-force is liberated by whatever disturbs the molecular equilibrium of nerve-substance. And this is all which it is necessary for us here to understand.

§ 22. The most important of these re-actions called forth from organisms by surrounding actions, remains to be noticed. To the above various forms of insensible motion thus caused, we have to add sensible motion. On the production of this mode of force, more especially depends the possibility of all vital phenomena. It is, indeed, usual to regard the power of generating sensible motion, as confined to one out of the two organic sub-kingdoms ; or, at any rate, as possessed by but few members of the other. On looking closer into the matter, however, we see that plant-life as well as animal-life, is uni- versally accompanied by certain manifestations of this power ; and that plant-life could not otherwise continue.

Through the humblest, as well as through the highest, ve- getal organisms, there are ever going on certain re-distribu- tions of matter. In protophytes the microscope shows us an internal transposition of parts, which when not active enough to be immediately visible, is proved to exist by the changes of arrangement that become manifest in the course of hours and days. In the individual cells of many higher plants, an active movement among the contained granules may be wit- nessed. And well-developed cryptogams in common with all phanerogams, exhibit this genesis of mechanical motion still more conspicuously in the circulation of sap. It might, in- deed, be concluded a priori, that through plants displaying much differentiation of parts, an internal movement must be going on ; since, without it, the mutual dependence of organs having unlike functions would seem impossible. Be-

sides these motions of fluids kept up internally, plants, espo-

154 THE DATA OF BIOLOGY.

cially of the lower orders, are able to move their external parts in relation to each other, and also to move about from place to place. Illustrations in abundance will occur to all students of recent Natural History — such illustrations as the active locomotion of the zoospores of many Algae, the rhyth- mical bendings of the Oscillatorice, the rambling progression of the DiatomacecB. In fact many of these smallest vegetals, and many of the larger ones in their early stages, display a mechanical activity not distinguishable from that of the simplest animals. Among well- organized plants, which are never locomotive in their adult states, we still not unfre- quently meet with relative motions of parts. To such fami- liar cases as those of the Sensitive plant and the Venus* fly-trap, many others may be added. When its base is irritated, the stamen of the Berberry flower leans over and touches the pistil. If the stamens of the common wild Cistus be gently brushed with the finger, they spread themselves — bending away from the seed-vessel. And some of the orchid- flowers, as Mr Darwin has recently shown, shoot out masses of pollen on to the entering bee, when its trunk is thrust down in search of honey.

Though the power of moving is not, as we see, a character- istic of animals alone, yet in them, considered as a class, it ia manifested to an extent so marked, as practically to become one of their distinctive characters — indeed, we may say, their most distinctive character. For it is by their immensely greater ability to generate mechanical motion, that animals are enabled to perform those actions which constitute their visible lives ; and it is by their immensely greater ability to generate mechanical motion, that the higher orders of animals are most obviously distinguished from the lower orders. Though, on remembering the seemingly active movements of infusoria, some will perhaps question this last-named con- trast ; yet, on comparing the quantities of matter propelled through given spaces in given times, they will see that the momentum evolved is far less in the protozoa than in the

THE RE-ACTIONS OF ORGANIC MATTER ON FORCES. 55

leleozoa. These sensible motions of animals are effected

by various organs under various stimuli. In the humblest forms, and even in. some of the more developed ones which inhabit the water, locomotion results from the vibrations of cilia : the contractility resides in these waving hairs that grow from the surface. Some of the AcalephtB, and their allies the Polypes, move when mechanically irritated : the long pendant tentacle of a Physalia is suddenly drawn up if touched ; and, as well as its tentacles, the whole body of a Hydra collapses if roughly handled, or jarred by some shock in its neighbourhood. In all the higher animals how- ever, and to a smaller degree in many of the lower, sensible motion is generated by a special tissue, under the special ex- citement of a neural discharge. Though it is not strictly true that such animals shoi^ no sensible motions otherwise caused ; since all of them have certain ciliated membranes, and since the circulation of fluid in them is partially due to osmotic and capillary actions ; yet, generally speaking, we may say that their movements are effected only by muscles that contract only through the agency of nerves.

What special transformations of force generate these various mechanical changes, we do not, in most cases, know. Those re-distributions of fluid, with the alterations of form sometimes caused by them, that result from osmose, are not, indeed, quite incomprehensible. Certain motions of plants which, like those of the " animated oat," follow contact with water, are easily interpreted ; as are also such other vegetal motions as those of the Touch-me-not, the Squirting Cucumber, and the Carpobolus. But we have as yet no clue to the mode in which molecular movement is transformed into the movement of masses, in animals. "We cannot refer to known causes the rhythmical action of a Medusa's disc, or that slow decrease of bulk that spreads throughout the mass of an Alcyomum, when one of its component individuals has been irritated. ~Nor are we any better able to say how the insensible motion transmitted through a nerve, crives rise to sensible motion in

56 THE DATA OF BIOLOGY.

a muscle. It is true that Science lias given to Art, several methods of changing insensible into sensible motion. By ap- plying heat to water we vaporize it ; and the movement of its expanding vapour, we transfer to solid matter ; but it is clear that the genesis of muscular movement is in no way analogous to this. The force evolved during chemical transformations in a galvanic battery, we communicate to a soft iron magnet through a wire coiled round it ; and it would be quite possi- ble, by placing near to each other several magnets thus excited, to obtain, through the attraction of each for its neighbours, an accumulated movement made up of their separate movements, and thus to mechanically imitate a mus- cular contraction ; but from what wre know of organic mat- ter, and the structure of muscle, there is no reason to suppose that anything analogous to this takes place in it. We

can, however, through one kind of molecular change, produce sensible changes of aggregation such as possibly might, when occurring in organic substance, cause sensible motion in it : I refer to allotropic change. Sulphur, for example, as- sumes different crystalline and non-crystalline forms at dif- ferent temperatures ; and may be made to pass backwards and forwards from one form to another, by slight variations of temperature : undergoing each time an alteration of bulk. We know that this allotropism, or rather its analogue iso- nierism, prevails among colloids — inorganic and organic. We also know that some of these metamorphoses among-col- loids, are accompanied by visible re- arrangements : instance hydrated silicic acid, which, after passing from its soluble state to the state of an insoluble jelly, begins, in a few days, to contract, and to give out part of its contained water. Now, considering that such isoineric changes of organic as well as inorganic colloids, are often very rapidly produced by very slio-ht causes, it seems not impossible that some of the colloids constituting muscle, may be thus changed by a nervous dis- char^e — resuming their previous condition when the dis- charge ceases. And it is conceivable that by structural

THE RE- ACTIONS OF ORGANIC MAITlvR ON FORCES. 57

arrangements, minute sensible motions so caused, may be ac- cumulated into large sensible motions. There is, however, no evidence to support this supposition.

§ 23. But the truths which it is here our business espe- cially to note, are quite independent of hypotheses or inter- pretations. It is sufficient for the ends we have in view, to observe that organic matter does exhibit these several conspi- cuous re- actions, when acted on by incident forces : it is not requisite that we should know how these re-actions originate.

In the last chapter were set forth the several modes in which incident forces cause re -distributions of organic mat- ter ; and in this chapter have been set forth the several modes in which is manifested the motion accompanying this re-dis- tribution. There we contemplated under its several aspects, the general fact, that in consequence of its extreme instability, organic matter undergoes extensive molecular re-arrange- ments, on very slight changes of conditions. And here we have contemplated under its several aspects, the correlative general fact, that during these extensive molecular re-arrange- ments, there are necessarily evolved large amounts of force. In the one case the atoms of which organic matter consists, are regarded as changing from positions of unstable equili- brium to positions of stable equilibrium ; and in the other case they are regarded as giving out in their falls from unstable to stable equilibrium, certain momenta — momenta that may be manifested as heat, light, electricity, nerve- force or mechanical motion, according as the conditions determine.

I will add only that these evolutions of force are rigor- ously dependent on these changes of matter. It is a corol- lary from that primordial truth which, as we have seen, underlies all other truths, (First Principles, §§ 76, 141,) that whatever amount of power an organism expends in any shape, is the correlate and equivalent of a power that was taken into it from without. On the one hand, it

68 .THE DATA OF BIOLOGY.

follows from the persistence of force, that each portion of mechanical or other energy which an organism exerts, im- plies the transformation of as much organic matter as con- tained this energy in a latent state. And on the other hand, it follows from the persistence of force that no such trans- formation of organic matter containing this latent energy can take place, without the energy being in one shape or other manifested.

CHAPTER IT *

PROXIMATE DEFINITION OF LIFE.

§ 24. To those who accept the general doctrine of Evolu- tion, it needs scarcely be pointed out that classifications are subjective conceptions, which have no absolute demarcations in Nature corresponding to them. They are appliances by which we limit and arrange the matters under investigation ; and so facilitate our thinking. Consequently, when we at- tempt to define anything complex, or make a generalization of facts other than the most simple, we can scarcely ever avoid including more than we intended, or leaving out some- thing that should be taken in. Thus it happens that on seeking a definition of Life, we have great difficulty in find- ing one that is neither more nor less than sufficient. Let us look at a few of the most tenable definitions that have been given. "\Yhile recognizing the respects in which they are defective, we shall see what requirements a more com- plete one must fulfil.

* This chapter and the following two chapters originally appeared in Part III. of the Principles of Psychology : forming a preliminary which, though indis- pensable to the argument there developed, was somewhat parenthetical. Having now to deal with the general science of Biology before the more special one of Psychology, it becomes possible to transfer these chapters to their proper place. The} have been carefully revised.

THE DATA OF BIOLOGY.

Schelling said tliat Life is the tendency to individuation. This formula, until studied, conveys little meaning1. But it

+> O

needs only to consider it as illustrated by the facts of develop- ment, or by the contrasts between lower and higher forms of life, to recognize its value ; especially in respect of compre- hensiveness. As before shown, however, (First Principles, § 56), it is objectionable, partly on the ground that it refers, not so much to the functional changes constituting Life, as to the structural changes of those aggregations of matter which manifest Life; and partly on the ground that it includes under the idea Life, much that we usually exclude from it : for instance — crystallization.

The definition of Pticherand, — " Life is a collection of phenomena which succeed each other during a limited timo in an organized body," — is liable to the fatal criticism, that it equally applies to the decay which goes on after death. For this, too, is " a collection of phenomena which succeed each other during a limited time in an organized body."

" Life," according to De Blaiiiville, " is the two-fold internal movement of composition and decomposition, at once general and continuous." This conception is in some re- spects too narrow, and in other respects too wide. On the one hand, while it expresses what physiologists distinguish as vegetative life, it excludes those nervous and muscular functions which form the most conspicuous and distinctive classes of vital phenomena. On the other hand, it describes not only the integrating and disintegrating processes going on in a living body, but it equally well describes those going on in a galvanic battery ; which also exhibits a " two-fold in- ternal movement of composition and decomposition, at once general and continuous."

Elsewhere, I have myself proposed to define Life as " the co-ordination of actions ; "* and I still incline towards this de- finition as one answering to the facts with tolerable precision.

* See Westminster Review for April, 1852.— Art. IV. "A Theory of Popu- fction."

PROXIMATE DEFINITION OF LIFE. 61

It includes all organic changes, alike of the viscera, the limbs, and the brain. It excludes the great mass of inor- ganic changes ; which displa}^ little or no co-ordination. By making co-ordination the specific characteristic of vitality, it involves the truths, that an arrest of co-ordination is death, and that imperfect co-ordination is disease. More- over, it harmonizes with our ordinary ideas of life in its dif- ferent gradations : seeing that the organisms which we rank as low in their degree of life, are those which display but little co-ordination of actions ; and seeing that from these up to man, the recognized increase in degree of life corresponds with an increase in the extent and complexity of co-ordina- tion. But, like the others, this definition includes too much ; for it may be said of the Solar System, with its regularly- recurring movements and its self-balancing perturbations, that it, also, exhibits co-ordination of actions. And how- ever plausibly it may be argued that, in the abstract, the motions of the planets and satellites are as properly compre- hended in the idea of life, as the changes going on in a motionless, unsensitive seed ; yet, it must be admitted that they are foreign to that idea as commonly received, and as here to be formulated.

It remains to add the definition since- suggested by Mr Gr. H. Lewes — " Life is a series of definite and successive changes, both of structure and composition, which take place within an individual without destroying its identity." The last fact which this statement has the merit of bringing into view — the persistence of a living organism as a whole, in spite of the continuous removal and replacement of its parts — is important. But otherwise it may be argued, that since changes of structure and composition, though probably the causes of muscular and nervous actions, are not the muscular and nervous actions themselves, the definition excludes the more visible movements with which our idea of life is most associated ; and further, that in describing vital changes as a series, it scarcely includes the fact that manv of them, a?

THE DATA OF BIOLOGY.

Nutrition, Circulation, Respiration, and Secretion, in their many subdivisions, go on simultaneously.

Thus, however well each of these definitions expresses the phenomena of life under some of its aspects, no one of them is more than approximately true. It may turn out, that to find a formula which will bear every test is impossible. Meanwhile, it is possible to frame a more adequate formula than any of the foregoing. As we shall presently find, these all omit an essential peculiarity of vital changes in. general — a peculiarity which, perhaps more than any other, distinguishes them from non- vital changes. Before specify- ing this peculiarity, however, it will be well to trace our way, step by step, to as complete an idea of Life as may be reached from our present stand- point : by doing which, we shall both gee the necessity for each limitation as it is made, and ulti- mately be led to feel the need for a further limitation

And here, as the best mode of determining what are those general characteristics which distinguish vitality from non- vitality, we shall do well to compare the two most unlike kinds of vitality, and see in what they agree. Manifestly, that which is essential to Life must be that which is common to Life of all orders. And manifestly, that which is common to all forms of Life, will most readily be seen on contrasting those forms of Life which have the least in common, or are the most unlike.*

§ 25. Choosing assimilation, then, for our example of bodily life, and reasoning for our example of that life known as intelligence ; it is first to be observed, that they are both processes of change. Without change, food cannot be taken into the blood nor transformed into tissue : without

* This paragraph replaces a sentence that, in The Principles of Psychology, referred to a preceding chapter on " Method;" in which the mode of procedure here indicated, was set forth as a mode to be systematically piirsued in the choice of hypotheses. Should opportunity ever permit, this chapter on Method will be embodied, along with other matter on the same topic, in a General Introduction to First Principles.

PROXIMATE DEFINITION OF LIFE. 63

change, there can be no getting from premisses to conclusion. And it is this conspicuous manifestation of change, which forms the substratum of our idea of Life in general. Doubt- less we see innumerable changes to which no notion of vital- ity attaches : inorganic bodies are ever undergoing changes of temperature, changes of colour, changes of aggregation. But it will be admitted that the great majority of the phe- nomena displayed by inorganic bodies, are statical and not dynamical ; that the modifications of inorganic bodies are mostly slow and unobtrusive ; that on the one hand, when, we see sudden movements in inorganic bodies, we are apt to assume living agency, and on the other hand, when we see no movements in organic bodies, we are apt to assume death. From all which considerations it is manifest, that be the requisite qualifications what they may, a definition of Life must be a definition of some kind of change or changes.

On further comparing assimilation and reasoning, with a view of seeing in what respect the change displayed in both differs from non- vital change, we find that it differs in being not simple change, but change made up of successive changes. The transformation of food into tissue, involves mastication, deglutition, chymification, chylification, absorption, and those various actions gone through after the lacteal ducts have poured their contents into the blood. Carrying on an argu- ment necessitates a long chain of states of consciousness ; each implying a change of the preceding state. Inorganic changes, however, do not in any considerable degree exhibit this peculiarity. It is true that from meteorologic causes, inanimate objects are daily, sometimes hourly, undergoing modifications of temperature, of bulk, of hygrometric and electric condition. Not only, however, do these modifications lack that conspicuousness and that rapidity of succession which vital ones possess, but vital ones form an additional series. Living as well as not-living bodies are affected by atmospheric influences ; and beyond the changes which these produce, living bodies exhibit other changes, more nu-

64 THE DATA OF BIOLOGY.

merous and more marked. So that though organic change

o o o

is not rigorously distinguished from inorganic change by presenting successive phases — though some inanimate objects, as watches, display phases of change both quick and nu- merous— though all objects are ever undergoing change of some kind, visible or invisible — though there is scarcely any object which does not, in the lapse of time, undergo a con- siderable amount of change that is fairly divisible into phases; yet, vital change so greatly exceeds other change in its dis- play of varying phases, that we may consider this as prac- tically one of its characteristics. Life, then, as thus roughly differentiated, may be regarded as change presenting succes- sive phases ; or otherwise, as a series of changes. And it should be observed, as a fact in harmony with this concep- tion, that the higher the life the more conspicuous the varia- tions. On comparing inferior with superior organisms, these last will be seen to display more rapid changes, or a more lengthened series of them, or both.

Contemplating afresh our two typical phenomena, we may see that vital change is further distinguished from non- vital change, by being made up of many simultaneous changes. Assimilation is not simply a series of actions, but includes many actions going on together. During mastication the stomach is busy with the food already swallowed ; on which it is both pouring out solvent fluids, and expending muscular efforts. While the stomach is still active, the intestines ara performing their secretive, contractile, and absorbent func- tions ; and at the same time that one meal is being digested, the nutriment obtained from a previous meal is undergoing that transformation into tissue which constitutes the final act of assimilation. So also is it, in a certain sense, with mental changes. Though the states of consciousness which make up an argument occur in series, yet, as each of these states is complex — implies the simultaneous excitement of those many faculties by which the perception of any object or relation has been effected; it is obvious that each such change in

PROXIMATE DEFINITION OF LIFE. 65

consciousness implies many component changes. In

this respect too, however, it must be admitted that the distinction between animate and inanimate is not precise. No mass of dead matter can have its temperature altered, without at the same time undergoing an alteration in bulk, and sometimes also in hygrometric state. An inorganic body cannot be oxidized, without being at the same time changed in weight, colour, atomic arrangement, temperature, and electric condition. And in some vast and mobile aggre- gates like the sea, the simultaneous as well as the successive changes displayed, outnumber those going on in an animal. Nevertheless, speaking generally, a living thing is distin- guished from a dead thing, by the multiplicity of the changes at any moment taking place in it. Add to which, that by this peculiarity, as by the previous one, not only is the vital more or less clearly marked oif from the non- vital ; but creatures possessing high vitality are marked off from those possessing low vitality. It needs but to contrast the many organs co-operating in a mammal, with the few in a polype, to see that the actions which are progressing together in the body of the first, as much exceed in number the actions pro- gressing together in the body of the last, as these do those in a stone. As at present analyzed, then, Life consists of simultaneous and successive changes.

Continuing the comparison, we next find that vital changes, both visceral and cerebral, differ from other changes in their heterogeneity. Neither the simultaneous acts nor the serial acts, which together constitute the process of digestion, are at all alike. The states of consciousness comprised in any ratiocination are not repetitions of each other, either in com- position or in modes of dependence. Inorganic processes, on the other hand, even wrhen like organic ones in the number of the simultaneous and successive changes they involve, are unlike them in the homoo-eneitv of these changes. In the

v

case of the sea, just referred to, it is observable that count- less as are the actions at any moment going on, they are

4

66 THE DATA OF BIOLOGi'.

mostly mechanical actions that are to a great degree similar; and in this respect widely differ from the actions at any mo- ment taking place ia an organism : which not only belong to the several classes, mechanical, chemical, thermal, electric, but present under each of these classes, innumerable unlike actions. Even where life is nearly simulated, as by the working of a steam-engine, we may see that considerable as is the number of simultaneous changes, and rapid as are the successive ones, the regularity with which they soon recur in the same order and degree, renders them unlike those varied changes exhi- bited by a living creature. Still, it will be found that this peculiarity, like the foregoing ones, does not divide the two classes of changes with precision ; inasmuch as there are inanimate things which exhibit considerable heterogeneity of change : for instance, a cloud. The variations of state which this undergoes, both simultaneous and successive, are many and quick ; and they differ widely from each other both in quality and quantity. At the same instant there may occur in a cloud, change of position, change of form, change of size, change of density, change of colour, change of tem- perature, change of electric state ; and these several kinds of change are continuously displayed in different degrees and combinations. Yet notwithstanding this, when we consider that very few inorganic objects manifest heterogeneity of change in a marked manner, while all organic objects mani- fest it ; and further, that in ascending from low to high forms of life, we meet with an increasing variety in the kinds and amounts of changes displayed ; we see that there is here a further leading distinction between organic and inorganic actions. According to this modified conception, then, Life is made up of heterogeneous changes both simultaneous and successive.

If now we look for some point of agreement between the assimilative and logical processes, by which they are distin- guished from those inorganic processes that are most like them in the heterogeneity of the simultaneous and successive

PROXIMATE DEFINITION OF LIFE.

changes they comprise, we discover that they are distinguish- ed by the combination subsisting among their constituent changes. The acts that make up digestion are mutually de- pendent. Those composing a train of reasoning are in close connection. And generally, it is to be remarked of vital changes, that each is made possible by all, and all are affected by each. Respiration, circulation, absorption, secretion, in their many sub-divisions, are bound up together. Muscular contraction involves chemical change, change of temperature, and change in the excretions. Active thought influences the operations of the stomach, of the heart, of the kidneys. But we miss this union among inorganic processes. Life-like as may seem the action of a volcano in respect of the heterogeneity of its many simultaneous and successive changes, it is not life- like in respect of their combination. Though the chemical, mechanical, thermal, and electric phenomena exhibited, have some inter-dependence ; yet the emission of stones, mud, lava, flame, ashes, smoke, steam, usually takes place irregularly in quantity, order, intervals, and mode of conjunction. Even

here, however, it cannot be said that inanimate things pre sent no parallels to animate ones. A glacier may be instanced as showing nearly as much combination in its changes as a plant of the lowest organization. It is ever growing and ever decaying ; and the rates of its composition and decom- position preserve a tolerably constant ratio. It moves ; and its motion is in immediate dependence on its thawing. .?* emits a torrent of water, which, in common with its motion, undergoes annual variations, as plants do. During part of the year the surface melts and freezes alternately ; and on these changes are dependent the variations in movement, and in efflux of water. Thus we have growth, decay, changes of t-emperature, changes of consistence, changes of velocity changes of excretion, all going on in connexion ; and it may be as truly said of a glacier as of an animal, that by cease- less integration and disintegration it gradually undergoes an entire change of substance without losing its individuality-

68 THE DATA OF BIOLOGY.

Tliis exceptional instance, however, will scarcely be held ta obscure that broad distinction from inorganic processes, which organic processes derive from the combination among their constituent changes. And the reality of this distinction becomes yet more manifest when we find that, in common with previous ones, it not only marks off the living from the not-living, but also things which live little from things which live much. For while the changes going on in a plant or a zoophyte are so imperfectly combined that they can continue after it has been divided into two or more pieces, the com- bination among the changes going on in a mammal is so close that no part cut off from the rest can live, and any con- siderable disturbance of one function causes a cessation of the others. Life, therefore, as we now regard it, is a com- bination of heterogeneous changes, both simultaneous and successive.

Once more looking for a characteristic common to these two kinds of vital action, we perceive that the combinations of heterogeneous changes which constitute them, differ from the few combinations which they otherwise resemble, in re- spect of definiteness. The associated changes going on in a glacier, admit of indefinite variation. Under a conceivable alteration of climate, its thawing and its progression may be stopped for myriads of years, without disabling it from again displaying these phenomena under appropriate conditions. By a geological convulsion, its motion may be arrested with- out an arrest of its thawing ; or by an increase in the in- clination of the surface it slides over, its motion may be accelerated without accelerating its rate of dissolution. Other things remaining the same, a more rapid deposit of enow may cause an indefinite increase of bulk ; or, conversely the accretion may entirely cease, and yet all the other actions continue until the mass disappears. Here, then, the combina- tion has none of that definiteness which, in a plant, marks the mutual dependence of assimilation, respiration, and cir- culation; much less has it that definiteness seen in the

PROXIMATE DEFINITION OF LIFE. 09

mutual dependence of tlie chief animal functions : no one of which can be varied without varying the rest: no one of which can go on unless the rest go on. It is this definiteness of combination which distinguishes the changes occurring in a living body from those occurring in a dead one. Decom- position exhibits both simultaneous and successive changes, which are to some extent heterogeneous, and in a sense com- bined ; but they are not combined in a definite manner. They vary according as the surrounding medium is air, water, or earth. They alter in nature with the temperature. If the local conditions are unlike, they progress differently in different parts of the mass, without mutual influence. They may end in producing gases, or adipocire, or the dry substance of which mummies consist. They may occupy a few days, or thousands of years. Thus, neither in their simultaneous nor in their suc- cessive changes, do dead bodies display that definiteness of combination which characterizes living ones. It is

true that in some inferior creatures the cycle of successive

•/

changes admits of a certain indefiniteness — that it may be apparently suspended for a long period by desiccation or freezing ; and may afterwards go on as though there had been no breach in its continuity. But the circumstance that only a low order of life permits the cycle of its changes to be thus modified, serves but to suggest that, like the pre- vious characteristics, this characteristic of definiteness in its combined changes, distinguishes high vitality from low vital- ky, as it distinguishes low vitality from inorganic processes. Hence, our formula as further amended reads thus : — Life is a definite combination of heterogeneous changes, both simul- taneous and successive.

Finally, we shall still better express the facts, if, instead oi saying a definite combination of heterogeneous changes, we say the definite combination of heterogeneous changes. As it at present stands, the definition is defective both in allow- ing that there may be other definite combinations of hetero- geneous changes, and in directing attention to the hetero-

70 THE DATA OF BIOLOGY.

geneoug changos rather than to the definiteness of their combination. Just as it is not so much its chemical elements which constitute an organism, as it is the arrangement of them into special tissues and organs ; so it is not so much its heterogeneous changes which constitute Life, as it is the de- finite combination of them. Observe what it is that ceases when life ceases. In a dead body there are going on hetero- geneous changes, both simultaneous and successive. What then has disappeared ? The definite combination has dis- appeared. Mark, too, that however heterogeneous the simul- taneous and successive changes exhibited by an inorganic object, as a volcano, we much less tend to think of it as living, than we do a watch or a steam-engine, which, though displaying homogeneous changes, displays them definitely combined. So dominant an element is this in our idea of Life, that even when an object is motionless, yet, if its parts be definitely combined, we conclude either that it has had life, or has been made by something having life. Thus then, we conclude that Life is — the definite combination of hetero- geneous changes, both simultaneous and successive.

§26. Such is the conception at which we arrive without changing our stand-point. It is, however, an incomplete conception. This ultimate formula (which is to a consider- able extent identical with one above given — " the co-ordina- tion of actions ; ' seeing that " definite combination ' ia synonymous with " co-ordination," arid " changes both si- multaneous and successive ' are comprehended under the term " actions ; " but which differs from it in specifying the fact, that the actions or changes are " heterogeneous ") — thia ultimate formula, I say, is after all but proxirnately correct. It is true that it does not fail by including the growth of a crystal ; for the successive changes this implies cannot be called heterogeneous. It is true that the action of a galvanic battery is not comprised in it ; since here, too, heterogeneity is not exhibited by the successive changes. It is true that by

PROXIMATE DEFI^TITTO^7 OF LIFE. 71

this same qualification the motions of the Solar System are excluded ; as are also those of a watch and a steam-engine. It is true, moreover, that while, in virtue of their heteroge- neity, the actions going on in a cloud, in a volcano, in a glacier, fulfil the definition ; they fall short of it in lacking dofiniteness of combination. It is further true that this de- finiteness of combination, distinguishes the changes taking place in an organism during life, from those which commence at death. And beyond all this it is true that, as well as serving to mark off, more or less clearly, organic actions from inorganic actions, each member of the definition serves to mark off the actions constituting high vitality from those constituting low vitality; seeing that life is high in propor- tion to the number of successive changes occurring between birth and death ; in proportion to the number of simultaneous changes ; in proportion to the heterogeneity of the changes ; in proportion to the combination subsisting among the changes ; and in proportion to the definiteness of their com- bination. Nevertheless, answering though it does to so many requirements, this definition is essentially defective. It does not convey a complete idea of the thing contem- plated. The definite combination of heterogeneous changes, both simultaneous and successive, is a formula which fails to call up an adequate conception. And it fails from omitting the most distinctive peculiarity — the peculiarity of which we have the most familiar experience, and with which our notion of Life is, more than with any other, associated. It remains now to supplement the definition by the addition of iiua peculiarity.

CHAPTER Y.

THE CORRESPONDENCE BETWEEN LIFE AND ITS

CIRCUMSTANCES.

§ 27. WE habitually distinguish, between a live object and a dead one, by observing whether a change which we make in the surrounding conditions, or one which Nature makes in them, is or is not followed by some perceptible change in the object. By discovering that certain things shrink when touched, or fly away when approached, or start when a noise is made, the child first roughly discriminates between the living and the not-living ; and the man when in doubt whether an animal he is looking at is dead or not, stirs it with his stick ; or if it be at a distance, shouts, or throws a stone at it. Vegetal and animal life are alike primarily recognized by this process. The tree that puts out leaves when the spring brings a change of temperature, the flower which opens and closes with the rising and setting of the sun, the plant that droops when the soil is dry, and re-erects itself when watered, are considered alive because of these in- duced changes ; in common with the zoophyte which contracts on the passing of a cloud over the sun, the worm that comes to the surface when the ground is continuously shaken, and the hedgehog that rolls itself up when attacked.

Not only, however, do we habitually look for some response when an external stimulus is applied to a living organism, but we perceive a fitness in the response. Dead as well as living things display changes under certain changes of con-

CORRESPONDENCE BETWEEN LIFE AND ITS CIRCUMSTANCES. 73

dition : instance, a lump of carbonate of soda that effervesces when dropped into sulphuric acid ; a cord that contracts when wetted ; a piece of bread that turns brown when held near the fire. But in these cases, we do not see a connexion between the changes undergone, and the preservation of the things that undergo them ; or, to avoid any teleological im- plication— the changes have no apparent relations to future external events which are sure or likely to take place. In vital changes, however, such relations are manifest. Light being necessary to vegetal life, we see in the action of a plant which, when much shaded, grows towards the unshaded side, an appropriateness which we should not see did it grow otherwise. Evidently the proceedings of a spider, which rushes out when its web is gently shaken and stays within when the shaking is violent, conduce better to the obtainment of food and the avoidance of danger than were they reversed. The fact that we feel surprise when, as in the case of a bird fas- cinated by a snake, the conduct tends towards self-destruction, at once shows how generally we have observed an adaptation of living changes to changes in surrounding circumstances.

Note further the kindred truth, rendered so familiar by infinite repetition- that we forget its significance, that there is invariably, and necessarily, a conformity between the vital functions of any organism, and the conditions in which it is placed — between the processes going on inside of it, and the processes going on outside of it. We know that a fish can- not live in air, or a man in water. An oak growing in the ocean, and a seaweed on the top of a hill, are incredible combinations of ideas. We find that every animal is limited to a certain range of climate ; every plant to certain zones of latitude and elevation. Of the marine flora and fauna, each species is found exclusively between such and such depths. Some blind creatures flourish only in dark caves ; the limpet only where it is alternately covered and uncovered by the tide ; the red-snow alga rarely elsewhere than in the arctic regions or among alpine peaks.

74 THE DATA OF BIOLOGY.

Grouping together the cases first named, in wliich a parti- cular change in the circumstances of an organism is followed by a particular change in it, and the cases last named, in which the constant actions occurring within an organism im- ply some constant actions occurring without it ; we see that in both, the changes or processes displayed by a living body are specially related to the changes or processes in its en- vironment. And here we have the needful supplement to our conception of Life. Adding this all-important charac- teristic, our conception of Life becomes — The definite com- bination of heterogeneous changes, both simultaneous and successive, in correspondence with external co-existences and sequences. That the full significance of this addition may be seen, it will be necessary to glance at the correspondence under some of its leading aspects.*

§ 28. Neglecting minor requirements, tne actions going

* Speaking of "the general idea of life" M. Comte says : — " Cette idee sup- pose, en effet, non-seulement celle d'un etre organise de maniere a comporter 1'etat vital, mais aussi celle, non moins indispensable, d'un certain ensemble d'infiuences exterieure propres a son accomplissement. line telle harmonic entre 1'etre vivant et le milieu correspondant, caracterise evidemment la condition f'on- damentale de la vie." Commenting on de Blainville's definition of life, which he adopts, he says : — " Cette lumineuse definition ne me parait laisser rien d'impor- tant a desirer, si ce n'est une indication plus directe et plus explicite de ces deux conditions fondamentales co-relatives, necessairement inseparables de 1'etat vivant, un organisme determine et un milieu convenable." It is strange that M. Comte should have thus recognized the necessity of a harmony between an organism and its environment, as a condition essential to life, and should not have seen that the continuous maintenance of such inner actions as will counterbalance outer actions, constitutes life. It is the more strange that he should have been so near this truth and yet missed it, since, besides his wide range of thought, M. Comte is oi'ton remarkable for his clear intuitions. Lest by saying this, I should deepen a misconception into which some have fallen, let me take the opportunity of stating, that though I believe some of M. Comte's minor generalizations to be true, and though I recognize the profundity of many incidental observations he makes, I by no means accept his system. Those general doctrines in which I agree with him, are those which he holds in common with sundry other thinkers. "With all those general doctrines which are distinctive of his philosophy, I disagree — with all those at least that I have definite knowledge of; for beyond the first half of his " Course of Positive Philosophy," I know his opinions ouly by hearsay.

CORRESPONDENCE BETWEEN LIFE AND ITS CIRCUMSTANCES. 75

on in a plant pre-suppose a surrounding medium containing at least carbonic acid and water, together with a due supply of light and a certain temperature. Within the leaves carbon is being assimilated and oxygen given off ; without them, is the gas from which the carbon is abstracted, and the imponderable agents that aid the abstraction. Be the nature of the process what it may, it is clear that there are external elements prone to undergo special re-arrangements under special conditions. It is clear that the plant in sunshine presents these conditions and so effects these re- arrange- ments. And thus it is clear that the changes which consti- tute the plant's life, are in correspondence with co-existences in its environment.

If, again, we ask respecting the lowest protozoon, how it lives ; the answer is, that while on the one hand its sub- stance is ever undergoing oxidation, it is on the other hand ever absorbing nutriment ; and that it may continue to exist, the assimilation must keep pace with, or exceed, the oxidation. If further we ask under what circumstances these combined changes are possible ; there is the obvious reply, that the medium in which the protozoon is placed, must contain oxy- gen and food — oxygen in such quantity as to produce some disintegration ; food in such quantity as to permit that dis- integration to be made good. In other words — the two antagonistic processes taking place internally, imply the pre- sence externally of materials having affinities that can give rise to these processes.

Leaving those lowest animal forms revealed by the mi- croscope, which simply take in through their surfaces the nutriment and oxygenated fluids coming in contact with them, we pass to those somewhat higher forms which have their tissues partially specialized into assimilative and re- spiratory. In these we see a correspondence between certain actions in the digestive sac, and the properties of certain sur- rounding bodies. That a creature of this order may continue to live, it is necessary not only that there be masses of sub-

76 THE DATA OF BIOLOGY.

stance in the environment capable of transformation into its own tissue ; but that the introduction of these masses into its stomach, shall be followed by the secretion of a solvent fluid that will reduce them to a fit state for absorption. Special outer properties must be met by special inner properties.

TVhen, from the process by which food is digested, we turn to the processes by which it is seized, we perceive the same general truth. The stinging and contractile power of a polype's tentacle, correspond to the sensitiveness and strength of the creatures serving it for prey. Unless that external change which brings one of these creatures in con- tact with the tentacle, were quickly followed by those inter- nal changes which result in the coiling and drawing up of the tentacle, the polype would die of inanition. The funda- mental processes of integration and disintegration within it, would get out of correspondence with the agencies and pro- cesses without it ; and the life would cease.

Similarly, it may be shown that when the creature be- comes so large that its tissue cannot be efficiently supplied with nutriment by mere absorption through its limiting membranes, or duly oxygenated by contact with the fluid that bathes its surface, there arises a necessity for a circu- latory system by which nutriment and oxygen may be dis- tributed throughout the mass ; and the functions of this sys- tem, being subsidiary to the two primary functions, form links in the correspondence between internal and external ac- tions. The like is obviously true of all those subordinate functions, secretory and excretory, that facilitate oxidation and assimilation — functions in which we may trace, both co- temporaneous changes answering to co-existences in the en- vironment, and successive changes answering to those changes of composition, of temperature, of light, of moisture, of pres- sure, which the environment undergoes.

Ascending from the visceral actions to the muscular and nervous actions, we find the correspondence displayed in a manner still more obvious. Every act of locomotion implies

CORRESPONDENCE BETWEEN LIFE AND ITS CIRCUMSTANCES. 77

the expenditure of certain internal mechanical forces, adapted in amounts and directions to balance or out-balance certain external ones. The recognition of an object is impossible without a harmony between the changes constituting per- ception, and particular properties co-existing in the environ- ment. Escape from enemies supposes motions within tho organism, related in kind and rapidity to motions without it.

O •* A. *

Destruction of prey requires a particular combination of sub- jective actions, fitted in degree and succession to overcome a group of objective ones. And so with those countless au- tomatic processes exemplified in works on animal instinct.

In the highest order of vital changes, the same fact is equally manifest. The empirical generalization that guides the farmer in his rotation of crops, serves to bring his actions into concord with certain of the actions going on in plants and soil. The rational deductions of the educated navigator who calculates his position at sea, constitute a series of mental acts by which .his proceedings are conformed to surrounding circumstances. Alike in the simplest inferences of the child, and the most complex ones of the man of science, we find a correspondence between simultaneous and successive changes in the organism, and co-existences and sequences in its envi- ronment.

f 29. This general formula, which thus includes the lowest vegetal processes as well as the highest manifestations of hu- man intelligence, will perhaps call forth some criticisms which, it is desirable here to meet.

It may be thought that there are still a few inorganic ac- tions included in the definition ; as for example that displayed by the mis-named storm-glass. The feathery crystallization which, on a certain change of temperature, takes place in the solution contained by this instrument, and which afterwards dissolves to reappear in new forms under new conditions, may be held to present simultaneous and successive changes that are to some extent heterogeneous, that occur with some de»

THE DATA OF BIOLOGY.

nniteness of combination, and, above all, occur in correspond- ence with external changes. In this case vegetal life is sim- ulated to a considerable extent ; but it is merely simulated. The relation between the phenomena occurring in the storm- glass and in the atmosphere respectively, is really not a cor- respondence at all, in the proper sense of the word. Outside there is a certain change ; inside there is a change of atomic arrangement. Outside there is another certain change ; in^ side there is another change of atomic arrangement. But subtle as is the dependence of each internal upon each ex- ternal change, the connexion between them does not, in the abstract, differ from the connexion between the motion of a straw and the motion of the wind that disturbs it. In either case a change produces a change, and there it ends. The alteration wrought by some environing agency on an inani- mate object, does not tend to induce in it a secondary altera- tion, that anticipates some secondary alteration in the en- vironment. But in every living body there is a tendency towards secondary alterations of this nature ; and it is in their production that the correspondence consists. The dif- ference may be best expressed by symbols. Let -A be a change in the environment ; and B some resulting change in an inorganic mass. Then A having produced B, the ac- tion ceases. Though the change A in the environment, is followed by some consequent change a in it ; no parallel se- quence in the inorganic mass simultaneously generates in it eorne change b that has reference to the change a. But if we take a living body of the requisite organization, and let the change A impress on it some change C ; then, while in the environment A is occasioning a, in the living body C will be occasioning c : of which a and c will show a certain con- cord in time, place, or intensity. And while it is in the con- tinuous production of such concords or correspondences that Life consists, it is by the continuous production of them that Life is maintained.

The further criticism that may be expected, concerns cer-

fJORHESPONDENCE BETWEEN LIFE AND ITS CIRCUMSTANCES. 79

tuin verbal imperfections in the definition, which it seems impossible to avoid. It may be fairly urged that the word correspondence will not include, without straining, the various relations to be expressed by it. It may be asked : — How can the continuous processes of assimilation and respiration, cor- respond with the co-existence of food and oxygen in the en- vironment? or asrain : — How can the act of secreting some

o o

defensive fluid, correspond with some external danger which may never occur ? or again : — How can the dynamical phe- nomena constituting perception, correspond with the statical phenomena of the solid body perceived ? The only reply to these questions, is, that we have no word sufficiently general to comprehend all forms of this relation between the organ- ism and its medium, and yet sufficiently specific to convey an adequate idea of the relation ; and that the word correspond- ence seems the least objectionable. The fact to be expressed in all cases, is, that certain changes, continuous or discon- tinuous, in the organism, are connected after such a manner that, in their amounts, or variations, or periods of occurrence, or modes of succession, they have a reference to external ac- tions, constant or serial, actual or potential — a reference such that a definite relation among any members of the one group, implies a definite relation among certain members of the other group ; and the word correspondence appears the best fitted to express this fact.

§ 30. The presentation of the phenomena under this ge- neral form, su Quests how our definition of Life mav be reduced

o O *

to its most abstract shape ; and perhaps its best shape. By regarding the respective elements of the definition as relations, we avoid both the circumlocution and the verbal inaccuracy ; and that wTe may so regard them with propriety is obvious. If a creature's rate of assimilation is increased in consequence of a decrease of temperature in the environment ; it is that the relation between the food consumed and heat produced, is sc> re- adjusted by multiplying both its members, that the

80 THE DATA OF BIOLOGY.

altered relation in the surrounding medium between tho quantity of heat absorbed from, and radiated to, bodies of a given temperature, is counterbalanced. If a sound or a scent wafted to it on the breeze, prompts the stag to dart away from the deer-stalker ; it is that there exists in its neighbour- hood a relation between a certain sensible property and cer- tain actions dangerous to the stag, while in its organism there exists an adapted relation between the impression this sensible property produces, and the actions by which danger is escaped. If inquiry has led the chemist to a law, enabling him to tell how much of any one element will combine with so much of another ; it is that there has been established in him specific mental relations, which accord with specific chemical relations in the things around. Seeing, then, that in all cases we may consider the external phenomena as simply in relation, and the internal phenomena also as simply in re- lation ; the broadest and most complete definition of Life will be — The continuous adjustment of internal relations to external relations*

While it is simpler, this modified formula has the further advantage of being somewhat more comprehensive. To say that it includes not only those definite combinations of simul- taneous and successive changes in an organism, which cor- respond to co-existences and sequences in the environment, but also those structural arrangements which enable the or- ganism to adapt its actions to actions in the environment, may perhaps be going too far ; for though these structural arrangements present internal relations adjusted to external relations, yet the continuous adjustment of relations can scarcely be held to include a fixed adjustment already made. Clearly, Life, which is made up of dynamical phenomena, cannot be defined in terms that shall at the same time define the apparatus manifesting it, which presents only statical phenomena. But while this antithesis serves to remind us that the fundamental distinction between the organism and

* In further elucidation of this general doctrine, see First Principles, § 25.

CORRESPONDENCE BETWEEN LIFE AND ITS CIRCUMSTANCES. 81

its actions, is as wide as that between Matter and Motion, it at the same time draws attention to the fact, that if the structural arrangements of the adult are not properly in- cluded in the definition, yet the developmental processes by which those arrangements were established, are included. For that process of evolution during which the organs of the embryo are fitted to their prospective functions, is from be- ginning to end the gradual or continuous adjustment of in- ternal relations to external relations. Moreover, those struc- tural modifications of the adult organism, which, under change of climate, change of occupation, change of food, slowly bring about some re-arrangement in the organic balance, must simi- larly be regarded as continuous adjustments of internal re- lations to external relations. So that not only does the de- finition, as thus expressed, comprehend all those activities, bodily and mental, which constitute our ordinary idea of Life ; but it also comprehends, both those processes of development by which the organism is brought into general fitness for •these activities, and those after-processes of adaptation by which it is specially fitted to its special activities.

Nevertheless, superior as it is in simplicity and comprehen- siveness, so abstract a formula as this is scarcely fitted for our present purpose. Reserving its terms for such use as oc- casion may dictate, it will be best commonly. to employ its more concrete equivalent — to consider the internal relations as " definite combinations of simultaneous and successive changes ; ' the external relations as " co-existences and se- quences ; ' and the connexion between them as a " corre- spondence."

CHAPTER YL

THE DEGREE OF LIFE VARIES AS THE DEGREE OF

CORRESPONDENCE.

31. ALREADY it has been shown respecting each other qualification included in the foregoing definition, that the life is high in proportion as that qualification is well fulfilled ; and it is now to be remarked, that the same thing is especially true respecting this last qualification — the correspondence be- tween internal and external relations. It is manifest a priori, that since changes in the physical state of the environment, as also those mechanical actions and those variations of available food which occur in it, are liable to stop the processes going on in the organism ; and since the adaptive changes in the organism have the effects of directly or indirectly counter- balancing these changes in the environment ; it follows that the life of the organism will be short or long, low or high, according; to the extent to which changes in the environment

o o

are met by corresponding changes in the organism. Allow- ing a margin for perturbations, the life will continue only while the correspondence continues ; the completeness of the life will be proportionate to the completeness of the corre- spondence ; and the life will be perfect only when the corre- spondence is perfect. Not to dwell in general statements, however, let us contemplate this truth under its concrete aspects.

§ 32. In life of the lowest order, we find that only the

THE LIFE VARIES AS THE CORRESPONDENCE. 83

most prevalent coexistences and sequences in the environ- ment, have any simultaneous and successive changes answer- ing to them in the organism. A plant's vital processes display adjustment solely to the continuous coexistence of certain elements and forces surrounding its roots and leaves ; and vary only with, the variations produced in these ele- ments and forces by the sun — are unaifected by the countless mechanical and other changes occurring around ; save when accidentally arrested by these, The life of a worm is made up of actions referring almost exclusively to the tangible pro- perties of adjacent things. All those visible and audible changes which happen near it, and are connected with other changes that may presently destroy it, pass unrecognized — produce in it no adapted changes : its only adjustment of in- ternal relations to external relations of this order, is seen when it escapes to the surface on feeling the vibrations pro- duced by an approaching mole. Adjusted as are the pro- ceedings of a bird, to a far greater number of coexistences and sequences in the environment, cognizable by sight, hearing, scent, and their combinations ; and numerous as are the dangers it shuns and the needs it fulfils, in virtue of this ex- tensive correspondence ; it exhibits no such actions as those by which a human being counterbalances variations in tem- perature and supply of food, consequent on the seasons. And when we see the plant eaten, the worm trodden on, tho bird dead from starvation ; we see alike that the death is an arrest of such correspondence as existed ; that it occurred when there was some change in the environment to which the or- ganism made no answering change ; and that thus, both in shortness and simplicity, the life was incomplete in propor- tion as the correspondence was incomplete. Progress towards more prolonged and higher life, evidently implies an ability to respond to less general coexistences and sequences. Each step upwards must consist in adding to the previously- adjusted relations which the organism exhibits, some further relation parallel to a further relation in the environment. And the

84 THE DATA OF BIOLOGY.

greater correspondence thus established, must, other things equal, show itself both in greater complexity of life, and greater length of life — a truth which will be duly realized on remembering that enormous mortality which prevails among lowly-organized creatures, and that gradual increase of longevity and diminution of fertility which we meet with on ascending to creatures of higher and higher development.

It must, however, be remarked, that while length and com- plexity of life are, to a great extent, associated — while a more extended correspondence in the successive changes, commonly implies increased correspondence in the simul- taneous changes ; yet it is not uniformly so. Between the two great divisions of life — animal and vegetal — this contrast by no means holds. A tree may live a thousand years, though the simultaneous changes going on in it answer only to the few chemical affinities in the air and the earth, and though its serial changes answer only to those of day and night, of the weather and the seasons. A tortoise, which exhibits in a given time nothing like the number of internal actions adjusted to external ones, that are exhibited by a dog, yet lives far longer. The tree by its massive trunk, and the tortoise by its hard carapace, a-re saved the necessity of re- sponding to those many surrounding mechanical actions which organisms not thus protected must respond to or die ; or rather — the tree and the tortoise display in their structures, certain simple statical relations adapted to meet countless dynamical relations external to them. But notwithstanding the qualifications suggested by such cases, it needs but to compare a microscopic fungus with an oak, an animalcule with a shark, a mouse with a man, to recognize the fact that this increasing correspondence of its changes with those of the environment, which characterizes progressing life, ha- bitually shows itself at the same time in continuity and in complication.

Even were not the connexion between length of life and complexity of life thus conspicuous, it would still be true

THE LIFE VARIES AS THE CORRESPONDENCE. 85

that the degree of life varies with the degree of correspond- ence. For if the lengthened existence of a tree be looked upon as tantamount to a considerable degree of life ; then it must be admitted that its lengthened display of correspond- ences is tantamount to a considerable degree of correspond- ence. If otherwise it be held, that notwithstanding its much shorter existence, a dog must rank above a tortoise in degree of life,, because of its superior activity ; then it is implied that its life is higher, because its simultaneous and successive

o

changes are more complex and more rapid — because the correspondence is greater. And since we regard as the high- est life, that which, like our own, shows great complexity in the correspondences, great rapidity in the succession of them, and great length in the series of them ; the equivalence

o ^

between degree of life and degree of correspondence, is un- questionable.

§ 33. In further elucidation of this general truth, and especially in explanation of the irregularities just referred to, it requires to be observed, that as the life becomes higher the environment itself becomes more complex. Though, literally, the environment means all surrounding space with the coexistences and sequences contained in it ; yet, practi- cally, it often means but a small part of this. The environ- ment of an entozoon, can scarcely be said to extend beyond the body of the animal in which the entozoon lives. That of a fresh- water alga is, virtually, limited to the ditch inhab- ited by the alga. And understanding the term in this re- stricted sense, we shall see that the superior organisms inhabit the more complicated environments.

Thus, contrasted with that found on land, the lower life is that found in the sea ; and it has the simpler environment. Marine creatures are affected by a smaller number of co- existences and sequences than terrestrial ones. Being very nearly of the same specific gravity as the surrounding Tnedium, they have to contend with less various mechanical

8ti THE DATA OF BIOLOGY.

actions. The zoophyte rooted to a stone, and the acalephe passively borne along in. the current, need to undergo no internal changes such as those by which the caterpillar meets the varying effects of gravitation, while creeping over and under the leaves. Again, the sea is liable to none

of those extreme and rapid alterations of temperature which the air suffers. Night and day produce no appreciable modifications in it ; and it is comparatively little affected by the seasons. Thus its contained fauna show no marked cor- respondences similar to those by which air-breathing crea- tures counterbalance thermal changes. Further, in respect to the supply of nutriment the conditions are more simple. The lower tribes of animals inhabiting the water, like the plants inhabiting the air, have their food brought to them. The same current which brings oxygen to the oyster, also brings it the microscopic organisms on which it lives : the disintegrating matter and the matter to be inte- grated, coexist under the simplest relation. It is otherwise with land animals. The oxygen is everywhere ; but that which is needed to neutralize its action is not everywhere : it has to be sought ; and the conditions under which it is to be obtained are more or less complex. So too with that liquid by the agency of which the vital processes are carried on. To marine creatures, water is ever present, and by the lowest is passively absorbed ; but to most creatures livino on the earth and in the air, it is made available onlv through

*/ O

those nervous changes constituting perception, and those muscular ones by which drinking is effected. Simi-

larly, the contrast might' be continued with respect to the electric and hygrometric variations ; and the greater multi- plicity of optical and acoustic phenomena with which ter- restrial life is surrounded. And tracing upwards from the amphibia the widening extent and complexity which the environment, as practically considered, assumes — observing further how increasing heterogeneity in the flora and fauna of the globe, itself progressively complicates the environment

THE LIFE VARIES AS THE CORRESPONDENCE. 87

of each species of organism — it might finally be shown, that the same general truth is displayed in the history of mankind : whose advance in civilization has been simultaneous with their advance from the less varied requirements of the torrid zone to the more varied requirements of the temperate zone ; whose chief steps have been made in regions presenting a complicated physical geography ; and who, in the course of their progress, have been adding to their physical environ- ment a social environment that has been growing even more involved. Thus, speaking generally, it is clear that those re- lations in the environment to which relations in the organism must correspond, themselves increase in number and intricacy as the life assumes a higher forui.

§ 34. To make yet more manifest the fact, that the degree of life varies as the degree of correspondence, I may here point out, that those other distinctions successively noted when contrasting vital changes with rion- vital changes, are all implied in this last distinction — their correspondence with external coexistences and sequences. And to this may be added the supplementary fact, that the increasing fulfil- ment of those other distinctions which we found to accompany increasing life, is involved in the increasing fulfilment of this last distinction. To descend to particulars : — We saw that living organisms are characterized by successive changes ; and that as the life becomes higher, the successive changes become more numerous. Well, the environment is full of successive changes, both positive and relative ; and the greater the correspondence, the greater the number of suc- cessive changes an organism must display. "We saw that lifo presents simultaneous changes ; and that the more elevated it is, the more marked the multiplicity of them. Well, besides countless phenomena of coexistence in the environ- ment, there are often many changes occurring in it at tho same moment ; and hence increased correspondence with ifc, supposes an increased display of simultaneous changes in the

S9 THE DATA OF BIOLOGY.

organism. Similarly with, the heterogeneity of the changes. In the environment the relations are very varied in their kinds ; and hence, as the organic actions come more and more into correspondence with them, they also must become very varied in their kinds. So again is it, even with clefiniteness of combination. For though the inorganic bodies of which

o o

the environment mainly consists, do not present definitely- combined changes, yet they present definitely-combined properties ; and though the minor meteorologic variations of the environment, do not show much definiteness of combination, yet those resulting from day and night and the seasons do. Add to which, that as the environment of each organism comprehends all those other organisms existing within its sphere of life — as the most important and most numerous surrounding changes with which each animal has to deal, are the definitely -combined changes exhibited by other animals, whether prey or enemies ; it results that definiteness of combination, is a general characteristic of the external changes with which internal ones have to correspond. Hence, increase of correspondence involves increased definiteness of combination. So that throughout, the correspondence of the internal relations with the external ones, is the essential thing ; and all the special characteristics of the internal relations, are but the collateral results of this correspondence.

§ 35. As affording the simplest and most conclusive proof that the degree of life varies as the degree of correspondence, it remains to point out that perfect correspondence would be perfect life. Were there no changes in the environment but such as the organism had adapted changes to meet ; and were it never to fail in the efficiency with which it met them ; there would be eternal existence and universal knowledge. Death by natural decay, occurs because in old age the relations be- tween assimilation, oxidation, and genesis of force going on in the organism, gradually fall out of correspondence with the relations between oxygen and food and absorption of heat by

THE LIFE VARIES AS THE CORRESPONDENCE. 89

the environment. Death from disease, arises either when the organism is congenitally defective in its power to balance th« ordinary external actions by the ordinary internal actions, or when there has taken place some unusual external action to which there was no answering internal action. Death by accident, implies some neighbouring mechanical changes of which the causes are either unobserved from inattention, or are so intricate that their results cannot be foreseen ; and consequently certain relations in the organism are not adjusted to the relations in the environment. Manifestly, if, to every outer coexistence and sequence by which it was ever in any degree affected, the organism presented an answering process or act ; the simultaneous changes would be indefinitely nu- merous and complex, and the successive ones endless — the correspondence would be the greatest conceivable, and the life the highest conceivable, both in degree and in length.

§ 36. Before closing the chapter, it will be useful to compare the definition of Life here set forth, with the defini- tion of Evolution set forth in First Principles. Living bodies being bodies which display in the highest degree the structural changes constituting Evolution ; and Life being made up of the functional changes that accompany these structural changes ; we ought to find a certain harmony between the definitions of Evolution and of Life. Such a harmony is not wanting.

The first distinction we noted between the kind of chansro

V ,

shown in Life, and other kinds of change, was its serial character : we saw that vital change is substantially unlike non- vital change, in being made up of successive changes. Now since organic bodies display in so much higher a de- gree than inorganic bodies, those continuous differentiations and integrations which constitute Evolution ; and since the re-distributions of matter thus carried so far in a compara- tively short period, imply concomitant re-distributions of mo- tion ; it is clear that in a given time, organic bodies must

5

90 1HE DATA OF BIOLOGY.

undergo changes so comparatively numerous as to render the successiveness of their changes a marked characteristic. And it will follow a priori, as we found it to do a posteriori, that the organisms exhibiting Evolution in the highest degree, exhibit the longest or the most rapid successions of changes, or both. Again, it was shown that vital change is

distinguished from non-vital change by being made up of many simultaneous changes ; and also that creatures possess- ing high vitality are marked off from those possessing low vitality, by the far greater number of their simultaneous changes. Here too there is entire congruity. In First Principles, § 116, we reached the conclusion, that a force falling on any aggregate is divided into several forces ; that when the aggregate consists of parts that are unlike, each part becomes a centre of unlike differentiations of the inci- dent force ; and that thus the multiplicity of such differen- tiations must increase with the multiplicity of the unlike

J- «/

parts. It follows necessarily, therefore, that organic aggre- gates, which as a class are distinguished from inorganic aggregates by the greater number of their unlike parts, must be also distinguished from them by the greater number of simultaneous changes they display ; and further that the higher organic aggregates, having more numerous unlike parts than the lower, must undergo more numerous simul- taneous changes. We next found that the changes occurring in living bodies, are contrasted with those occurring in other bodies, as being much more heterogeneous ; and that the changes occurring in the superior living bodies, are similarly contrasted with those occurring in inferior ones. Well, heterogeneity of function is the correlate of hetero- geneity of structure ; and heterogeneity of structure is the leading distinction between organic and inorganic aggre- gates, as well as between the more highly organized and the more lowly organized. By reaction, an incident force must be rendered multiform in proportion to the multiformity of the aggregate on which it falls ; and hence those most mul-

THE LIFE VARIES AS THE CORRESPONDENCE. 91

tiform aggregates which, display in the highest degree the phenomena of Evolution structurally considered, must at the same time be aggregates which display in the highest de- gree the multiform actions which constitute Evolution functionally considered. These heterogeneous changes,

exhibited simultaneously and in succession by a living or- ganism, prove, on further inquiry, to be distinguished by their combination from certain non-vital changes which simulate them. Here, too, the parallelism is maintained. It was shown in § 56 of First Principles, that an essential characteristic of Evolution is the integration of parts, which accompanies their differentiation — an integration that is shown both in the consolidation of each part, and in the consolidation of all the parts into a whole. Now, manifestly, combination among the changes going on in different com- bined parts, must be proportionate to the degree of com- bination among these parts : the more mutually-dependent the parts, the more mutually-dependent must be their actions.. Hence, animate bodies having greater co-ordin- ation of parts than inanimate ones, must exhibit greater co-ordination of changes. And this greater co-ordination of their changes must not only distinguish organic from inor- ganic aggregates ; but must, for the same reason, distinguish higher organisms from lower ones, as we found that it did. Yet once more, it was pointed out that the

changes constituting Life, differ from other changes in the- dcfiniteness of their combination ; and that a distinction like in kind, though less in degree, holds between the vital chart 2:03

* O O 7 O

of superior creatures and those of inferior creatures. These, also, are contrasts in harmony with the contrasts disclosed by the analysis of Evolution. "We saw (First Principles, §§ 54, 55) that during Evolution, there is an increase of defmiteness as well as an increase of heterogeneity. We saw that the integration accompanying differentiation, has necessarily the effect of increasing the distinctness with which the parts are marked off from each other ; and that so, out of the inco-

THE DATA OF BIOLOGY.

herent and indefinite, there arises the coherent and definite. But a coherent whole made up of definite parts definitely combined, must exhibit more definitely combined changes than a whole made up of parts that are neither definite in themselves nor in their combination. Hence, if living bodies display more than other bodies this structural definiteness, then, definiteness of combination must be a characteristic of the changes constituting Life ; and must also distinguish the vital changes of higher organisms from those of lower organ- isms. Finally, however, we discovered that all these peculiarities are subordinate to the one fundamental pecu- liarity, that vital changes take place in correspondence with external co-existences and sequences ; and that the highest possible Life is reached, when there is some inner relation of actions fitted to meet every outer relation of actions by which the organism can be affected. But this conception of the highest possible Life, is in perfect harmony with the con- ception, before arrived at, of the ultimate limit of Evolution. When treating of equilibration as exhibited in organic phenomena (First Principles, §§ 133, 134), it was pointed out, that the continual tendency is towards the establishment of a balance between inner and outer changes. It was shown that " the final structural arrangements must be such as will meet all the forces acting on the aggregate, by equivalent antagonistic forces,'5 and that " the maintenance of such a moving equilibrium ' as an organism displays, " requires the habitual genesis of internal forces correspond- ing in number, directions, and amounts, to the external incident forces — as many inner functions, single or com- bined, as there are single or combined outer actions to be met." It was shown, too, that the relations among concep- tions and ideas, are ever in progress towards a better balance between mental actions and those actions in the environment to which conduct must be adjusted. So that that main- tenance of a correspondence between inner and outer rela- tions, which \vc have here found to constitute Life, and the

THE LIFE VARIES AS THE CORRESPONDENCE. 93

perfection of wliicli is the perfection of Life, answers com- pletely to that state of organic moving equilibrium which we saw arises in the course of Evolution, and tends ever to become more complete.

There is much significance in this complete parallelism. That two inquiries starting from different points and carried on in different ways, should lead to conclusions so entirely harmonizing with each other, cannot fail further to conn mi these conclusions; if further couiirmation of them be needed,

CHAPTER VIL

THE SCOPE OF BIOLOGY.

§ 37. WE are now in a position to map-out the boundaries and divisions of our subject. Grouping together the general results arrived at in the first three chapters, and joining with them the results which the last three chapters have brought us to, we shall be prepared to comprehend the science of Biology as a whole ; and to see how its truths may best be classified.

In the chapters treating of Organic Matter, the Actions of Forces on it, and its Reactions on Forces, the generalizations reached were these : — that organic matter is specially sensi- tive to surrounding agencies ; that in consequence of the extreme instability of the compounds it contains, minute dis- turbances can cause in it large amounts of re-distribution ; and that during the fall of its unstably-arranged atoms into stable arrangements, there are given out proportionately large amounts of motion. We saw that organic matter is so constituted, that small incident actions are capable of initiat- ing great reactions — setting up extensive structural modifica- tions, and liberating large quantities of power. In the chapters just concluded, the changes of which Life is made up, were shown to be so adjusted as to balance outer changes. And the general process of the adjustment we found resolves itself into this ; that if in the environment there are any related actions, A and B, by which the or-

THE SCOPE OF BIOLOGY. 9f>

pranism is affected, then if A produces in the organism some change a, there follows in the organism some change b, fitted in time, direction, and amount to meet the action B — a change which is often required to be much larger than its antecedent. Mark, now, the relation between these

two final results. On the one hand, for the maintenance of that correspondence between inner and outer actions which constitutes Life, an organism must be susceptible to small changes from small external forces (as in sensation), and must be able to initiate large changes in opposition to large external forces (as in muscular action). On the other hand, organic matter is at once extremely sensitive to disturbing agencies of all kinds, and is capable of suddenly evolving motion in great amounts. That is to say, the constitution of organic matter specially adapts it to receive and produce the internal changes required to balance external changes.

This being the general character of the vital Functions, and of the Matter in which they are performed, the science of Biology becomes an account of all the phenomena attend- ant on the performance of such Functions by such Matter — an account of all the conditions, concomitants, and conse- quences, under the various circumstances fallen into by living bodies. If all the functional phenomena which living bodies present, are, as we have concluded, incidents in the main- tenance of a correspondence between inner and outer ac- tions ; and if all the structural phenomena which living bodies present, are direct or indirect concomitants of func- tional phenomena ; then the entire Science of Life, must con- sist in a detailed interpretation of all these functional and structural phenomena in their relations to the phenomena of the environment. Immediately or mediately, proximately or remotely, every trait exhibited by organic bodies, as distinguished from inorganic bodies, must be referable to this continuous adjustment between their actions and the actions going on around them. Such being the extent and nature of our subject-matter, it may be thus divided.

96 THE DATA OF BIOLOGY.

1. An account of the structural phenomena presented bv organisms. And this subdivides into : —

a. The structural phenomena presented by individual

organisms.

b. The structural phenomena presented by successions

of organisms.

2. An account of the functional phenomena which or- ganisms present. And this, too, admits of sub-division into : —

a. The functional phenomena of individual organisms.

b. The functional phenomena of successions of organisms. 8. An account of the actions of Structure on Function,

and the re-actions of Function on Structure. And like the others, this is divisible into : —

a. The actions and re- actions as exhibited in individual

organisms.

b. The actions and re- actions as exhibited in successions of organisms.

4. An account of the phenomena attending the production of successions of organisms : in other words — the phenomena of Genesis.

There is, indeed, another mode of grouping the facts of Biology, with which all are familiar. According as they are facts of animal or vegetal life, they may be classed under the heads of Zoology and Botany. But this di- vision, though convenient and indeed necessary for practi- cal purposes, is one that does not here concern us. Dealing with organic structures and functions in connexion with their causes, conditions, concomitants, and consequences, Biology cannot divide itself into Animal-Biology and Vege- tal-Biology ; since the same fundamental classes of phe- nomena are common to both. Recognizing this familiar distinction only as much as convenience obliges us to do, let, us now pass on to consider, more in detail, the classification of biologic phenomena, above set down in its leading outlines,

38. The facts of structure which an individual or-

THE SCOPE OF BIOLOGY. 97

ganism exhibits, are of two chief kinds. In order of con« spicuousness, though not in order of time, there come first those ultimate arrangements of parts which characterize the organism in its mature state — an account of which, commonly called Anatomy, is more properly called Morphology. And second, there come those successive modifications through which the organism passes in its development from the germ to the adult form — an account of which is called Embryology.

The facts of structure which any succession of individual organisms exhibits, admit of similar classification. On the one hand, we have those inner and outer differences of shape, that are liable to arise between the adult members of suc- cessive generations descended from a common stock — differ- ences which, though usually not marked between adjacent generations, may in course of many generations become great. And on the other hand, we have those developmental modi- fications through which such modifications of the descended forms are reached.

The interpretation of structure, as exhibited in individual organisms and successions of organisms, is aided by two sub- sidiary divisions of biologic inquiry, named Comparative Anatomy (properly Comparative Morphology) and Compara- tive Embryology. These cannot properly be regarded as in

i

themselves parts of Biology ; since the facts embraced under them are not substantive phenomena, but are simply inci- dental to substantive phenomena. All the facts of structural Biology are comprehended under the two foregoing sub- divisions ; and the comparison of these facts as presented in different classes of organisms, is simply a method of inter- preting the real relations and dependencies of the facts compared.

Kevertheless, though Comparative Morphology and Com- parative Embryology do not disclose additional series of con- crete or special facts, they lead to the establishment of certain abstract or general facts. By them it is made manifest, that underneath the superficial differences of groups and classes

THE DATA OF BIOLOGY.

and types of organisms, there are hidden fundamental simi- larities ; and that the courses of development in such groups and classes and types, though in many respects divergent, are in some essential respects, coincident. The wide truths thus disclosed, come under the heads of General Morphology and General Embryology.

By contrasting the structures of organisms, there is alsc achieved that grouping of the like and separation of the unlike, called Classification. First by observation of ex- ternal characters ; second by observation of internal charac- ters ; and third by observation of the phases of development ; it is ascertained what organisms are most similar in all particulars ; what organisms are like each other in every important attribute ; what organisms have common primor- dial characters. Whence there finally results such an ar- rangement of organisms, that if certain structural attributes of any one be given, its other structural attributes may be empirically predicted ; and which prepares the way for that interpretation of their relations and genesis, which forms an important part of rational Biology.

§ 39. The second main division of Biology, above de- scribed as embracing the functional phenomena of organisms, is that which is in part signified by Physiology : the remain- der being what we distinguish as Psychology. Both of these fall into subdivisions that may best be treated separ- ately. That part of Physiology which is concerned with the molecular changes going on in organisms, is known as Organic Chemistry. An account of the modes in which the force generated in organisms by chemical change, is trans- formed into other forces, and made to work the various or- gans that carry on the functions of Life, comes under the head of Organic Physics. Psychology, which is mainly concerned with the adjustment of vital actions to actions in the environment (in contrast with Physiology, which is mainly concerned with vital actions apart from

THE SCOPE OF BIOLOGY. 89

actions in the environment) consists of two quite distinct por- tions. Objective Psychology deals with those functions of the nervo-muscular apparatus by which such organisms as possess it, are enabled to adjust inner to outer relations ; and includes also, the study of the same functions as externally manifested in conduct. Subjective Psychology deals with the sensations, perceptions, ideas, emotions, and volitions that are the direct or indirect concomitants of this visible adjustment of inner to outer relations — considers these several kinds of conscious- ness in their genesis, and their connexions of co-existence and succession. Consciousness under its different modes and forms, being a subject-matter radically distinct in nature from the subject-matter of Biology in general ; and the method of self- analysis, by which alone the laws of dependence among changes of consciousness can be found, being a method un- paralleled by anything in the rest of Biology ; we are obliged to regard Subjective Psychology as a separate study — not absolutehr, of course, but relatively to the mind of each student. And since it would be very inconvenient to dis- sociate Objective Psychology from Subjective Psychology, we are practically compelled to deal with the two as forming an independent sub-science, to be treated apart from the lower divisions of Biology.

Obviously, the functional phenomena presented in succes- sions of organisms, similarly divide into physiological and psychological. Under the physiological, come the

modifications of bodily actions that arise in the course of generations, as concomitants of structural modifications ; and these may be modifications, qualitative or quantitative, in the molecular changes classed as chemical, or in the organic actions classed as physical, or in both. Under the

psychological, come the qualitative and quantitative modifica- tions of instincts, feelings, conceptions, and mental changes in general, that occur in creatures having more or less intelligence, when certain of their conditions are changed. This, like the preceding department of Psychology, has in

100 THE DATA OF BIOLOGY.

the abstract two different aspects — the objective and the sub- jective. Practically, however, the objective, which don Is with these mental modifications as exhibited in the changh:<* habits and abilities of successive generations of creatures, is the only one that admits of scientific investigation ; since tho corresponding alterations in consciousness, cannot be im- mediately known to any but the subjects of them. Evidently, convenience requires us to class this part of Psychology along with the other parts, in a distinct sub-science.

Light is thrown on functions, as wrell as on structures, by comparing organisms of different kinds. Comparative Physiology and Comparative Psychology, are the names given to those collections of facts respecting the homologies and analogies, bodily and mental, that are brought to light by this kind of inquiry. These classified observations concern- ing likenesses and differences of functions, are helpers to interpret functions in their essential natures and relations. Hence Comparative Physiology and Comparative Psychology are names of methods, rather than names of true subdivisions of Biology.

Here, however, as before, the comparison of special truths, besides facilitating their interpretation, brings to light certain general truths. Contrasting bodily and mental functions as exhibited in various orders of organisms, shows that there exists, more or less extensively, a community of processes and methods. Hence result two groups of abstract proposi- tions, constituting General Physiology and General Psy- chology.

§ 40. In these various divisions and sub-divisions of the first two great departments of Biology, the phenomena of Structure are considered separately from the phenomena of Function, so far as separate treatment of them is possible. The third great department of Biology deals with them in their necessary connexions. It comprehends the determin-

THE SCOPE OF BIOLOGY. 101

ation of functions by structures, and the determination oi structures by functions.

As displayed in individual organisms, the action of struc- tures on functions is to be studied, not only in the broad and familiar fact that the general kind of life an organism leads is necessitated by the main characters of its organization, but in the more special and less conspicuous fact, that between members of the same species, minor differences of structure lead to minor differences of power to perform certain kinds of action, and of tendency to perform such kinds of action. Con-

versely, under the re- actions of function on structure as displayed in individual organisms, come the facts showing that functions, when fulfilled to their normal extents, main- tain integrity of structure in their respective organs ; and that within certain limits, the increase of functions is followed by such structural changes in their respective organs, as enables the organs to discharge better their extra functions.

Inquiry into the action of structure on function as dis- played in successions of organisms, introduces us to such phenomena as Mr Darwin's "Origin of Species" deals with. In this category come all proofs of the general truth, that when an individual is enabled by a certain structural pecu- liarity, to perform better than others of its species some advantageous action ; and when it bequeaths more or less of its structural peculiarity to descendants, among whom those which have it most markedly, are best able to thrive and propagate ; there arises through this continuous action of structure on function, a visibly modified type of structure, having a more or less distinct function. In the cor-

relative class of facts, which come under the category of re- actions of function on structure as exhibited in successions of organisms, are to be placed all those modifications of struc- ture which arise in races, when changes of conditions entail changes in the balance of their functions. Here is to ha studied the way in which altered function externally necc£si»

102 THE DATA OF BIOLOGY.

tated, works, by re-action, altered structure ; and howinsucceed- ing generations, this altered structure may be made continu- ally more marked by this altered function. Though logically distinct, these two sub-divisions of biologic inquiry cannot in practice be carried on apart. A speciality of struc- ture which leads to an excess of function in any direction, is, by the perpetual re-action of function, rendered ever more decided. A speciality of function, by calling forth a corre- sponding speciality of structure, produces an increasingly efficient discharge of such function. Whichever of the two initiates the change, there goes on between them an unceas- ing action and re-action, producing in them co-ordinate mo difi cations.

§ 41. The fourth great division of Biology, comprehend- ing the phenomena of Genesis, may be conveniently separated into three sub-divisions.

Under the first, comes a description of all the special modes whereby the multiplication of organisms is carried on: which modes range themselves under the two chief heads 01 sexual and asexual. An account of Sexual Multiplication in- eludes the various methods by which germs and ova are fertilized, and by which, after fertilization, they are furnished with the materials, and maintained in the conditions, needful for their development. An account of Asexual Multiplica- tion includes the various methods by which, from the same fertilized germ or ovum, there are produced many organisms that are partially or totally independent of each other.

The second of these sub-divisions deals with the phenomena of Genesis in the abstract. It takes for its subject-matter, such general questions as — What is the end subserved by the union of sperm-cell and germ -cell ? Why cannot all multi- plication be carried on after the asexual method? What are the laws of hereditary transmission ? What are tho causes of variation ?

The third sub-division is devoted to still more abstract

THE SCOPE OF BIOLOGY. 103

aspects of the phenomena. Recognizing the general facts of multiplication, without reference to their modes or immediate causes, it concerns itself simply with the different rates of multiplication in different kinds of organisms, and different individuals of the same kind. Generalizing the numerous contrasts and variations of fertility, it seeks a rationale of them in their relations to other organic phenomena.

§ 42. Such appears to be the natural arrangement of divisions and sub-divisions which Biology presents, when re- garded from the highest point of view, as the Science of Life — the science which has for its subject-matter, the cor- respondence of organic relations, with the relations amid which organisms exist. This, however, is a classification of the parts of Biology when fully developed ; rather than a classification of the parts of Biology as it now stands. Several of the sub-divisions above named have no recognized existence ; and sundry of the others are in quite rudimentary states. It is therefore impossible now to fill in, even in the roughest way, more than a part of the outlines here sketched.

Our course of inquiry being thus in great measure de- termined by the present state of knowledge, we are com- pelled to follow an order widely different from this ideal one. It will be necessary first to give an account of those empiri- cal generalizations which naturalists and physiologists have established : arranging them rather with a view to facility of comprehension than to logical sequence ; and append- ing to those which admit of it, such deductive interpreta- tions as First Principles furnish us with. Having done this, We shall be the better prepared for dealing with the lead- ing truths of Biology, in connexion with the doctrine cf Evolution.

PART II.

THE INDUCTIONS OF BIOLOGY.

CHAPTER I.

GROWTH.

43. PERHAPS the widest and most familiar inductiuf. if Biology, is that organisms grow. While, however, this is a characteristic so habitually and markedly displayed by plants and animals, as to be carelessly thought peculiar to them, it is really not so. Under appropriate conditions, increase of size takes place in inorganic aggregates, as well as in organic aggregates. Crystals grow ; and often far more rapidly than living bodies. Where the requisite materials are supplied in the requisite forms, growth may be witnessed in non-crystal- line masses : instance the fungus-like accumulation of carbon that takes place on the wick of an unsnuffed candle. On an immense!}^ larger scale, we have growth in geologic formations : the slow accumulation of deposited sediment into a stratum, is not distinguishable from growth in its widest acceptation. And if we go back to the genesis of celestial bodies, assuming them to have arisen by Evolution, these, too, must have gradually passed into their concrete shapes through processes of growth. Growth is indeed a concomi- tant of Evolution ; and if Evolution of one kind or other is universal, growth is universal — universal, that is, in the sense that all aggregates display it in some way at some period.

The essential community of nature between organic growth and inorganic growth, is, however, most clearly seen

108 THE INDUCTIONS OF BIOLOGY.

•

on oLserving that they both result in the same way. The segregation of different kinds of detritus from each other, as vvell as from the water carrying them, and their aggregation into distinct strata, is but an instance of a universal tend- ency towards the union of like units and the parting of un- like units (First Principles, § 123). The deposit of a crystal from a solution, is a differentiation of the previously mixed atoms ; and an integration of one class of atoms into a solid body, and the other class into a liquid solvent. Is not the growth of an organism a substantially similar process ? Around a plant there exist certain elements that are like the elements which form its substance ; and its increase of size is effected by continually integrating these surrounding like elements with itself. Nor does the animal fundament- ally differ in this respect from the plant or the crystal. Its food is a portion of the environing matter, that contains some compound atoms like some of the compound atoms constitut- ing its tissues ; and either through simple imbibition or through digestion, the animal eventually integrates with it- self, units like those of which it is built up, and leaves behind the unlike units. To prevent misconception, it may

be well to point out that growth, as here defined, must bo distinguished from certain apparent and real augmentations of bulk which simulate it. Thus, the long, white potato- shoots thrown out in the dark, are produced at the expense of the substances which the tuber contains : they illustrate not the accumulation of organic matter, but simply its re- arrangement. Certain animal-embryos, again, during their early stages, increase considerably in size without assimil- ating any solids from the environment ; and they do this by absorbing the surrounding water. Even in the highest organisms, as in children, there appears sometimes to occur a rapid gain in dimensions, that does not truly measure the added quantity of organic matter ; but is in part due to Changes analogous to those just named. Alterations of tins

GROWTH. 109

kind must not be confounded with that growth, properly so called, of which we have here to treat.

The next general fact to be noted respecting organic growth, is, that it has limits. Here there appears to be a distinction between organic and inorganic growth ; but this distinction is by no means definite. Though that aggrega- tion of inanimate matter which simple attraction produces, may go on without end ; yet there appears to be an end to that more definite kind of aggregation which results from polar attraction. Different elements and compounds, habitu- ally form crystals more or less unlike in their sizes ; and each seems to Lave a size that is not usually exceeded without a

tf

tendency arising to form new crystals rather than to increase the old. On looking at the organic kingdom as a

whole, we see that the limits between which growth ranges, are very wide apart. At the one extreme, we have monads BO minute as to be rendered but imperfectly visible by micro- scopes of the highest power ; and at the other extreme, we have trees of 300 feet high, and animals of 100 feet long. It is true that though in one sense this contrast may be legitimately drawn, yet in another sense it may not ; since these largest organisms are made by the combination of units that are individually like the smallest. A single plant of the genus Protococcus, is of the same structure as one of the many cells united together to form the thallus of some higher Alga, or the leaf of a phsenogam. Each separate shoot of a phsenogam is usually the bearer of many leaves. And a tree is an assemblage of numerous united shoots. One of these great teleophytes is thus an ag- gregate of aggregates of aggregates of units, which sever- ally resemble protophytes in their sizes and structures ; and a like building up is traceable throughout a consider- able part of the animal kingdom. Even, however, when we bear in mind this qualification, and make our coin. parisons between, organisms of the same degree of compo-

110 THE INDUCTIONS OF BIOLOGY.

sition, we still find the limit of growth to have a great range. The smallest branched flowering plant is extremely insignificant by the side of a forest tree ; and there is an enormous difference in bulk between the least and the great- est mammal. But on comparing members of the samo species, we discover the limit of growth to be much less vari- able. Among the Protozoa and Protophyta, each kind has a tolerably constant adult size ; and among the most complex organisms, the differences between those of the same kind that have reached maturity, are usually not very great. The compound plants do, indeed, sometimes present marked contrasts between stunted and well- grown individuals ; but the higher animals diverge but inconsiderably from the average standards of their species.

On surveying the facts with a view of empirically general- izing the causes of these differences, we are so -m made aware that by variously combining and conflicting with each other, these causes produce great irregularities of result. It be- comes manifest that no one of them can be traced to its consequences, unqualified by the rest. Hence the several statements contained in the following paragraphs, must be taken as subject to mutual modification.

Let us consider first, the connexion between degree of growth and complexity of structure. This connexion being involved with many others, becomes apparent only oil so averaging the comparisons, as to eliminate differences among the rest. Nor does it hold at all where the conditions are radically dissimilar ; as between plants and animals. But bearing in mind these qualifications, we shall see that organization has a determining influence on increase of mnss. Of plants the lowest, classed as Thallogens,

usually attain no considerable size. Lichens, Algoe, and Fun- gi, count among their numbers but few bulky species: the largest, such as certain Al^se found in antartic seas, not

o * ^

serving greatly to raise the average. Though among Acro<rens there are some, as the Tree-ferns, whiuli attain a

GROWTH. 1H

»

considerable height, the majority are but of humble growth. The Endogens, including at one extreme small grasses and iit the other tall palms, show us an average and a maximum greater than that reached by the Acrogens. And the En- dogens are exceeded by the Exogens ; among which are found the monarchs of the vegetal kingdom. Pass-

ing to animals, we meet the fact that the size attained by Vertebrata is usually much greater than the size attained by Imcrtebrata. Of invertebrate animals the smallest, classed as Protozoa, are also the simplest ; and the largest, be- longing to the Annulosa and Mollusca. are among the most

O O O

complex of their respective types. Of vertebrate animals we see that the greatest are Mammals ; and that though, in past epochs, there were reptiles of vast bulk, their bulk did not equal that of the whale. Between reptiles arid birds, and between land -vertebrates and aquatic vertebrates, the relation does not hold : the conditions of existence be- ing in these cases widely different. But among fishes as a class, and among reptiles as a class, it is observable that, speaking generally, the larger species are framed on the higher types. The critical reader, who has men-

tally checked these statements in passing them, has doubtless already seen that this relation is not a dependence of or- ganization on growth, but a dependence of growth on or- ganization. The majority of Exogens are smaller than some Endogens ; many Endogens are exceeded in size by certain Acrogens ; and even among Thallogens, the least developed of plants, there are kinds of a size which many plants of the highest order do not reach. Similarly among animals : there are plenty of Crustaceans less than Actinia ; numerous reptiles are smaller than some fish ; the majority of mam- mals are inferior in bulk to the largest reptiles ; and in the contrast between a mouse and a well-grown Medusa, we see a creature that is elevated in the scale of organization, ex- coerUxl in mass by one that is extremely degraded. Clearly then, it pjmnot be held that high organization is habitually

112 THE INDUCTIONS OF BIOLOGY.

*

accompanied by great size. The proposition hero illustrated is the converse one, that great size is habitually accompanied by high organization. The conspicuous fact that the largest species of both animals and vegetals belong to the highest classes ; and that throughout their various sub-classes the higher usually contain the more bulky forms ; shows this connexion as clearly as we can expect it to be shown, amid so many modifying causes and conditions.

The relation between growth and supply of available nutriment, is too familiar a relation to need proving. There are, however, some aspects of it that must be contemplated be- fore its implications can be fully appreciated. Among plants, which are all constantly in contact with the gaseous, liquid, and solid matters to be incorporated with their tissues ; and which, in the same locality, receive not very unlike amounts of light and heat ; differences in the supplies of available nutriment, have but a subordinate connexion with differences of growth. Though in a cluster of herbs spring- ing up from the seeds let fall by a parent, the greater size of some than of others is doubtless due to better nutrition, consequent on accidental advantages ; yet no such inter- pretation can be given of the contrast in size between these herbs and an adjacent tree. Other conditions here come into play : one of the most important probably being, an absence in the one case, and presence in the other, of an ability to se- crete such a quantity of ligneous fibre as will produce a stem capable of supporting a large growth. Among animals, however, which (excepting some Entozoa] differ from plants in this, that instead of bathing their surfaces, the matters they subsist on are dispersed, and have to be obtained ; the relation between available food and growth f is shown with more regularity. The Protozoa, living ou microscopic fragments of organic matter contained in the surrounding water, are unable, during their brief lives, to accumulate any considerable quantity of nutriment. Polypes and Molluscoida, having for food these scarcely visible mem-

GROWTH. 1 1 H

bers of the animal kingdom, are, though, large compared with their prey, small as measured by other standards : even when aggregated into groups of many individuals, which severally catch food for the common weal, they are often so inconspicuous as readily to be passed over by the unobservant. And if from this point upwards we survey the successive grades of animals, it becomes manifest that, in proportion aa the size is great, the masses of nutriment are either large, or, what is practically the same thing, are so abundant and so grouped as that large quantities may be readily taken in. Though, for example, the greatest of mammals, the arctic whale, feeds on such comparatively small creatures as the acalephes and molluscs floating in the seas it inhabits, its method of gulping in whole shoals of them and filtering away the accompanying water, enables it to secure great quantities of food. We may then with safety say, that, other things equal, the growth of an animal depends on the abundance and sizes of the masses of nutriment which its powers enable it to appropriate. Perhaps it may be

needful to add that, in interpreting this statement, the number of competitors must be taken into account. Clearly, not the absolute, but the relative, abundance of fit food is the point ; and this relative abundance very much depends on how many individuals are competing for the food. Thus all who have had experience of fishing in Highland lochs, know that where the trout are numerous thev are small, and

•/

that where they are comparatively large they are compara- tively few.

What is the relation between growth and expenditure of force ? is a question which next presents itself. Though there is reason to believe such a relation exists, it is not very readily traced : involved as it is with so many other rela- tions. Some contrasts, however, may be pointed out, that appear to give evidence of it. Passing over the vegetal kingdom, throughout which the expenditure of force is too small to allow of such a relation being visible ; let us seek ia

6

[14 THE INDUCTIONS OF B1OLOG\.

the animal kingdom, some case where classes otherwise allied, are contrasted in their locomotive activities. Let us compare birds on the one hand, with reptiles and mammals 311 the other. It is an accepted doctrine that birds are organized on a type closely allied to the reptilian type, but superior to it ; and though in many respects the organization of birds is inferior to that of mammals, yet in other respects, as in the greater heterogeneity and integration of the skeleton, the more complex development of the respiratory system, and the higher temperature of the blood, it may be held that birds stand above mammals. Hence were growth de- pendent only on organization, we might infer that the limit of growth among birds should not be much short of that among mammals ; and that the bird- type should admit of a larger growth than the reptile-type. Again, we see no mani- fest disadvantages under which birds labour in obtaining food, but from which reptiles and mammals are free. On the contrary, birds are able to get at food that is fixed beyond the reach of reptiles and mammals ; and can catch food that is too swift of movement to be ordinarily caught by reptiles and mammals. Nevertheless, the limit of growth in birds, falls far below that reached by reptiles and mammals. With what other contrast between these classes, is this contrast connected ? May we not suspect that it is connected with the contrast between their amounts of locomotive exertion ? Whereas mammals (excepting bats, which are small), are during all their movements supported by solid surfaces or dense liquids; and whereas reptiles (excepting the ancient pterodactyles, which were not very large), are similarly re- stricted in their spheres of movement ; the majority of birds move more or less habitually through a rare medium, in which they cannot support themselves without relatively great efforts. The conclusion that there exists this inverse

ratio between growth and expenditure of force, is enforced by the significant fact, that those members of the class Ares, as the Dinornis and Epiornis, which approached in size to

GROWTH. 115

the larger Mammalia and Reptttia, were creatures incapable of flight — creatures wrhich did not expend this excess of force in locomotion. Further evidence that there is

an antagonism between the increase of bulk and the quantity of motion evolved by an organism, is supplied by the ge- neral experience, that human beings and domestic animals, when overworked while growing, are prevented from attain- ing the ordinary dimensions.

One other general truth concerning degrees of growth, must be set down. It is a rule, having exceptions of no great importance, that large organisms commence their separate existences as masses of organic matter more or less considerable in size, and commonly with organizations more or less advanced; and that throughout each organic sub-kingdom, there is a certain general, though irregular, relation between the initial and the final bulks. Yegetals exhibit this

relation much less clearly and constantly than animals. Yet though, among the plants that begin life as minute spores, there are some which, under their special conditions, grow to considerable sizes, the immense majority of them remain small. "While, conversely, the great Endogens and Exogens, when thrown off from their parents, have already the formed organs of young plants, to which are attached large stores of highly nutritive matter. That is to say, where the young plant consists merely of a centre of development, the ultimate growth is commonly insignificant ; but where the growth is to become great, there exists to start with, a well-developed embryo and a stock of assimilable matter. Through-

out the animal kingdom, this relation is tolerably regular. Save among classes that escape the ordinary requirements of animal life, small germs or eggs do not give rise to bulky creatures. Where great bulk is to be reached, the young proceeds from an egg of considerable bulk, or is born of con- siderable bulk ready-organized and partially active. In the c -itiss fishes, for instance, a certain average proportion obtains bctwoeu the sizes of the ova and the sizes of the adult indi-

118 THE INDUCTIONS OF BIOLOGY.

dividuals ; and among the highest fishes, as sharks, the eggs are comparatively few and comparatively large. Rep- tiles have eo^o-s that are smaller in number, and relatively

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greater in mass, than those of fishes ; and throughout this class, too, there is a general ratio between the bulk of the egg and the bulk of the adult creature. As a group, birds show us a further limitation in the number of their eggs, and a further increase in their relative sizes ; and from the minute eggs of the humming-bird up to the immense ones of the Epiornis, holding several quarts, we see that, speaking ge- nerally, the greater the eggs, the greater the birds. Finally, among mammals (omitting the marsupials) the young are born, not only of comparatively large sizes, but with ad- vanced organizations ; and throughout this sub-division of the vertebrata, as throughout the others, there is a mani- fest connexion between the sizes at birth and the sizes at maturity. As having a kindred meaning, there

must finally be noted the fact, that the young of these highest animals, besides starting in life with bodies of considerable sizes, almost fully organized, are, during sub- sequent periods of greater or less length, supplied with nutri- ment— in birds by feeding, and in mammals by suckling and afterwards by feeding. That is to say, beyond the mass and organization directly bequeathed, a bird or mammal obtains a further large mass at but little cost to itself.

Were an exhaustive treatment of the topic intended, it would be needful to give a paragraph to each of the many incidental circumstances by which growth may be aided or restricted. Such facts as that an entozoon is limited by the size of the creature, or even the organ, in which it thrives ; that an epizoon, though getting abundant nutriment with- out appreciable exertion, is restricted to that small bulk at which it escapes ready detection by the animal it infests ; i hat sometimes, as in the weazel, smallness is a condition to Buccessful pursuit of the animals preyed upon ; and that at other times, the advantage of resembling certain other crea-

GROWTH. H7

tures, and so deceiving enemies or prey, becomes an indirect cause of restricted size. But the present purpose is simply to set down those most general relations between growth and other organic phenomena, which induction leads us to. Having done this, let us go on to inquire whether these general relations can be deductively established.

§ 44. That there must exist a certain dependence of growth on organization, may be shown a priori. When wo consider the phenomena of Life, either by themselves or in their relations to surrounding phenomena, we see that, other things equal, tho larger the aggregate the greater is the needful complexity of structure.

In plants, even of the highest type, there is a com- paratively small mutual dependence of parts : a gathered flower-bud will unfold and flourish for days, if its stem be immersed in water ; and a shoot cut off from its parent-tree and stuck in the ground, will grow. The respective parts having vital activities that are not widely unlike, it is pos- sible for great bulk to be reached without that structural

o

complexity required for combining the actions of parts. Even here, however, we see that for the attainment of great bulk, there requires such a degree of organization as shall co-ordinate the functions of roots and branches — we see that such a size as is reached by trees, is not possible without an efficient vascular system enabling the remote organs to utilize each other's products. And we see that such a co-existence of large growth with low organization, as occurs in some of the marine Alcjce, occurs where the conditions of existence do not necessitate any considerable mutual dependence of parts — where the near approach of the plant to its medium in specific gravity, precludes the need of a well-developed stem, and where all the materials of growth being derived from the water by each portion of the thallus, there requires no apparatus for transferring materials from part to part. Among animals which, with but few

118 THE INDUCTIONS OF BIOLOGY.

X.

exceptions, are, by the conditions of their existence, required to take in nutriment through one specialized part of the body, it is clear that there must be a means whereby other parts of the body, to be supported by this nutriment, must have it conveyed to them. It is clear that for an equally efficient maintenance of their nutrition, the parts of a large mass must have a more elaborate propelling and conducting apparatus ; and that in proportion as these parts undergo greater waste, a yet higher development of the vascular system is necessitated. Similarly with the pre-requisites to those mechanical motions which animals are required to perform. The parts of a mass cannot be made to move, and have their movements so co-ordinated as to produce locomo- tive and other actions, without certain structural arrange- ments ; and, other things equal, a given amount of such activity requires more involved structural arrangements in a large mass than in a small one. There must at least be a co-ordinating apparatus presenting greater contrasts in its central and peripheral parts.

The qualified dependence of growth on organization, is equally implied when we study it in connexion with that adjustment of inner to outer relations which constitutes LitV. In plants this is not conspicuous, because the adjustment of inner to outer relations is but small. Still, it is visible in tl e fact that the condition on which only a plant can grow to a great size, is, that it shall, by the development of a massive trunk, present inner relations of forces fitted to counter- balance those outer relations of forces, which tend continually and occasionally to overthrow it ; and this formation of a core of regularly- arranged woody fibres, is an advance in organization. Throughout the animal kingdom, this

connexion of phenomena is manifest. To obtain materials for growth ; to avoid injuries, which interfere with growth ; am? to escape those enemies which bring growth to a sudden end ; implies in the organism, the means of fitting its movements to meet numerous external co-existences and sequences —

GROTVTTT. H(J

implies such various structural arrangements as shall make possible these variously-adapted actions. It cannot be questioned that, everything else remaining constant, a more complex animal, capable of adjusting its conduct to a greater number of surrounding contingences, will be the better able to secure food and evade damage, and so to increase bulk. And evidently, without any qualification, we may say that a large animal, living under such complex conditions of exist- ence as everywhere obtain, is not possible without compara- tively high organization.

"While, then, this relation is traversed and obscured by sundry other relations, it cannot but exist. Deductively we see that it must be modified, as inductively we saw that it is modified, by the circumstances amid which each kind of or- ganism is placed ; but that it is always a factor in determin- ing the result.

§ 45. That growth is, cceterisparibus, dependent on the sup- ply of assimilable matter, is a proposition so continually illus- trated by special experience, as well as so obvious from general experience, that it would scarcely need stating, were it not re- quisite to notice the qualifications with which it must be taken.

The materials which each organism requires for building itself up, are not of one kind, but of several kinds. As a vehicle for transferring matter through their structures, all organisms require water as well as solid constituents ; and how- ever abundant the solid constituents, there can be no growth in the absence of water. Among the solids supplied, there must be a proportion ranging within certain limits. A plant round which carbonic acid, water, and ammonia exist in the right quantities, may yet be arrested in its growth by a deficiency of silica. The total absence of lime from its food, may stop the formation of a mammal's skeleton : thus dwarfing, if not eventually destroying, the mammal ; and this, no matter what quantities of other needful colloids and crystalloids are furnished.

i?0 THE INDUCTIONS OF BIOLOGY.

Again, the truth that, other things equal, growth according to the supply of nutriment, has to be qualified by the condition, that the supply shall not exceed the ability to appropriate it. In the vegetal kingdom, the assimilating surface being external, and admitting of rapid expansion by the formation of new roots, shoots, and leaves, the effect of this limitation is not conspicuous : by artificially supplying plants with those materials which they have usually the most difficulty in obtaining, we can greatly facilitate their growth ; and so can produce striking differences of size in the same species. Even here, however, the effect is confined within the limits of the ability to appropriate ; since in the absence of that solar light and heat, by the help of which the chief appropriation is carried on, the additional materials of growth are useless. In the animal kingdom this

restriction is rigorous. The absorbent surface being, in the great majority of cases, internal ; having a comparatively small area, which cannot be greatly enlarged without re- construction of the whole body ; and being in connexion with a vascular system, which mu'st also be re-constructed before any considerable increase of nutriment can be made available ; it is clear that beyond a certain point, very soon reached, increase of nutriment will not cause increase of growth. On the contrary, if the quantity of nutriment taken in, is greatly beyond the absorbent power, the excess, becoming an obstacle to the regular working of the organism, mav retard growth rather than advance it,

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While then it is certain, a priori, that there cannot bo growth in the absence of such substances as those of which an organism consists ; and while it is equally certain that the amount of growth must primarily be governed by the supply of these substances ; it is not less certain that extra supply will not produce extra growth, beyond a point very soon reached. Deduction shows to be necessary, as induction makes familiar, the truths that, the value of food for purposes of growth depends not on the quantity of the various

GROWTH. ICi

organizable materials it contains, but on the quantity of the material most needed ; that given a right proportion of materials, the pre-existing structure of the organism limits their availability ; and that the higher the structure, the sooner is this limit reached.

§ 46. But why should the growth of every organism be finally arrested ? Though the rate of increase may, in each

t/ »/ '

case, be necessarily restricted within a narrow range of varia- tion— though the increment that is possible in a given time, cannot exceed a certain amount ; yet why should the incre- ments decreasej and finally become insensible ? TThy should not all organisms, when supplied with sufficient materials,

continue to grow as loner as they live ? To find an answer

•/

to this question, we must first revert to the nature and functions of organic matter.

In the first three chapters of Part I., it was shown that plants and animals mainly consist of substances in states of unstable equilibrium — substances which have been raised to this unstable equilibrium by the expeDditure of the forces we know as solar radiations, and which give out these forces in other forms, on falling into states of stable equilibrium. Lea vins: out the water, which serves as a vehicle for these

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materials and a medium for their changes; and excluding

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those mineral matters that play either passive or subsidiary parts ; organisms are built up of compounds which are stores of force. Those complex colloids and crystalloids- which, as united together, form organized bodies, are the same colloids

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and crystalloids which give out, on their decomposition, the forces expended by organized bodies. Thus these

nitrogeneous and carbonaceous substances, being at once the materials for organic growth and the sources of organic

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force ; it results that as much of them as is used up for the genesis of force, is taken away from the means of growth ; and as much as is economized by diminishing the genesis of force, is available for growth. Given that limited quantity

122 1HE INDUCTIONS OF BIOLOGY.

of nutritive matter which, the pre-existing structure of an organism enables it to absorb ; and it is a necessary corollary from the persistence of force, that the matter accumulated as growth, cannot exceed that surplus which remains uncle - composed, after the production of the required amounts of sensible and insensible motion. This, which would

be rigorously true under all conditions, if exactly the same substances were used in exactly the same proportions, for the production of force and for the formation of tissue, requires, however, to be taken with the qualification, that some of the force-evolving substances are not constituents of tissue ; and that thus, there may be a genesis of force which is not at the expense of potential growth. But since organisms (or at least animal organisms, with which we are here chiefly concerned,) have a certain power of selective absorption, which, partially in an individual and more completely in a race, adapts the proportions of the substances absorbed to the needs of the system ; then if a certain habitual expenditure of force, leads to a certain habitual absorption of force- evolving matters that are not available for growth ; and if, were there less need for such matters, the ability to absorb matters available for growth would be increased to an equi- valent extent ; it follows that the antagonism described, does, in the long run, hold even without this qualification. Hence, growth is substantially equivalent to the absorbed nutriment, minus the nutriment used up in action.

This, however, is no answer to the question — why has individual growth a limit ? The antagonism described, does not manifestly account for the fact, that in every domestic animal the increments of growth bear continually decreasing ratios to the mass, and finally come to an end. Nevertheless, it is demonstrable that the excess of absorbed over expended nutriment, must, other things equal, become less as the size of the animal becomes greater. In similarly-shaped bodies,

the masses vary as the cubes of the dimensions ; whereas the strengths vary as the squares of the dimensions. See here

GROWTH. 123

the solution of the problem. Supposing a creature which a year ago was one foot high, has now become two feet high, while it is unchanged in proportions and structure ; what are the necessary concomitant changes that have taken place in it ? It is eight times as heavy ; that is to- say, it has to re- sist eight times the strain which gravitation puts on its structure ; and in. producing, as well as in arresting, every one of its movements, it has to overcome eight times the inertia. Meanwhile, the muscles and bones have sever- ally increased their contractile and resisting powers in pro- portion to the areas of their transverse sections ; and hence are severally but four times as strong as they were. Thus, while the creature has doubled in height, and while its ability to overcome forces has quadrupled, the forces it has to overcome have grown eight times as great. Hence, to raise its body through a given space, its muscles have to be contracted with twice the intensity, at a double cost of matter expended. This necessity will be seen still more clearly if we leave out the motor apparatus, and consider only the forces required and the means of supplying them. For since, in similar bodies, the areas vary as the squares of the dimensions, and the masses vary as the cubes ; it follows that the absorbing sur- face has become four times as great, while the weight to be moved by the matter absorbed has become eight times as

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great. If then, a year ago, the absorbing surface could take up twice as much nutriment as was needed for expenditure, thus leaving one-half for growth, it is now able only just to meet expenditure, and can provide nothing for growth. How- ever great the excess of assimilation over waste, may be dur- ing the early life of an active organism, we see that because a series of numbers increasing as the cubes, overtakes a series increasing as the squares, even though starting from a much smaller number, there must be reached, if the organism lives long enoiigl*, a point at which the surplus assimilation is brought down to nothing — a point at which expenditure ba- lances nutrition — a state of moving equilibrium. This,

124 THE INDUCTIONS OF BIOLOGY.

however, though, the chief, is not the sole, varying relation ba- t\veen degrees of growth and amounts of expended force. There are two more ; one of which conspires with the last, while the other conflicts with it. Consider in the first place, the cost at which nutriment is distributed through the body, and effete matters removed from it. Each increment of growth being added at the periphery of the organism, the force ex- pended in the transfer of matter must increase in a rapid progression — a progression more rapid than that of the mass. But as the dynamic expense of distribution is small compared with the dynamic value of the materials distributed, this item in the calculation is unimportant. Now consider, in the second place, the changing proportion between production and loss of heat. In similar organisms, the quantities of heat generated by similar actions going on throughout their sub- stance, must increase as the masses, or as the cubes of the dimensions. Meanwhile, the surfaces from which loss of heat by radiation takes place, increase only as the squares of the dimensions. Though the loss of heat does not therefore in- crease only as the squares of the dimensions, it certainly in- creases at a smaller rate than the cubes. And to the extent that augmentation of mass results in a greater retention of heat, it effects an economization of force. This advantage is not, however, so important as at first appears. Organic heat is a concomitant of organic action, and is so abundantly pro- duced during action, that the loss of it is then of no conse- quence : indeed the loss is often not rapid enough to keep the supply from rising to an inconvenient excess. It is only in respect of that maintenance of heat which is needful during quiescence, that large organisms have an advantage over small ones in this relatively diminished loss. Thus these two subsidiary relations between degrees of growth and amounts of expended force, being in antagonism with each other, we may conclude that their differential result does not greatly •aiodify the result of the chief relation previously set forth. Any one who proceeds to test this deduction, will find some

GROWTH. 125

seeming incongruities between it and certain facts inductively established. Lest these should mislead him, it will be well to explain them. Throughout the vegetal kingdom,

he may remark that there is no limit of growth except what death entails. Passing over a large proportion of plants which never exceed a comparatively small size, because they wholly or partially die down at the end of the year ; and pointing to trees that annually send forth new shoots, even, when their trunks are hollowed out by decay ; he may ask — How does growth happen here to be unlimited ? The answer is, that plants are only accumulators ; they are in no apprecia- ble degree expenders. As they do not undergo a waste which increases as the cubes of the dimensions, while assimilation increases as their squares ; there is no reason why their growth should be arrested by the equilibration of assimilation and waste. Again, should he look among animals for an

exact correspondence between the decreasing increments of growth as ascertained by observation and as determined by de- duction, he will not find it. And there are sufficient reasons why the correspondence cannot be more than approximate. Besides the fact above noted, that there are other varying relations which complicate the chief one, he must bear in mind that the bodies compared are not truly similar : the proportions of trunk to limbs and trunk to head, vary con- siderably. The comparison is still more seriously vitiated by the inconstant ratio between the constituents of which the body is composed. In the flesh of adult mammalia, water forms from 68 to 71 per cent., organic substance from 24 to 28 per cent., and inorganic substance from 3 to 5 per cent.; whereas in the foetal state, the water amounts to 87 per cent., and the solid organic constituents to only 11 per cent. Clearly this change from a state in which the force-evolving matter forms one tenth of the whole, to a state in which it forms two and a half tenths, must greatly interfere with the parallelism between the actual and the theoretical progression. Yet

another difficulty may come under his notice. The crocodilo

126 THE INDUCTIONS OF BIOLOGY.

is said to grow as long as it lives ; and there appears reason to think that some predaceous fishes, such as the pike, do the same. That these animals of comparatively high organ- ization, have no definite limits of growth, is, however, an ex- ceptional fact due to the exceptional non-fulfilment of those conditions which entail limitation. What kind of life does a crocodile lead? It is a cold-blooded, or almost cold- blooded, creature ; that -is, it expends very little for the main- tenance of heat. It is habitually inert: not chasing prey, bur lying in wait for it ; and undergoes considerable exertion only during its occasional brief contests with prey. Such other exertion as is, at intervals, needful for moving from place to place, is rendered small by the small difference between the animal's specific gravity and that of water. Thus the crocodile expends in muscular action, an amount of force that is insignificant compared with the force commonly expended by land- animals. Hence its habitual assimilation is diminished much less than usual by habitual waste ; and beginning with an excessive disproportion between the two, it is quite possible for the one never quite to lose its advance over the other while life continues. On looking closer into such cases as this and that of the pike, which is similarly cold-blooded, similarly lies in wait, and is similarly able to obtain larger and larger kinds of prey as it increases in size ; we discover a further reason for this absence of a definite limit. The mechanical causes necessitating a limit, are here only partially in action. For a' creature living in a medium of nearly the same density as its body, has not constantly to overcome that gravitative force which is the chief resistance to be met by terrestrial animals : it has not to expend for this purpose, a muscular power that is large at the outset, and increases as the cubes of its dimensions. The only force in- creasing as the cubes of its dimensions, which it has thus to overcome, is the inertia of its parts. The exceptional con- tinuance of growth observed in creatures so circumstanced, ia therefore perfectly explicable.

GROWTH. 127

§ 47. Obviously this antagonism, between accumulation and expenditure, must be a leading cause of the contrasts in size between allied organisms that are in many respects similarly conditioned. The life followed by each kind of animal, is one involving a certain average amount of exertion for the obtainment of a given amount of nutriment — an exertion, part of which goes to the gathering or catching of food, part to the tearing and mastication of it, and part to the after- processes requisite for separating the nutritive atoms — an exertion which therefore varies according as the food is abund- ant or scarce, fixed or moving, according as it is- mechani- cally easy or difficult to deal with when secured, and accord- ing as it is, or is not, readily soluble. Hence, while among animals of the same species having the same mode of life, there will be a tolerably constant ratio between accumulation and expenditure, and therefore a tolerably constant limit of growth ; there is every reason to expect that different species, following different modes of life, will have unlike ratios be- tween accumulation and expenditure, and therefore unlike limits of growth.

Though the facts as inductively established, show a general harmony with this deduction, we cannot usually trace this harmony in any specific way ; since the conflicting and con- spiring causes which affect growth are so numerous. The only contrast which seems fairly to the point, is the before- named one between the vertebrates which fly, and the most nearly-allied vertebrates which do not fly : the differences in degrees of organization and relations to food, being not such as seriously to affect the comparison. If it be admitted that birds habitually expend more force than mammals and rep- tiles, then it will follow a priori, that, other things being tolerably equal, they should have a lower limit of growth than mammals and reptiles ; and this we know to be the fact

2 posteriori.

\ 48. One of the chief causes, if not the chief cause, of

128 THE INDUCTIONS OF BIOLOGY.

the differences between the sizes of organisms, has yet to hi- considered. We are introduced to it by pushing the above inquiry a little further. Small animals have been shown to possess an advantage over large ones, in the greater ratio which, other things equal, assimilation bears to expenditure; and we have seen that hence, small animals in becoming large ones, gradually lose that surplus of assimilative power which they had, and eventually cannot assimilate more than is required to balance waste. But how come these animals while young and small, to have surplus assimilative powers ? Have all. animals equal surplus of assimilative powers ? And if not, how far do differences between the surpluses de- termine differences between the limits of growth ? We shall find in the answers to these questions, the interpretation of many marked contrasts in growth that are not due to any of the causes above assigned. For example, an ox immensely exceeds a sheep in mass. Yet the two live from generation to generation in the same fields, eat the same grass and tur- nips, obtain these aliments with the same small expenditure of force, and differ scarcely at all in their degrees of organiz- ation. Whence arises, then, their striking unlikeness of bulk ?

We noted when studying the phenomena of growth in- ductively, that organisms of the larger and higher types, com- mence their separate existences, as masses of organic matter having tolerable magnitudes. Speaking generally, we saw that throughout each organic sub-kingdom, the acquire- ment of great bulk occurs only where the incipient bulk and organization are considerable ; and that they are the more considerable in proportion to the complexity of the life which the organism is to lead.

The deductive interpretation of this induction may best bo commenced by an analogy. A street orange-vendor makes but a trifling profit on each transaction; and unless more than ordinarily fortunate, he is unable to realize during the day a larger amount than will meet his wants : leav- ing him to start on the morrow in the same condition aa

GROWTH. 129

before. The trade of the huxter in ounces of tea and half- pounds of sugar, is one similarly entailing much labour for fc.iiuil returns. Beginning with a capital of a few pounds, it is impossible for him to have a shop large enough, or goods sufficiently abundant and various, to permit an extensive business : he must be content with the half-pence and pence which he makes by little sales to poor people ; and if, avoid- ing bad debts, he is able by strict economy to accumulate anything, it can be but a trifle. A large retail trader is obliged to lay out much money in fitting up an adequate establishment ; he must invest a still greater sum in stock ; and he must have a further floating capital to meet the charges that fall due before his returns come in. Setting

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out, however, with means enough for these purposes, he is able to make numerous and comparatively large sales ; and so to got greater and more numerous increments of profit. Similarly, to get returns in thousands, merchants and manu- facturers must make their investments in tens of thousands. In brief, the rate at which a man's wealth accumulates, is measured by the surplus of income over expenditure ; and this, save in exceptionally favourable cases, is determined by the capital with, which he begins business. Now ap-

plying the analogy, we may trace in the transactions of an organism, the same three ultimate elements. There is the expenditure required for the obtainment and digestion of food ; there is the gross return in the shape of nutriment as- similated, or fit for assimilation ; and there is the difference between this gross return of nutriment and the nutriment *hat was used up in the labour of securing it — a difference which may be a profit or a loss. Clearly, however, a surplus implies that the force expended is less than the force latent in the assimilated food. Clearly, too, the increment of growth is limited to the amount of this surplus of income over expenditure ; so that large growth implies both that the excess of nutrition over waste shall be relatively considerable, and that the waste and nutrition shall be on extensive scales.

130 THE INDUCTIONS OF BIOLOGY.

And clearly, the ability of an organism to expend largely and assimilate largely, so as to make a large surplus, presupposes u large physiological capital, in the shape of organic matter more or less complete in its structural arrangements.

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Throughout the vegetal kingdom, the illustrations of this truth are not conspicuous and regular : the obvious reason being, that since plants are accumulators and in so small a degree expenders, the premises of the above argument are but very partially fulfilled. The food of plants (excepting Fungi and certain parasites) being in a great measure the same for all, and bathing all so that it can be absorbed with- out effort, their vital processes result almost entirely in profit. Once fairly rooted in a fit place, a plant may thus from the outset add its entire returns to capital ; and may soon be able to carry on its processes on a large scale, though it does not at first do so. When, however, plants are expenders, namely, during their germination and first stages of growth, their degrees of growth are determined by their amounts of vital capital. It is because the young tree commences life with a ready-formed embryo and store of food sufficient to last for some time, that it is enabled to strike root and lift its head above the surrounding herbage. Throughout the

animal kingdom, however, the necessity of this relation is everywhere obvious. The small carnivore preying on small herbivores, can increase in size only by small increments : its organization unfitting it to digest larger creatures, even if it ran kill them, it cannot profit by amounts of nutriment ex- ceeding a narrow limit ; and its possible increments of growth being small to set out with, and rapidly decreasing, must come to an end before any considerable size is attained. Manifestly the young lion, born of tolerable bulk, suckled un- til much bigger, and fed until half- grown, is enabled by tho power and organization which he thus gets gratis, to catch and kill animals of size enough to give him the large supply of nutriment needed to meet his large expenditure, and y:t leave a large surplus for growth. Thus then is explained

GROWTH. 131

(lie above-named contrast between the ox and the sheep. A calf and a lamb commence their physiological transactions on widely different scales ; their first increments of growth arc similarly contrasted in their amounts ; and the two diminish- ing series of such increments, end at similarly-contrasted limits.

§ 49. Such are the several conditions by which the phe- nomena of growth are governed. Conspiring and conflicting in endless different ways and degrees, they in every case qualify more or less differently each other's effects. Hence it happens that we are obliged to state each generalization as true on the average, or to make the proviso — other things equal.

Understood, in this qualified form, our conclusions are these. First, that growth being an integration with the organism, of such environing matters as are of like nature

O * *— '

with the matters composing the organism, its growth is de- pendent on the available supply of such matters : this is alike a truth established by experience, and an inference from the truth given in our forms of thought (First Principles, § 67). Second, that the available supply of assimilable matter being the same, and other conditions not dissimilar, the degree of growth varies according to the surplus of nutrition over ex- penditure— a generalization which is illustrated in some of the broader contrasts between different divisions of orsran-

o

isms, and is a direct corollary from the persistence of force. Third, that in the same organism, the surplus of nutrition over expenditure is a variable quantity ; and that growth is unlimited or has a definite limit, according as the surplus does or does not progressively decrease. This proposition we found on the one hand exemplified by the unceasing growth of organisms that do not expend force ; by the growth, slowly diminishing but never completely ceasing, of organisms that expend comparatively little force ; and by the definitely limited growth of organisms that expend much force ; and

132 THE INDUCTIONS OF BIOLOGY.

on the other hand, we found it to follow from a certain rela tive increase of expenditure that necessarily accompanies in crease of bulk, and to be therefore an indirect corollary from the persistence of force. Fourth, that among organisms which, are large expenders of force, the size ultimately at- ^ained is, other things equal, determined by the initial size : in proof of which conclusion we have abundant facts, as weU as the a priori necessity that the sum-totals of analogous diminishing series, must depend upon the amounts of their initial terms. Fifth, that where the likeness of other cir- cumstances permits a comparison, the possible extent of growth depends on the degree of organization : an inference testified to by the larger forms among the various divisions and sub-divisions of organisms ; and inferable a r>^iovt from tne conditions of existence.

CHAPTER II.

DEVELOPMENT.*

§ 50. CERTAIN general aspects of Development may bo studied apart from any examination of internal structures. These fundamental contrasts between the modes of arrange- ment of parts, originating, as they do, the leading external distinctions among the various forms of organization, will be best dealt with at the outset. If all organisms have arisen by Evolution, it is of course not to be expected that such several modes of development can be absolutely demarcated : we may be sure of finding them united by transitional modes. But premising that a classification of modes can but approx- imately represent the facts, we shall find our general con- ceptions of Development aided by one.

Development is primarily central. All organic forms of which the entire history is known, set out with a symmetri-

•/ K

cal arrangement of parts round a centre. In organisms of the lowest grade, no other mode of arrangement is ever definitely established ; and in the highest organisms, central development, though subordinate to another mode of de- velopment, continues to be habitually shown in. the changes of

* In ordinary speech, Development is often used as synonymous with Growth. It hence seems needful to say, that Development as here and hereafter used, means increase of structure, and not increase of bulk. It may be added, that the word Evolution, comprehending Growth as well as Development, is to be reserved foi occasions \vhenboth ,,vc implied

THE INDUCTIONS OF BIOLOGY.

minute structure. Let us glance at these propositions in the concrete. Leaving out those Rhizopods which are

wholly structureless, every plant and animal in its earliest stage, consists of a spherical sac, full of liquid containing organic matter, in which is suspended a nucleated cell, more or less distinct from the rest ; and the first changes that occur in the germ thus constituted, are changes that take place round centres produced by division of the original centre. From this type of structure, the simplest organisms do not depart ; or depart in no definite or conspicuous ways. Among plants, the Uredo and the several tribes of Protococci permanently maintain such a central distribution ; while among animals, it is permanently maintained by crea- tures like the Gregarina, and in a different manner by the Amceba, Actiiiophrys, and their allies. In larger organisms, made up chiefly of units that are analogous in structure to these simplest organisms, the formation of units ever continues to take place round points or nuclei ; though the arrangement of these units into groups and wholes may proceed after another method.

Central development may be distinguished into unicentral and muUicentral ; according as the product of the original germ, develops symmetrically round one centre, or develops without subordination to one centre — develops, that is, in subordination to many centres. Unicentral de-

velopment, as displayed not in the formation of single cells but in the formation of aggregates, is not common. The animal kingdom, shows it only in the small group named ThalassicollcB : inert, spherical masses of jelly, with scarcely any organization, which are found floating in southern seas. It is feebly represented in the vegetal kingdom by the Vol- vox yfobator. On the other hand, multicentral devel-

opment, or development round insubordinate centres, is va- riously exemplified in both divisions of the organic worhl. It is exemplified in two distinct ways, according as the insubor- dination among the centres of development is partial or total

DEVELOPMENT. 135

rmy most conveniently consider it under the heads hence

at ising.

Total insubordination among the centres of development, is shown where the units or cells, as fast as they are severally formed, part company and lead independent lives. This, in the vegetal kingdom, habitually occurs among the Proto- phyta; and in the animal kingdom, among the Proto- zoa. Partial insubordination is seen in those somewhat advanced organisms, that consist of units which, though they have not separated, have so little mutual depend- ence that the aggregate they form is irregular. Among plants, the Thallogens very generally exemplify this mode of development. Lichens, spreading with flat or corrugated edges in this or that direction, as the conditions determine, have no manifest co-ordination of parts. In the Algce, the Nostocs similarly show us an unsymmetrical structure. Of Fungi, the sessile and creeping kinds display no further dependence of one part on another, than is implied by their cohesion. And even in such better-organized plants as the Marchantia, the general arrangement shows no reference to a directive centre. Among animals, many of the Sponges may be cited as being thus devoid of that co-ordination implied by symmetry: the Amseba-like units composing them, though they have some subordination to local centres, have no subor- dination to a general centre. To distinguish that kind of development in which the whole product of a germ coheres in one mass, from that kind of development in which it does not, Professor Huxley has introduced the words " con- tinuous " and " discontinuous;" and these seem the best fitted for the purpose. Multicentral development, then, is divisible into continuous and discontinuous.

From central development we pass insensibly to that higher kind of development for which axial seems the most appro- priate name. A tendency towards this is vaguely manifested almost everywhere. The great majority even of Protophyta 1'iotozoa have different longitudinal and transverse di-

136 THE INDUCTIONS OF BIOLOGY.

mensioiis — have an obscure if not a distinct axial structure. The originally cellular units out of which higher organisms are mainly built up, usually pass into shapes that are subordi- nated to lines rather than to points. And in the higher organ- isms, considered as wholes, an arrangement of parts in rela- tion to an axis is distinct and nearly universal. We see it in the superior orders of Thallogens ; and in all the Acrogens, Endogens, and Exogens. With few exceptions the Caelente- rata clearly exhibit it ; it is traceable, though less conspicu- ously, throughout the Mollusca; and the Anmilosa and Vertebrata uniformly show it with perfect definiteness.

This kind of development, like the first kind, is of two orders. The whole germ-product may arrange itself round a single axis, or it may arrange itself round many axes ; the structure may be uniaxlal or multiaxial. Each division of the organic kingdom furnishes examples of both these or- ders. In such Fungi as exhibit axial development at all, we commonly see development round a single axis. Some of the Algce, as the common tangle, show us this arrange- ment. And of the higher plants, many Endogens and small Exogens are uniaxial. Of animals, the advanced are without exception in this .category. There is no known ver- tebrate in which the whole of the germ- product is not subor- dinated to a single axis. In the more fully-organized Annu- losa, the like is almost universal ; as it is also in the superior orders of Mollusca. Multiaxial development occurs in most of the plants we are familiar with — every branch of a shrub or tree being an independent axis. But while in tho vegetal kingdom, multiaxial development prevails among tlio highest types; in the animal kingdom, it prevails only among the lowest types. It is extremely general, if not universal, among the Ccelenterata ; it is characteristic of the Mollus- coida ; among Molluscs the compound Ascidians exhibit it; and it is seen, though under another form, in the inferior An nu losa.

Development that is axial, like development that is central,

DEVELOPMENT. 137

may be either continuous or discontinuous : the parts havino- different axes may continue united, or they may separate. Instances of each alternative are supplied by both plants and animals. Continuous, multiaxial development, is

that which plants usually display ; and need not be illustrated further than by reference to every garden. As cases of it in animals may be named, all the compound Hydrozoa and Ac- tinozoa; and such molluscous forms as the Botryllidce. Of

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 ] or r Continuous

(• Multicentral < or

D. j C Discontinuous

EVELOPMENT IS < Or

Uniaxial

Axial ^ or C Continuous

Multiaxial j or

^ Discontinuous

133 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, ho 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, 55, 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. 139

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 under-

goes 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. " Gradually," 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. Nor do we fail to find an essentially

parallel i-et 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 cylindrical, some prismatic, somn spindle- shaped. Tho.se develop spiral fibres

140 THE INDUCTIONS OF BIOLOGY.

in 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 hyaline substance which unites them into a translucent mass. TVTien first perceptible, the muscles are gelatinous, pale, yellowish, transparent, and indistinguish- able from their tendons. The various other tissues of which tie arm consists, beginning with very faintly-marked differ- ences, become day by day more definite in their outlines and appearances. In like 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 trteir curious structure of branching canals. And so in their

»^

respective ways with the units of skin ana 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 niDst remarkable inductions of embry-

142 THE INDUCTIONS OF BIOLOGY.

comes next in order. Yon Baer found that in its

> *.

earliest stage, every organism has 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 displayed 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 wrill 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 which 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 bv

•)

all Vertebrata; but never exhibited by Annnlosa, Molltisca, or Ccelenterata. 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 irnplacental 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 footuses 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.

form that distinguish the variety of man to which it be- longs.

The 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 the more advanced types. Suppos- ing the serins of all kinds of organisms to be simultaneously

O «/

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 successively 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 with, 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. In form, a^ain.

»' n

DEVELOPMENT. 145

we see the same fact. An ordinary characteristic of inor- ganic matter is its indefiniteness 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 bat amorphous : 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 Foramini- fera, 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 differ very little in size. A parallel increase of contrast is seen in chemi-

cal 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 Profophyta 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 — wre 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

146 THE INDUCTIONS OF BIOLOG*.

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, differ 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

O »/

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 tea 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-mobility a pro-

gressive 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. 1 4?

thus are less differentiated from the inorganic world than animals. Though in those microscopic Protophyta and Pro- tozoa inhabiting the water — the spores of algce, the gemmules 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 Ahe water. Among tne nigher aquatic Invertebrata, — cuttle- fishes and lobsters, for instance, — there is a very considerable power of locomotion ; and the aquatic Vertebrata 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 t eristic of self- mobility manifested in the highest degree. Flving 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, in 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 thp admission that the several aspects uide?

148 THE INDUCTIONS OF BIOLOGY.

which 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 counter- acting such changes, it exhibits greater unlikeness to its en- vironment.*

If now, from this same point of view, we consider the rela- tion borne to its environment by any superior organism in its successive stages, we find an analogous series of con- trasts. Of course in respect of degrees of structure, the parallelism is complete. The difference, at first small, be- tween the comparatively structureless germ and the com- paratively structureless inorganic world, becomes necessarily greater, step by step, as the differentiations of the germ be- come more numerous and definite. How of form the like holds, is equally manifest. The sphere, which is

* This paragraph originally formed part of a review-article on " Transcenden- tal Physiology," published in ] 857

DEVELOPMENT. 149

tiie point of departure common to all organisms, is the most generalized of figures ; and one that is, under various circum- stances, assumed by inorganic matter. V/hile the incipient organism is spherical, it is not only like many particular in- organic masses ; but it is like the rest, in the sense that it has the shape which would result, were all their irregularities averaged. But as it develops, it loses all likeness to inor- ganic objects in the environment ; and eventually becomes distinct even from all organic objects in its environ- ment. In specific gravity, the alteration, though not very marked, is still in the same direction. Development being habitually accompanied by a relative decrease in the quantity of water, and an increase in the quantity of consti-

tuents that are heavier than water, there results a small au»-

-••>

mentation of relative weight. In power of maintain-

ing a temperature above that of surrounding things, the differentiation from the environment that accompanies deve- lopment, is marked. All ova are absolutely dependent for their heat on external sources. Like inorganic bodies, they gain or lose heat according as neighbouring bodies are colder or hotter. The mammalian young is, during its uterine life, dependent on the maternal heat ; and at birth has but a par- tial power of making good the loss by radiation. But as it advances in development, it gains an ability to maintain a constant temperature above that of surrounding things : so becoming markedly unlike all surrounding things, save or- ganisms of allied nature. Lastly, in self-mobility this increasing contrast is not less decided. Save in a few aber- rant tribes, chiefly parasitic, we find the general fact to be, that the locomotive power, totally absent or very small at the outset, increases with the advance towards maturity. The more highly developed the organism becomes, the stronger grows the contrast between its activity and the inertness of the objects amid which it moves.

Thus we may say that the development of an individual organism, is at the same time a differentiation of its parts

150 THE INDUCTIONS OF BIOLOGY.

from each other, and a differentiation of the consolidated whole from the environment ; and that in the last as in the first respect, there is a general analogy between the progres- sion of an individual organism, and the progression from, the lowest orders of organisms to the highest orders. It

may be remarked that some kinship seems to exist between these generalizations and the doctrine of Schelling, that Life is the tendency to individuation. For evidently, in becom- ing more distinct from each other, and from their environ- ment, organisms acquire more marked individualities. As far as I can gather from outlines of his philosophy, however, it appears that Schelling entertained this conception in a general and transcendental sense, rather than in a special and scientific one.

§ 54. The deductive interpretations of these general facts- of development, in so far as they are at present possible, must be postponed until we arrive at the fourth and fifth divisions of this work ; which will be chiefly occupied with them. There are, however, one or two general aspects of these inductions, which may be here most conveniently dealt with deductively.

The general law of development as displayed in organisms, is readily shown to be necessary, if the initial and terminal stages are such as we know them to be. Grant that each organism is at the outset homogeneous, and that when com- plete it is relatively heterogeneous ; and of necessity it fol- lows that development is a change from the homogeneous to the heterogeneous — a change during which there must be gone through all the infinitesimal gradations of heterogeneity that lie between these extremes. If, again, there is at first mdefiniteness, and at last definiteness, the transition cannot but be from the one to the other of these, through all intermedi- ate degrees of definiteness. Further, if the parts, originally incoherent or uncombined, eventually become relatively co- herent or combined ; there must be a continuous increase of coherence or combination. Hence the general truth that

DEVELOPMENT- 151

development is a change from incoherent, indefinite homo- geneity, to coherent, definite heterogeneity, becomes a self- evident one, when observation has shown us the state in which organisms begin, and the state in which they end.

^j o * *•

Just in the same way that the growth of an entire organ- ism, is carried on by abstracting from the environment substances like those composing the organism ; so the pro- duction of each organ within the organism, is carried on bv

o «/

abstracting from the substances contained in the organism, those required by this particular organ. Each organ at the expense of the organism as a whole, integrates with itself certain special kinds and proportions of the matters circulat- ing around it ; in the same way that the organism as a whole, integrates with itself certain special kinds and propor- tions of matters at the expense of the environment as a whole. So that the organs are qualitatively . differentiated from each other, in a way analogous to that by which the en- tire organism is qualitatively differentiated from things around it. Evidently this selective assimilation

illustrates the general truth, demonstrable a priori, that like units tend to segregate. It illustrates, moreover, the further aspect of this general truth, that the pre-existence of a mass of certain units, produces, probably by polar attraction, a tendency for diffused units of the same kind to aggregate

«/ o o o

with this mass, rather than elsewhere. It has been shown of particular salts, A and B, co-existing in a solution not suf- ficiently concentrated to crystallize, that if a crystal of the salt A be put iu ' o the solution, it will increase by uniting with itself the dissolved atoms of the salt A ; and that similarly, though there otherwise takes place no deposition of the salt B, yet if a crystal of the salt B is placed in the solution, it will exercise a coercive force on the diffused atoms of this salt, and grow at their expense. No doubt much organic assimilation occurs in the same way. Particular parts of the organism are com- posed of special units, or have the function of secreting special units, which are ever present in them in large quan-

152 THE INDUCTIONS OF BIOLOGY.

tities. The fluids circulating through tlie body contain special units of this same order. And these diffused units are continually being deposited along with the groups of like units that already exist. How purely physical are the causes of this selective assimilation, is, indeed, conclusively shown by the fact, that abnormal constituents of the blood are segregrated in the same way. Cancer-cells having begun to be deposited at a particular place, continue to be deposited at that place. Tubercular matter, making its appearance at particular points, collects more and more round those points. And similarly in numerous pustular diseases. \Yhere

the component units of an organ, or some of them, do not exist as such in the circulating fluids, but are formed out of elements or compounds that exist separately in the circulat- ing fluids ; it is clear that the process of differential assimil- ation is of a more complex kind. Still, however, it seems not impossible that it is carried on in an analogous way. If there be an aggregate of compound atoms, each of which contains the constituents A, B, C ; and if round this aggre- gate the constituents A and B and C are diffused in uncoin- bined states ; it may be suspected that the coercive polar force of these aggregated compound atoms A, B, C, may not only bring into union with themselves adjacent compound atoms A, B, C, but may cause the adjacent constituents A and B and 0 to unite into such compound' atoms, and then aggre- gate with the mass. Should this be so, the process of differ- ential assimilation, which plays so important a part in organic development, will not be difficult to understand. At present, however, chemical inquiry appears to have furnished no evidence either for or against such an hypothesis.

CHAPTER III.

FUNCTION.

§ 55. DOES Structure originate Function, or does Func- tion originate Structure ? is a question about which there has been disagreement. Using the word Function in its widest signification, as the totality of all vital actions, the question amounts to this — does Life produce Organization, or does Organization produce Life ?

To answer this question is not easy, since we habitually find the two so associated that neither seems possible without the other ; and they appear uniformly to increase and decrease together. If it be said that the arrangement of or- ganic substances in particular forms, cannot be the ultimate cause of vital changes, which must depend on the properties of such substances ; it may be replied that, in the absence of structural arrangements, the forces evolved cannot be so directed and combined as to secure that correspondence between inner and outer actions which constitutes Life. Again, to the allegation that the vital activity of every germ whence an organism arises, is obviously antecedent to the development of its structures ; there is the answer that such germ is not absolutely structureless, but consists of a mass of :-<>lls, containing a cell that differs from the rest, and initiates the developmental changes. There is, however, one

fact implying that Function must be regarded as taking pre- cedence of Structure. Of the lowest Rhizopods, which pre-

[54: 'HIE INDUCTIONS OF BIOLOGY.

sent no distinctions of parts, and nevertheless toed and grow and move abuut, Prof. Huxley has remarked that they exhibit Life without Organization. The perpetual changes of form which alone distinguish one of these creatures from an inanimate fragment, are no doubt totally irregular and un- directed. Still they do, through an average of accidents, subserve the creatures' nutrition ; and they do imply an ex- penditure of force that in some way depends on the consump- tion of nutriment. They do, therefore, though in the rudest way, display a vital adjustment of internal to external relations.

8 56. Function falls into divisions of several kinds, ac-

o

cording to our point of view. Let us take these divisions in the order of their simplicity.

Under Function in i'.s widest sense, are included both the statical and the dynamical distributions of force which an organism opposes to the forces brought to bear on it. In a tree, the woody core of trunk and branches, and in an animal, the skeleton, internal or external, may be regarded as pas- sively resisting the gravity and momentum which tend habitually or occasionally to derange the requisite relations between the organism and its environment ; and since they resist these forces simply by their cohesion, their functions may be classed as statical. Conversely, the leaves and sap- vessels in a tree, and those organs which in an animal similarly carry on nutrition and circulation, as well as those which generate and direct muscular motion, must be con- sidered as dynamical in their actions. From another point of view, Function is divisible 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. In plants we see little beyond the first of these : expenditure being inappreciable, and transfer required only to facilitate

FUNCTION. 155

accumulation. In animals, the function of accumulation comprehends those processes by which the materials contain- ing latent force are taken in, digested, and separated from other materials ; the function of transfer comprehends those processes by which these materials, and such others as are needful to liberate the forces they contain, are conveyed throughout the organism ; and the function of expenditure comprehends those processes by which the forces are liberated from these materials, and transformed into properly co-ordin- ated motions. Each of these three most general divisions, includes several more special divisions. The accu- mulation of force may be separated into alimentation and aeration ; of which the first is again separable into the various acts gone through between prehension of food and the transformation of part of it into blood. By the transfer of force is to be understood what we call circulation; if the meaning of circulation be extended to embrace the duties of both the vascular system and the lymphatics. Under the head of expenditure of force, come nervous actions and mus- cular actions : though not absolutely co-extensive with ex- penditure, these are almost so. Lastly, there are the subsidiary functions which do not properly fall within any of these general functions, but subserve them by removing the obstacles to their performance : those, namely, of ex- cretion and exhalation, whereby waste products are got vid of. Again, disregarding their purposes and considering them analytically, the general physiologist may consider functions in their widest sense as the correlatives of tissues — the actions of epidemic tissue, cartilaginous tissue, elastic tissue, connective tissue, osseous tissue, muscular tissue, nervous tissue, glandular tissue. Once more, physiology in its concrete interpretations, recognizes special functions as the ends of special organs — regards the teeth as having the office of mastication ; the heart as an apparatus

^3

to propel blood ; this gland as fitted to produce one requisite

156 THE INDUCTIONS OF BIOLOGY.

secretion and that to produce another ; each muscle as the agent of a particular motion ; each nerve as the vehicle of a special sensation or a special motor impulse.

It is clear that dealing with Biology only in. its larger aspects, specialities of function do not concern us ; except in so far as they serve to illustrate, or to qualify, its general- ities

§ 57. The first induction to be here set down, is a iamiliar and obvious one : the induction, namely, that com- plexity of function, is the correlative of complexity of struc- ture. The leading aspects of this truth must be briefly noted.

"Where there are no distinctions of structure, there are no distinctions of function. One of the Rhizopocls above instanced as exhibiting life without organization, will serve as an illustration. From the outside of this creature, which has not even a limiting membrane, there are protruded numerous thread-like processes. Originating from any point of the surface, each of these may contract again and disap- pear ; or it may touch some fragment of nutriment, which it draws with it, when contracting, into the general mass — thus serving as hand and mouth ; or it may come in contact with its fellow-processes at a distance from the body, and become confluent with them ; or it may attach itself to an adjacent fixed object, and help by its contraction to draw the body into a new position. In brief, this structureless speck of animated jelly, is at once all stomach, all skin, all mouth, all limb, and doubtless, too, all lung. In organisms

/ j 7 O O

having a fixed distribution of parts, there is a concomitant fixed distribution of actions. Among plants we see that when, instead of a uniform tissue like that of the Algce, everywhere devoted to the same process of assimilation, there arise, as in the Exogens, root and stem and leaves, there arise correspondingly unlike processes. Still more con- spicuously among animals, do there result varieties of function when the originally homogeneous mass is replaced by hetero-

FUNCTION. 157

geneous organs; since both singly and by their combinations, do modified parts generate modified changes. Up to

the highest-organic types, this dependence continues mani- fest ; and it may be traced not only under this most general form, but also under the more special form, that in animals having one set of functions developed to more than usual heterogeneity, there is a correspondingly heterogeneous ap- paratus devoted to them. Thus among birds, which have more varied locomotive powers than mammals, the limbs are more widely differentiated ; while mammals, which rise to more numerous and more involved adjustments of inner to outer relations than birds, have more complex nervous systems.

§ 58. It is a generalization almost equally obvious with the last, that functions, like structures, arise by progressive differentiations. Just as an organ is first an indefinite rudi- ment, having nothing but some most general characteristic in common with the form it is ultimately to take ; so a function begins as a kind of action that is like the kind of action it will eventually become, only in a very vague way. And in functional development, as in structural development, the leading trait thus early manifested, is followed success- ively by traits of less and less importance This holds equally throughout the ascending grades of organisms, and throughout the stages of each organism. Let us look at cases : confining our attention to animals, in which func- tional development is better displayed than in plants

The first differentiation established, separates the two fundamentally- opposed functions above named — the accumu- lation of force and the expenditure of force, Passing over the, (Protozoa among which, however, such tribes as present fixed distributions of parts show us substantially the same thing), and commencing with the lowest Coelenterata, where definite tissues make their first appearance, we observe that the onlv marked functional distinction is between the endo-

15 £ THfi INDUCTIONS OF BIOLOGIC.

derm, which absorbs nutriment, and the ectoderm, which, b\ its own contractions and those of the tentacles it bears, pro- duces motion. That the functions of accumulation and ex- penditure are here very incompletely distinguished, may be admitted without affecting the position that this is the first specialization which begins to appear. These two

most general and most radically- opposed functions, become, in the Po/yzoa, much more clearly marked-off from each other ; at the same time that each of them becomes partially divided into subordinate functions. The endoderm and ectoderm are no longer merely the inner and outer walls of the same simple sac into which the food is drawn ; but the endoderm forms a true alimentary canal, separated from the ectoderm by a peri- visceral cavity, containing the nutritive matters absorbed from the food. That is to say, the function of accumulating force is exercised by a part distinctly divided from the part mainly occupied in expending force : the space between them, full of absorbed nutriment, effecting in a vague way that transfer of force which, at a higher stage of evolution, becomes a third leading function. Meanwhile, the endoderm no longer discharges the accumulative function in the same way throughout its whole extent ; but its differ- ent portions, sesophagus, stomach and intestine, perform different portions of this function. And instead of a con- tractility uniformly diffused through the ectoderm, there have arisen in it, some parts which have the office of con- tracting (muscles), and some parts which have the office of making them contract (nerves and ganglia). As we

pass upwards, the transfer of force, hitherto effected quite incidentally, comes to have a special organ. In the ascidion molluscs, circulation is produced by a muscular tube, open at both ends, which, by a wave of contraction passing along it, sends out at one end the nutrient fluid drawn in at the other ; and which, having thus propelled the fluid for a time in one direction, reverses its movement and propels it in the apposite direction. By such means does this rudimentary

FUNCTION. 159

Ueart generate alternating currents in the crude and dilute nutriment occupying the peri- visceral cavity. How the func- tion of transferring force, thus vaguely indicated in these in- ferior forms, comes afterwards to be the definitely-separated office of a complicated apparatus made up of many parts, each of which has a particular portion of the general duty, need not be described. It is sufficiently manifest that this general function becomes more clearly marked-off from the others. at the same time that it becomes itself Darted into subordinate Junctions

In a developing emorvo, tne functions or more strictly the structures whicn are to perform tnem. arise in the same general order. A like primarv distinction very early ap- pears between the endoderm and the ectoderm — the part which has the office of accumulating force, and the part out ;>f which grow those organs that are the great expenders of force Between these two there presently becomes visible the rudiment of that vascular system, which has to fulfil the intermediate duty of transferring force. Of these three ijeneral functions, that of accumulating force is carried on from the outset : the endoderm, even while yet incompletely differentiated from the ectoderm, absorbs nutritive matters from the subjacent yelk The transfer of force is also to some extent effected by the rudimentary vascular system, as soon as its central cavity and attached vessels are sketched out But the expenditure of force (in the higher animals at least) is not appreciably displayed by the ectodermic struc- tures that are afterwards to be mainly devoted to it : there is no sphere for the actions of these parts. Similarly

with the chief subdivisions of these fundamental functions. If we look at those discharged by the ectoderm, potentially if not actually, we see that the distinction first established

v '

separates the office of transforming other force into mechani cal motion, from the office of liberating the force to be so transformed — in the midst of the part out of which the mus- cular system is to be developed, there is marked-out the

IbO THE INDUCTIONS OF BIOLOGY.

rudiment of the nervous system. This indication of struc- tures which are to share between them the general duty of expending force, is soon followed by changes that foreshadow further specializations of this general duty. In the incipient nervous system, there begins to arise that contrast between the cerebral mass and the spinal cord, which, in the main, answers to the division of nervous actions into directive and executive ; and at the same time, the appearance of vertebral laminae foreshadows the separation of the osseous system, which has to resist the strains of muscular action, from the muscular system, which, in generating motion, en- tails these strains. Simultaneously there have been going on similar actual and potential specializations in the functions of accumulating force and transferring force. And through- out all subsequent phases, the method is substantially the same.

This progress from general, indefinite, and simple kinds of action, to special, definite, and complex kinds of action, has been aptly termed by "Milne-Edwards, the "physio- logical division of labour." Perhaps no metaphor can more truly express the nature of this advance from vital activity in its lowest forms to vital activity in its highest forms. And probably the general reader cannot in any other way obtain so clear a conception of functional development in organisms, as he can by tracing out functional development in societies : noting how there first comes a distinction between the governing class and the governed, class ; how while in the governing class there slowly grow up such differences of duty as the civil, military, and ecclesiastical, there arise in the governed class, fundamentally industrial differences like those between agriculturists and artizans ; and how there is a continual multiplication of such specialized occupations, and specialized shares of each occupation.

§ 59. Fully to understand this change from homogeneity to heterogeneity of function, which accompanies the change

FUNCTION. 161

from homogeneity to heterogeneity of structure, it is needful to contemplate it under a converse aspect. Standing alone, the above exposition conveys both an inadequate and an erroneous idea. The divisions and subdivisions of function, becoming definite as they become multiplied, do not lead to a more and more complete independence of functions ; as they would do were the process nothing beyond that just de- scribed ; but by a simultaneous process they are rendered more mutually dependent. While in one respect they are separating from each other, they are in another respect com- bining with each other. At the same time that they aro bein^ differentiated, they are also being integrated. Some illustrations will make this plain.

In animals which display little beyond the primary dif- ferentiation of functions, the activity of that part which absorbs nutriment or accumulates force, is not immediately bound up with the activity of that part which, in producing motion, expends force. In the higher animals, however, the performance of the alimentary functions depends on the per- formance of various muscular and nervous functions. Masti- cation and swallowing are nervo-muscular acts ; the ryth- mical contractions of the stomach and the allied vermicular motions of the intestines, result from the stimulation of cer- tain muscular coats by the nerve- fibres distributed through them ; the secretion of the several digestive fluids by their respective glands, is due to nervous excitation of them ; and digestion, besides requiring these special aids, is not properly performed in the absence of a continuous discharge of energy from the great nervous centres. Again, the function

of transferring nutriment or latent force, from part to part, though at first not closely connected with the other functions, eventually becomes so. The short contractile tube which propels backwards and forwards the crude dilute blood con- tained in the perivisceral cavity of an inferior mollusc, is neither structural! v nor functionallv much entangled with

r »

the creature's other organs. But on passing upwards through 8

i(>2 THE INDUCTIONS OF BIOLOGY

the higher molluscs, in which this simple tube is replaced Ly a system of branched tubes, that deliver their contents through their open ends into the tissues at distant parts ; and on coming to those advanced types of animals which have closed arterial and venous systems, ramifying minutely in every corner of every organ ; we find that the vascular apparatus, while it has become structurally interwoven with the whole body, has become unable to fulfil its office without the help of offices that are quite separated from its own. The heart is now a complex pump, worked by powerful muscles that are excited by a local nervous system ; and the general nervous system also, takes a share in regu- lating the contractions both of the heart and of all the arteries. On the due discharge of the respiratory function, too, the function of circulation is directly dependent : if the aeration of the blood is impeded, the vascular activity is lowered ; and arrest of the one very soon causes stoppage of the other. Similarly with the duties of the nervo-

muscular system. Animals of low organization, in which the differentiation and integration of the vital actions have not been carried far, will move about for a considerable time after being eviscerated, or deprived of those appliances by which force is accumulated and transferred. But animals of high organization are instantly killed by the removal of these appliances, and even by the injury of minor parts of them : a dog's movements are suddenly brought to an end, by cutting one of the main canals along which the materials that evolve movements are conveved. Thus while

t>

in well- developed creatures the distinction of functions is very marked, the combination of functions is very close. From instant to instant, the aeration of blood implies that certain respiratory muscles are being made to contract by certain nerves ; and that the heart is duly propelling the blood to be aerated. From instant to instant digestion pro- ceeds only on condition that there is a supply of aerated blood, and a due current of nervous energy through the digestive

FUNCTION. j 63

organs. That the heart may act, it must from instant to in- ettint be excited by discharges from certain ganglia ; and rho discharges from these ganglia are made possible, only by the conveyance to them, from instant to instant, of the blood which tho heart propels.

It is not easy to find an adequate expression for this double re-distribution of functions. It is not easy to realize a trans- formation through which the functions thus become in one sense separated and in another sense combined, or even in- terfused. Here, however, as before, an analogy drawn from social organization helps us. If we observe how the increas- ing division of labour in societies, is accompanied by a closer co-operation ; and how the agencies of different social actions, while becoming in one respect more distinct, become in an- other respect more minutely ramified through each other ; we shall understand better the increasing physiological co- operation that accompanies increasing physiological division of labour. Note, for example, that while local

divisions and classes of the community have been grow- ing unlike in their several occupations, the carrying on of their several occupations has been growing dependent on the due activity of that vast organization by which sus- tenance is collected and diffused. During the early stages of social development, every small group of people, and often every family, obtained separately its own necessaries ; but now, for each necessary, and for each superfluity, there ex- ists a combined body of wholesale and retail distributors, which brings its branched channels of supply within reach of all. While each citizen is pursuing a business that does not immediately aim at the satisfaction of his personal wants, his personal wants are satisfied by a general agency that brings from all places commodities for him and his fellow-citizens — an agency which could not cease its special duties for a few days, without bringing to an end his own special duties and those of most others. Consider, again, how each

of these differentiated functions is everywhere pervaded by

16-1 THE INDUCTIONS OF BIOLOGY.

certain oilier differentiated functions. Merchants, manu- facturers, wholesale distributors of their several species, to- gether with lawyers, bankers, &c., all employ clerks. In clerks we have a specialized class dispersed through various other classes ; and having its function fused with the differ- ent functions of these various other classes. Similarly commercial travellers, though having in one sense a separate occupation, have in another sense an occupation forming part of each of the many occupations which it aids. As it is here with the sociological division

of labour, so is it with the physiological division of la- bour above described. Just as we see in an advanced com- munity, that while the magisterial, the clerical, the medical, the legal, the manufacturing, and the commercial activities, have grown distinct, they have yet their agencies mingled together in every locality ; so in a developed organism, we see that while the general functions of circulation, secretion, absorption, excretion, contraction, excitation, &c., have be- come differentiated, yet through the ramifications of the sys- tems apportioned to them, they are closely combined with each other in every organ.

§ 60. The physiological division of labour, is usually not carried so far as wholly to destroy the primary physiological community of labour. As in societies the adaptation of special classes to special duties, does not entirely disable these classes from performing each others' duties on an emergency ; so in organisms, tissues and structures that have become fitted to

O 7

the particular offices they have ordinarily to discharge, often remain partially able to discharge other offices. It has been pointed out by Dr Carpenter, that " in cases where the differ ent functions are highly specialized, the general structure retains, more or less, the primitive community of function which originally characterized it." A few instances will

O »/

bring home this generalization

The roots and leaves of plants are widely differenti-

FUNCTION. 185

ated in their functions : by the roots, water and mineral substances are absorbed ; while the leaves take in. and de- compose, carbonic acid. Nevertheless, leaves retain a con- siderable power of absorbing water : and in what are popu- larly called " air-plants," the absorption of water is wholly carried on by them and bv the stems. Converselv, the under-

V •/ */ '

ground parts can partially assume the functions of leaves : the exposed tuber of a potato develops chlorophyll on its surface, and in other cases, roots, properly so called, do the like. In trees, the trunks, which have in great measure ceased to produce buds, recommence producing them if the branches are cut off; and under such circumstances the roots, though not in the habit of developing leaf-bearing organs, send up numerous suckers. Much more

various examples of vicarious function may be found among animals. Starting with the extreme case of the common hydra, which can live when the duties of skin and stomach have been interchanged by turning it inside out, we find in all grades, even up to the highest, that absorbent and excret- ing organs can partially supply each others' places. Among well-organized animals, the taking in of nutriment is ef- fected exclusively by an internal membrane ; but the external membrane is not wholly without the power to take in nutri- ment : when food cannot be swallowed, life may be pro- longed by immersing the body in nutritive fluids. The ex- cretion of carbonic acid and absorption of oxvsren, are mainly

JL v O *•

performed by the lungs, in creatures which have lungs ; but in such creatures there continues a certain amount of cutane- ous respiration, and in soft-skinned batrachians like the frog, this cutaneous respiration is important. Again, when the kidneys are not discharging their duties, a notable quantity of urea is got rid of by perspiration. Other

instances are supplied by the higher functions. In man, the limbs, which among lower vertebrates are almost wholly organs of locomotion, are specialized into organs of locomo- tion and organs of manipulation. Nevertheless, the human

166 THE INDUCTIONS OF I IOLOGY.

arms and legs do, when needful, fulfil, to some extent, each others' offices. Not only in childhood and old age are the arms used for purposes of support, but on occasions of emerg- ency, as when mountaineering, they are so used by men in full vigour. And that legs are to a considerable degree capable of performing the duties of arms, is proved by the great amount of manipulatory skill reached by them when the arms are absent. Among the perceptions, too, there are ex- amples of partial substitution. The deaf Dr Kitto described himself as having become excessively sensitive to vibrations propagated through the body ; and as so having gained the power of perceiving, through his general sensations, those neighbouring concussions of which the ears ordinarily give notice. Blind people make hearing perform, in part, the office of vision. Instead of identifying the positions and sizes of neighbouring objects by the reflection of light from their surfaces, they do this in a rude way by the reflection of sound from, their surfaces.

We see, as we might expect to see, that this power of per- forming more general functions, is great in proportion as the parts have been but little adapted to their special func- tions. In the hydra, where complete transposition of functions is possible, the histological differentiation that has been estab- lished, is extremely slight, or even inappreciable. Those parts of plants which show so considerable a power of discharging each others' offices, are not widely unlike in their minute structures. And the tissues that in animals are to some extent mutually vicarious, are tissues in which the original cellular composition is still conspicuous. But we do not find evidence that the muscular, nervous, or osseous tissues are able in any degree to perform those processes which the less differentiated tissues perform. Nor have we anv proof that nerve can partially fulfil the duty of muscle, or muscle that of nerve. We must sav, therefore, that

v *

the ability to resume the primordial community of function^.

FUNCTION. 167

varies inversely as the established specialization of function ; and that it disappears when the specialization of function becomes great*

§ 61. Something approaching to d priori reasons may be given for the conclusions thus reached d posteriori. They must be accepted for as much as they seem worth.

It may be argued that on the hypothesis of Evolution, Life necessarily comes before organization. On this hypo- thesis, organic matter in a state of homogeneous aggregation, must precede organic matter in a state of heterogeneous ag- gregation. But since the passing from a structureless state to a structured state, is itself a vital process, it follows that vital activity must have existed while there was vet no

V V

structure : structure could not else arise. That

function takes precedence of structure, seems also implied in the definition of Life. If Life consists of inner actions so adjusted as to balance outer actions — if the actions are the substance of Life, while the adjustment of them constitutes its form ; then, may we not say that the actions to be formed must come before that which forms them — that the continu- ous change which is the basis of function, must come before the structure which brings function into shape ? Or

again, since throughout all phases of Life up to the highest, every advance is the effecting of some better adjustment of inner to outer actions; and since the accompanying new com- plexity of structure is simply a means of making possible this better adjustment ; it follows that function is from beginning to end the determining cause of structure. IXot only is this manifestly true where the modification of struc- ture arises bv reaction from modification of function : but it

t,

is also true where a modification of structure otherwise pro- duced, apparently initiates a modification of fiuiction. For it is only when such so-called spontaneous moditication of structure subserves some advantageous action, that it is per«

168 THE INDUCTIONS OF BIOLOGY.

manently established : if it is a structural' modification that happens to facilitate the vital activities, "natural selection ' retains and increases it ; but if not, it disappears.

The connexion which we noted between heterogeneity of structure and heterogeneity of function — a connexion made so familiar by experience as to appear scarcely worth specifying — is clearly a necessary one. It follows from the general truth that in proportion to the heterogeneity of any aggregate, is the heterogeneity it will produce in any inci- dent force (First Principles, § 116). The force continually liberated in the organism by decomposition, is here the inci- dent force; the functions are the variously modified forms produced in its divisions by the organs they pass through ; and the more multiform the organs the more multiform must be the differentiations of the force passing through them.

It follows obviously from this, that if structure progresses from the homogeneous, indefinite, and incoherent, to the heterogeneous, definite, and coherent, so too must function. If the number of different parts in an aggregate must deter- mine the number of differentiations produced in the forces passing through it — if the distinctness of these parts from each other, must involve distinctness in their reactions, and there- fore distinctness between the divisions of the differentiated force ; there cannot but be a complete parallelism between the development of structure and the development of func- tion. If structure advances from the simple and general to the complex and special, function must do the same.

CHAPTER IV.

WASTE AND EEPAIR.

§ 62. THROUGHOUT the vegetal kingdom, the processes of Waste and Repair are comparatively insignificant in their amounts. Though plants, and especially certain parts of them, do, in the absence of light or under particular con- ditions, give out carbonic acid ; yet this carbonic acid, assuming it to indicate consumption of tissue, indicates but a small consumption. Of course if there is little waste, there can be but little repair — that is, little of the interstitial repair which restores the integrity of parts worn by functional acti- vity. Nor, indeed, is there displayed by plants in any con- siderable degree, if at all, that other species of repair whiuh consists in the restoration of lost or injured organs. Torn leaves and the shoots that are shortened by the pruner, do not reproduce their missing parts ; and though when the branch of a tree is cut off close to the trunk, the place is in the course of years covered over, it is not by any reparative action in the wounded surface, but by the lateral growth of the adjacent bark. Hence, without saying that Waste and Repair do not go on at all in plants, we may fitly pass them over as of no importance.

There are but slight indications of waste in those lower orders of animals which, by their comparative inactivity, show themselves least removed from vegetal life. Actinias knpt in an aquarium, do not appreciably di:mni>h in bulk

170 THE INDUCTIONS OF BIOLOGY.

from prolonged abstinence. Even fish, though much more active than most other aquatic creatures, appear to undergo but little loss of substance when kept unfed during con- siderable periods. Reptiles, too, maintaining no great temperature, and passing their lives mostly in a stute of torpor, suffer but little diminution of mass by waste. When, however, we turn to those higher orders of animals which are active and hot-blooded, we see that waste is rapid : producing when unchecked, a notable decrease in bulk and weight, ending very shortly in death. Besides

finding that waste is inconsiderable in creatures that pro- duce but little insensible and sensible motion, and that it becomes conspicuous in creatures that produce much insen- sible and sensible motion ; we find that in the same crea- tures there is most waste when most motion is generated. This is clearly proved by hybernating animals. " Ya- lentin found that the waking marmot excreted in the average 75 times more carbonic acid, and inhaled 41 times more oxygen than the same animal in the most complete state of hybernation. The stages between waking and most pro- found hybernation yielded intermediate figures. A waking hedgehog yielded about 20 '5 times more carbonic acid, and consumed 1S'4 times more oxygen than one in the state of hy- bernation." If we take these quantities of absorbed oxygen and excreted carbonic acid, as indicating something like the relative amounts of consumed organic substance, we see that there is a striking contrast between the waste ac- companying the ordinary state of activity, and the waste accompanying complete quiescence and reduced temperature. This difference is still more definitely shown by the fact, that the mean daily loss from starvation in rabbits and guinea-pigs, bears to that from hybernation, the proportion of 18 '3 : 1. Among men and domestic animals, the relation between degree of waste and amount of expended force, though one respecting which there is little doubt, is less distinctly demonstrable ; since waste is not allowed to go on

WASTE AND REPAIR. 171

uninterfered with. We have however in the lingering lives of invalids who are able to take scarcely any nutriment, but are kept warm, and still, an illustration of the extent to which waste diminishes as the expenditure of force declines. Besides the connexion between the waste of the organism as a whole, and the production of sensible and insensible motion by the organism as a whole ; there is a traceable connexion between the waste of special parts and the activi- ties of such special parts. Experiments have shown that " the starving pigeon daily consumes in the average 40 times more muscular substance than the marmot in the state of torpor, and only 11 times more fat, 33 times more of the tissue of the alimentary canal, 18'3 times more liver, 15 times more lung, 5 times more skin." That is to say, in the hybernating animal the parts least consumed are the almost totally quiescent motor-organs, and the part most consumed is the hydro-carbonaceous deposit serving as a store of force ; whereas in the pigeon, similarly unsupplied with food but awake and active, the greatest loss takes place in the motor-organs. The relation between special

activity and special waste, is illustrated too in the daily experiences of all : not indeed in the measurable decrease of the active parts in bulk or weight, for this we have no means of ascertaining ; but in the diminished ability of such parts to perform their functions. That legs exerted for many hours in walking, and arms long strained in rowing, lose their powers — that eyes become enfeebled by reading or writing without intermission — that concentrated attention unbroken by rest, so prostrates the brain as to incapacitate it for think- ing ; are familiar truths. And though we have no direct evidence to this effect, there is little danger in concluding that

muscles exercised until thev ache or become stiff, and nerves

•j

of sense rendered weary or obtuse by work, are organs so much wasted by action as to be partially incompetent.

Repair is everywhere and always making up for waste Though the two processes vary in their relative rates, both

172 THE INDUCTIONS OF BIOLOGY

are constantly going on. Though during the active, waking state of an animal, waste is in excess of repair, yet repair is in progress ; and though during sleep, repair is in excess of waste, yet some waste is necessitated by the carrying on of certain never-ceasing functions. The organs of these never- ceasing functions furnish, indeed, the most conclusive proofs of the simultaneity of repair and waste. Day and night the heart never stops beating, but only varies in the rapidity and vigour of its beats ; and hence the loss of substance which its contractions from moment to moment entail, must from moment to moment be made good. Day and night the lungs dilate and collapse ; and the muscles which make them do this, must therefore be ever kept in a state of integ- rity by a repair which keeps pace with waste, or which alternately falls behind and gets in advance of it to a very slight extent.

On a survey of the facts, we see, as we might expect to see, that repair is most rapid when activity is most reduced. Assuming that the organs which absorb and circulate nutri- ment are in proper order, the restoration of the organism to a state of integrity, after the disintegration consequent on expenditure of force, is proportionate to the diminution in expenditure of force. Thus we all know that those who are in health, feel the greatest return of vigour after profound sleep — after complete cessation of motion. "We know that a night during which the quiescence, bodily and mental, has been less decided, is usually not followed by that spontaneous overflow of energy that indicates a high state of efficiency throughout the organism. We know, again, that long- continued recumbency, even with wakeful- ness (providing the wakefulness is not the result of disorder), is followed by a certain renewal of strength ; though a re- newal less than that which would have followed the greater inactivity of slumber. "We know, too, that when exhausted by labour, sitting brings a partial return of vigour. And we also know that after the violent exertion of running.

O"

WASTE AND REPAIR 173

a lapse into the less violent exertion of walking, results in a gradual disappearance of that prostration which the run- ning produced. This series of illustrations conclusively proves that the rebuilding of the organism is ever making up for the pulling down of it caused by action ; and that the effect of this rebuilding becomes manifest, in proportion as the pulling down is less rapid. From each digested meal, there is every few hours absorbed into the mass of prepared nutriment circulating through the body, a fresh supply of the needful organic compounds ; and from the blood thus occasionally re-enriched, the organs through which it passes are ever taking up materials to replace the materials used up in the discharge of functions. During activity, the reinte- gration falls in arrear of the disintegration ; until, as a conse- quence, there presently comes a general state of functional languor ; ending, at length, in a quiescence which permits the reintegration to exceed the disintegration, and restore the parts to their state of integrity. Here, as wherever there are antagonistic actions, we see rhythmical divergences on opposite sides of the medium state — changes which equilibrate each other by their alternate excesses. (First Principles, §§ 96, 133.)

Illustrations are not wanting of special repair, that is similarly ever in progress, and similarly has intervals during which it falls below waste and rises above it, Everv one

*

knows that a muscle, or a set of muscles, continuously strain- ed, as bv holding out a weight at arnrs length . soon loses its

«. C7 G

power ; and that it recovers its power more or less fully after a short rest. The several organs of special sensation yield us like experiences : strong tastes, powerful odours, and loud sounds, temporarily unfit the nerves impressed by them, for appreciating faint tastes, odours, or sounds ; but these inca- pacities are remedied by brief intervals of repose. Vision still better illustrates this simultaneity of waste and repair Looking at the sun so affects the eye that, for a short time, it cannot perceive the ordinary contrasts of light and shade.

174 THE INDUCTIONS OF BIOLOGY.

After gazing at a bright light of a particular colour, we see on turning the eyes to adjacent objects, an image of the complementary colour; showing that the retina has, for the moment, lost the power to feel small amounts of those rays which have strongly affected it. Such inabilities disappear in a few seconds or a few minutes, according to circumstances. And here, indeed, we are introduced to a conclusive proof that special repair is ever neutralizing special waste. For the rapidity with which the eyes recover their sensitiveness, varies with the reparative power of the individual. In youth, the visual apparatus is so quickly restored to its state of in- tegrity, that many of these photogenes, as they are called, cannot be perceived. When sitting on the far side of a room, and gazing out of the window against a light sky, a person who is debilitated by disease or advancing years, perceives, on transferring the gaze to the adjacent wall, a momentary negative image of the window — the sash-bars appearing lighfc and the squares dark ; but a young and healthy person has no such experience. With a rich blood and vigorous circu- lation, the repair of the visual nerves after impressions of moderate intensity, is nearly instantaneous.

Function carried to excess, may produce waste so great, that repair cannot make up for it during the ordinary daily periods of rest ; and there may result incapacities of the overtaxed organs, lasting for considerable periods. We know that eyes strained by long-continued minute work, lose their power for months or years : perhaps suffering an injury which they never wholly recover. Brains, too, are often so unduly worked that permanent relaxation fails to restore them to vigour. Even of the motor organs the like holds. The most frequent cause of what is called " wasting palsy,'*' or atrophy of the muscles, is habitual excess of exertion : the proof being, that the disease occurs most frequently among those engaged in laborious handicrafts, and usually attacks 6rst the muscles that have been most worked.

There has yet to be noticed another kind of repair ; — that

WASTE AND REPAIR. 175

namely, by which, injured or lost parts are restored. Among the Hydrozoa it is common for any portion of the body to re- produce the rest ; even though the rest to be so reproduced is the greater part of the whole. In the more highly-organ- ized Actinozoa, the half of an individual will grow into a complete individual. Some of the lower Annelids, as the Neds, may be cut into thirty or forty pieces, and each piece will eventually become a perfect animal. As we ascend to higher forms, we find this reparative power much diminished, though still considerable. The reproduction of a lost claw by a lobster or crab, is a familiar instance. Some of the inferior Vertebrata also, as lizards, can develop new limbs or new tails, in place of those that have been cut off ; and can even do this several times over, though with decreasing complete- ness. The highest animals, however, thus repair themselves to but a very small extent. Mammals and birds do it only in the healing of wounds ; and very often but imperfectly even in this. For in muscular and glandular organs, the tissues destroyed are not properly reproduced, but are re- placed by tissue of an irregular kind, which serves to hold the-parts together. So that the power of reproducing lost parts is greatest where the organization is lowest ; and almost dis- appears where the organization is highest. And though we cannot say that between these extremes there is a constant in- verse relation between reparative power and degree of organ- ization ; yet we may say that there is some approach to such a relation.

§ 63. There is a very obvious and complete harmony be- tween the first of the above inductions, and the deduction that follows immediately from first principles. We have already seen (§ 23) " that whatever amount of power an organism expends in any shape, is the correlate and equi- valent of a power that was taken into it from without." Motion, sensible or insensible, generated by an organism, is insensible motion which was absorbed in producing certain

176 THE INDUCTIONS OF BIOLOGY.

chemical compounds appropriated by trie organism under the form of food. As much power as was required to raise the elements of these complex atoms to their state of unsta- ble equilibrium, is given out in their falls to a state of stable equilibrium ; and having fallen to a state of stable equilib- rium, they can give out no further power, but have to be got rid of as inert and useless. It is an inevitable corollary " from the persistence of force, that each portion of mechanical or other energy which an organism exerts, implies the trans- formation of as much organic matter as contained this energy in a latent state;'3 and that this organic matter in yielding up its latent energy, loses its value for the purposes of life, and becomes waste matter needing to be excreted. The loss of these complex unstable substances must hence be proportionate to the quantity of expended force. Here then is the rationale of certain general facts lately indicated. Plants do not waste to any considerable degree, for the obvi- ous reason that the sensible and insensible motions they generate are inconsiderable. Between the small waste, small activity, and low temperature of the inferior animals, the rela- tion is similarly one admitting of a priori establishment. Con- versely, the rapid waste of energetic, hot-blooded animals might be foreseen with equal certainty. And not less mani- festly necessary is the variation in waste which, in the same organism, attends the variation in the heat and mechanical motion produced.

Between the activity of a special part and the waste of that part, a like relation may be deductively inferred ; though it cannot be inferred that this relation is equally defi- nite. Were the activity of every organ quite independent of the activities of other organs, we might expect to trace out this relation distinctly ; but since one part of the force which any organ expends, is derived from materials brought to it by the blood from moment to moment in quantities varying with the demand, and since another part of the force which such organ expends, comes to it in the shape of

AND REPAIR. 177

nervous discharges from distant organs ; it is clear that spe- cial waste and general waste are too much, entangled to admit of a definite relation being established between special waste and special activity. We may fairly say, however, that this relation is quite as manifest as we can reasonably anticipate.

§ 64. Deductive interpretation of the phenomena of Re- pair, is by no means so easy. The tendency displayed by an animal organism, as well as by each of its organs, to return to a state of integrity bv the assimilation of new matter,

V V

when it has undergone the waste consequent on activity, is a tendency which is not manifestly deducible from first princi- ples ; though it appears to be in harmony with them. If in the blood there existed ready-formed units exactly like- in kind to those of which each organ consists, the sorting of these units, ending in the union of each kind with already existing groups of the same land, would be merely a good example of Differentiation and Integration (First Principles, § 123). It would be analogous to the process by which, from a mixed solution of salts, there are deposited segregated masses of these salts, in the shape of different crystals. But as already said (§ 54), though the selective assimilation by which the repair of organs is effected, no doubt results in part from an action of this kind, which is consequent on the persistence of force (First Principles, § 129), the facts cannot be thus wholly accounted for ; since organs are in part made up of units that do not exist as such in the circulating fruidu. The pro- cess becomes comprehensible however, if it be shown that, as suggested in § 54, groups of compound units have a certain power of moulding adjacent fit materials into units of their own form*. Let us see whether there is not reason to think such a power exists.

" The poison of small-pox or of scarlatina," remarks ]\ir Paget, " being once added to the blood, presently affects th*j composition of the whole : the disease pursues its course,

178 THE INDUCTIONS OF BIOLOGY.

and, if recovery ensue, the blood will seem to have returned to its previous condition : yet it is not as it was before ; for now the same poison may be added to it with impunity." * " The change once effected, may be maintained through life. And herein seems to be a proof of the assimil- ative force in the blood ; for there seems no other mode of explaining these cases than by admitting that the altered particles have the power of assimilating to themselves all those by which they are being replaced : in other words, all the blood that is formed after such a disease deviates from the natural composition, so far as to acquire the peculiarity engendered by the disease : it is formed according- to the

Cj «/ cj

altered model." Now if the compound molecules of the blood, or of an organism considered in the aggregate, have the power of moulding into their own type, the matters winch they absorb as nutriment ; and if, as Mr Pa get points out, they have the power when their type has been changed by disease, of moulding all materials afterwards received into the modified type ; may we not reasonably suspect that the more or less specialized molecules of each organ, have, in like manner, the power of moulding the materials which the blood brings to them, into similarly specialized molecules ? The one conclusion seems to be a corollary from the other.- Such a power cannot be claimed for the component units of the blood, without being con- reded to the component units of every tissue. Indeed the assertion of this power is little more than an assertion of the fact, that organs composed of specialized units are capable of resuming their structural integrity, after they have been wasted by function. For if they do this, they must do it by forming from the materials brought to them, certain special- ized units like in kind to those of which they are composed , and to say that they do this, is to say that their component units have the power of moulding tit materials into othei units of the same order.

The repair of a wasted tissue may therefore be considered

WASTE AND REPAIR. 179

as due to forces analogous to those by which a crystal repro- duces its lost apex, when placed in a solution like that from which it was formed. In either case, a mass of units of a given kind, shows a power of integrating with itself diffused units of the same kind : the only difference being, that the organic mass of units arranges the diffused units into special compound forms, before integrating them with itself. In the case of the crystal, this reintegration is ascribed to polarity — a power of whose nature we know nothing. What- ever be its nature, however, it appears probable that the power by which organs repair themselves from the nutritive matters circulating through them, is of the same order.

§ Go. That other kind of repair which shows itself in the regeneration of lost members, is comprehensible only as an e fleet of actions like those just referred to. The ability of an organism to recomplete itself when one of its parts has been cut off, is of the same order as the ability of an injured crystal to recomplete itself. In either case, the newly^ assimi- lated matter is so deposited as to restore the original outline. And if in the case of the crystal, we say that the whole aggregate exerts over its parts, a force which constrains the newly-integrated atoms to take a certain definite form ; we must in the case of the organism, assume an analogous force. This is, in truth, not an hypothesis : it is nothing more than a generalized expression of the facts. If when the leg of a lizard has been amputated, there presently buds out the germ of a new one, which, passing through phases of development like those of the original leg, eventually assumes a like shape and structure ; we assert nothing more than what we see, when we assert that the organism as a whole exercises such power over the newly- forming limb, as makes it a repetition of its predecessor. If a leg is reproduced where there was a leg, and a tail where there was a tail ; we have no alternative but to conclude that the aggregate forces of the body, con- trol the formative processes going on in each part. And on

180 THE INDUCTIONS OF BIOLOGY.

contemplating these facts in connexion with various kindred ones, there is suggested the hypothesis, that the form of each species of organism is determined by a peculiarity in the con- stitution of its units — that these have a special structure in which they tend to arrange themselves ; just as have the simpler units of inorganic matter. Let us glance at the evi- dences which more especially thrust this conclusion upon us.

A fragment of a Begonia-leaf, imbedded in fit soil and kept at an appropriate temperature, will develop a young Bego- nia ; and so small is the fragment which is thus capable of originating a complete plant, that something like a hundred plants might be produced from a single leaf. The friend to whom I owe this observation, tells me that various succulent plants have like powers of multiplication. Illustrating a similar po\ver among animals, we have the often-cited exper- iments of Trembley on the common polype. Each of the four pieces into which one of these creatures was cut, grew into a perfect individual. In each of these again, bisection and tri-section effected a like result. And so with their segments, similarly produced, until as many as fifty polypes had resulted from the original one. Bodies when cut off regenerated heads : heads regenerated bodies ; and when a

O 7 O *

polype had been divided into as many pieces as was practica- ble, nearly every piece survived and became a complete animal. What, now, is the implication ? We

cannot say that in each portion of a Begonia-leaf, and in every fragment of a Hydra's bod}*, there exists a ready- formed model of the entire organism. Even were there warrant for the now abandoned doctrine, that the germ of every organism contains the perfect organism in miniature, it still could not be contended that each considerable part of the perfect organism resulting from such a germ, contains another such miniature. Indeed the one hypothesis obviously nega- tives the other. We have therefore no alternative but to Kay, that the living particles composing one of these frag- ments, have an innate tendency to arrange themselves into

WASTE AND REPAIR. 181

the shape of the organism to which they belong. We must infer that a plant or animal of any species, is made up of special units, in all of which there dwells the intrinsic apti- tude to aggregate into the form of that species : just as in the atoms of a salt, there dwells the intrinsic aptitude to crystal- lize in a particular way. It seems difficult to conceive that this can be so ; but we see that it is so. Groups of units taken from an organism (providing they are of a certain bulk and not much differentiated into special structures) have this power of re-arranging themselves ; and we are thus compelled to recognize the tendency to assume the specific form, as inherent in all parts of the organism. Mani-

festly too, if we are thus to interpret the reproduction of an organism from one of its amorphous fragments, we must thus interpret the reproduction of any minor portion of an organism by the remainder. When in place of its lost claw, a lobster puts forth from the same spot a cellular mass, which, while increasing in bulk, assumes the form and structure of the original claw ; we can have no

hesitation in ascribing this result to a plav of forces like

j. i

that which moulds the materials contained in a piece of Begonia-leaf into the shape of a young Begonia. In the one case as in the other, the vitalized molecules composing the tissues, show their proclivity towards a particular arrange- ment ; and whether such proclivity is exhibited in repro- ducing the entire form, or in completing it when rendered imperfect, matters not.

For this property there is no fit term. If we accept the word polarity, as a name for the force by which inorganic units are aggregated into a form peculiar to them ; we may apply this word to the analogous force displayed by organic units. But, as above admitted, polarity, as ascribed to atoms, is but a name for something of which we are ignorant— a

o o

name for a hypothetical property which as much needs ex- planation as that which it is used to explain. Nevertheless, in default of another word, we must employ this : taking

182 THE INDUCTIONS OF BIOLOGY.

care, however, to restrict its meaning. If we simply substi- tute the term polarity, for the circuitous expression — the power which certain units have of arranging themselves into a special form, we may, without assuming anything more than is proved, use the term organic polarity or po- larity of the organic units, to signify the proximate cause of the ability which organisms display of reproducing lost parts.

§ 66. As we shall have frequent occasion hereafter to refer to these units, which possess the property of arranging themselves into the special structures of the organisms to which they belong ; it will be well here to ask what these units are, and by what name they may be most fitly called.

On the one hand, it cannot be in those proximate chemical compounds composing organic bodies, that this specific polar- ity dwells. It cannot be that the atoms of albumen, or fibrine, or gelatine, or the hypothetical protein-substance, possess this power of aggregating into specific shapes ; for in such case, there would be nothing to account for the unlike- nesses of different organisms. Millions of species of plants and animals, more or less contrasted in their structures, are all mainly built up of these complex atoms. But if the polarities of these atoms determined the forms of the or- ganisms they composed, the occurrence of such endlessly varied forms would be inexplicable. Hence, what we may call the chemical units, are clearly not the possessors of this property.

On the other hand, this property cannot reside in what may be roughly distinguished as the morphological writs. The g-erm of every organism is a microscopic cell. It is by multiplication of cells that all the early developmental changes tire effected. The various tissues which successively arise in the unfolding organism, are primarily cellular ; and in many of them the formation of cells continues to be, through-

WASTE AND REPAIR. 1£3

out life, the process by which repair is carried on. But though cells are so generally the ultimate visible components of organisms, that they may with some show of reason be called the morphological units ; yet, as they are not uni- versal, we cannot say that this tendency to aggregate into specified forms dwells in them. Finding that in many cases a fibrous tissue arises out of a structureless blastema, without cell- formation ; and finding that there are creatures, such as Rhizopods, which are not cellular, but nevertheless exhibit vital activities, and perpetuate in their progeny certain specific distinctions ; we are forbidden to ascribe to cells this peculiar power of arrangement. Xor, indeed, were cells universal, would such an hypothesis be acceptable ; since the formation of a cell is, to some extent a manifesta- tion of this same peculiar power.

If, then, this organic polarity can be possessed neither b} the chemical units nor the morphological units, we must conceive it as possessed by certain intermediate units, which we may term physiological. There seems no alternative but to suppose, that the chemical units combine into units immensely more complex than themselves, complex as they are ; and that in each organism, the physiological units produced by this further compounding of highly compound atoms, have a more or less distinctive character. TTe must conclude that in each case, some slight difference of com- position in these units, leading to some slight difference in their mutual play of forces, produces a difference in the form which the aggregate of them assumes.

The facts contained in this chapter, form but a small part of the evidence which thrusts this assumption upon us. TTe shall hereafter find various reasons for inferring that such physiological units exist, and that to their specific properties, more or less unlike in each plant and animal, various organic phenomena are due.

CHAPTER V.

ADAPTATION.

§ 67. IN plants, waste and repair being scarcely appre- ciable, there are not likely to arise appreciable changes in the proportions of already- formed parts. The only divergences from the average structure of a species, which we may expect particular conditions to produce, are those producible by the action of these conditions on parts in course of formation ; and such divergences we do find. We know that a tree which, standing alone in an exposed position, has a short and thick stem, has a tall and slender stem when it grows in a wood ; and that its branches then take a diiferent inclin- ation. AVe know that potato-sprouts which, on reaching the light, develop into foliage, will, in the absence of light, grow to a length of several feet without foliage. And every in-door plant furnishes proof, that shoots and leaves, by habitually turning themselves to the light, exhibit a certain adaptation — an adaptation due, as we must suppose, to the special effects of the special conditions on the still grow- ing parts. In animals, however, besides analogous structural changes wrought during the period of growth, by subjection to circumstances unlike the ordinary circum- stances ; there are structural changes similarly wrought, after maturity has been reached. Organs that have arrived at their full size, possess a certain modifiability ; BO that white the organism as a whole, retains pretty

ADAPTATION. 185

nearly the same bulk, the proportions of its parts may be considerably varied. Their variations, here treated of under the title Adaptation, depend on specialities of individual action. We saw in the last chapter, that the actions of or- ganisms entail re-actions on them ; and that specialities of action entail specialities of re-action. Here it remains to be pointed out, that the special actions and re-actions do not end with temporary changes, but work permanent changes.

If, in an adult animal, the waste and repair in all parts were exactly balanced — if each organ daily gained by nutrition, exactly as much as it lost daily by the discharge of its function — if excess of function were followed only by sucn excess of nutrition as balanced the extra waste ; it is clear that there would occur no change in the relative sizes of organs. But there is no such exact balance. If the excess of function, and consequent excess of waste, is moderate, it is not simply compensated by repair, but more than compensated — there is a certain increase of bulk. This is true to some decree of the organism as a whole, when the organism is

o o * o

framed for activity. A considerable waste giving considerable power of assimilation, is more favourable to accumulation of tissue, than is quiescence with its comparatively feeble assimi- lation : whence results a certain adaptation of the whole organism to its requirements. But it is more especially true of the parts of an organism in relation to each other. The illustrations fall into several groups. The growth

of muscles exercised to an unusual degree, is a matter of com- mon observation. In the often-cited blacksmith's arm, the dancer's legs, and the jockey's crural adductors, we have marked examples of a modifiability which almost every one has to some extent experienced. It is needless to multi- ply proofs. The occurrence of changes in the struc- ture of the skin, where the skin is exposed to a stress of function, is also familiar. That thickening of the epidermis on a labourer's palm, results from continual pressure and friction, is certain : those who have not before exerted their 9

186 THE INDUCTIONS OF BIOLOGY.

hands, find that such an exercise as rowing, soon begins to produce a like thickening1. This relation of cause and effect is still better shown by the marked indurations at the ends of

f

a violinist's fingers. Even in mucous membrane, which, ordinarily is not subject to mechanical forces of any intensity, similar modifications are possible : witness the callosity of the gums which arises in those who have lost their teeth, and have to masticate without teeth. The vascular

svstem furnishes good instances of the increased growth

v O O

that follows increased function. When, because of some permanent obstruction to the circulation, the heart has to exert a greater contractile force on the mass of blood which it propels at each pulsation into the arteries, and when there re- sults the laboured action known as palpitation ; there usually occurs dilatation, or hypertrophy, or a mixture of the two : the dilatation, which is a yielding of the heart's structure under the increased strain, implying a failure to meet the emergency ; but the hypertrophy, which consists in a thick- ening of the heart's muscular walls, being an adaptation of it to the additional effort required. Again, when an aneurism in some considerable artery has been obliterated, either arti- ficially or by a natural inflammatory process ; and when this artery has consequently ceased to be a channel for the blood ; some of the adjacent arteries which anastomose with it, become enlarged, so as to carry the needful quantity of blood to the parts supplied. Though we have no direct

proof of analogous modifications in nervous structures ; yet indirect proof is given by the greater efficiency that fol- lows greater activity. This is manifested alike in the senses and the intellect. The palate may be cultivated in- fo extreme sensitiveness, as in professional tea-tasters. An orchestral conductor gains by continual practice, an unusually great ability to discriminate differences of sound. And in the finger-reading of the blind, we have evidence that the sense of touch may be brought by exercise to a far higher capability than is ordinary. The increase of power which

ADAPTATION. 187

habitual exertion gives to mental faculties, needs no illustra- tion : every person of education has personal experience of it. Even from the osseous structures, evidence

may be drawn. The bones of men accustomed to great mus- cular action, are more massive and have more strongly marked processes for the attachment of muscles, than the bones of men who lead sedentary lives ; and a like contrast holds between the bones of wild and tame animals of the same species. Adaptations of another order, in which there is a qualitative rather than a quantitative modification, arise after certain accidents to which the skeleton is liable. When the hip-joint has been dislocated, and long delay has made it impossible to restore the parts to their proper places, the head of the thigh-bone, imbedded in the surrounding muscles, becomes fixed in its new position by attachments of fibrous tissue, which afford support enough to permit a halting walk. But the most remarkable modification of this order occurs in ununited fractures. "False joints'- are often formed — • joints which rudely simulate the hinge structure or the ball- and-socket structure, according as the muscles tend to pro- duce a motion of flexion and extension or a motion of rota- tion. In the one case, according to Rokitansky, the two ends of the broken bone become smooth and covered with perios- teum and fibrous tissue, and are attached by ligaments that allow a certain backward and forward motion; and in the other case, the ends, similarly clothed with the appropriate membranes, become the one convex and the other concave, are inclosed in a capsule, and are even occasionally supplied with synovial fluid !

The general truth that extra function is followed by extra growth, must be supplemented by the equally general truth, that beyond a limit, iisually soon reached, very little, if any, further modification can be produced. The experiences from which wre draw the one induction thrust the other upon us. After a time, no training makes the pugilist or the athlete any stronger. The adult gymnast at last acquires the power

1 88 THE INDUCTIONS OF BIOLOGY.

to perform certain difficult feats ; but certain more difficult feats, no additional practice enables him to perform. Years of discipline givre the singer a particular loudness and range of voice, beyond which further discipline does not give greater loudness or wider range : on the contrary, increased vocal ex- ercise, causing a waste in excess of repair, is often followed by decrease of power. In the perceptions we see

similar limits. The culture which exalts the susceptibility of the ear to the intervals and harmonies of notes, will not turn a bad ear into a good one. Life-long effort fails to make this artist a correct draftsman, or that a fine colourist : each does better than he did at first, but each falls short of the power attained by some other artists. Nor is

this truth less clearly illustrated among the more complex mental powers. Each man has a mathematical faculty, a poetical faculty, or an oratorical faculty, which special educa- tion improves to a certain extent. But unless he is unusually endowed in one of these directions, no amount of education will make him a first-rate mathematician, a first-rate poet, or a first-rate orator. Thus the general fact appears to

be, that while in each individual, certain changes in the proportions of parts, may be caused by variations of function, the congenital structure of each individual puts a limit to the rnodifiability of every part. Nor is this true of

individuals only : it holds, in a sense, of species. Leaving open the question whether, in indefinite time, indefinite modi- fication may not be produced ; experience proves that within assigned times, the changes wrought in races of organisms by changes of conditions fall within narrow limits. We see, for instance, that though by discipline, aided by selective breeding, one variety of horse has had its locomotive power increased considerably beyond the locomotive powers of otheff varieties ; yet that further increase takes place, if at all, at an inappreciable rate. The different kinds of dogs, too, in which different forms and capacities have been established, do not show aptitudes for diverging in the same directions %t

ADAPTATION. 189

considerable rates. In domestic animals generally, certain accessions of intelligence have been produced by culture ; but accessions beyond these are inconspicuous. It seems that in each species of organism, there is a margin for functional oscillations on all sides of a mean state, and a consequent margin of structural variations ; that it is possible rapidly to push functional and structural changes towards the extreme of this margin in any direction, both in an individual and in a race ; but that to push these changes further in any direction, and so to alter the organism as to bring its mean state up to the extreme of the margin in that direction, is a comparatively slow process.*

We have also to note that the limited increase of size pro- duced in any organ by a limited increase of its function, is not maintained unless the increase of function is permanent. A mature man or other animal, led by circumstances into exerting particular members in unusual degrees, and acquir- ing extra- size and power in these members, begins to lose such extra size and power on ceasing to exert these members ; and eventually lapses more or less nearly into the original state. Legs strengthened by a pedestrian tour, become weak again after a prolonged return to sedentary life. The acquired ability to perform feats of skill, disappears in course of time, if the performance of them is given up. For compara- tive failure in executing a piece of music, in playing a game at chess, or in anything requiring special culture, the being out of practice is a reason of which every one recognizes the validity. It is observable, too, that the rapidity and com- pleteness with which an artificial power is lost, is proportionate to the shortness of the cultivation which evoked it. One who has for many years persevered in habits which exercise special muscles or special faculties of mind, retains the extra

* Here, as in sundry places throughout this chapter, the necessities of the argu- ment have obliged me to forestall myself, by assuming the conclusion reached in a \ subsequent chapter, that modifications of structure produced by modifications of function, are transmitted to offspring.

190 THE INDUCTIONS OF BIOLOGY.

capacity produced, to a very considerable degree, even after a long period of desistancc ; but one who has persevered in such habits for but a short time, has, at the end of a like period, scarcely any of the facility he had gained. Here,

1oo, as before, successions of organisms present an analogous fact. A species in which domestication, continued through many generations, has organized certain peculiarities ; and which afterwards, escaping domestic discipline, returns to something like its original habits ; soon loses, in great mea- sure, such peculiarities. Though it is not true, as alleged, that it resumes completely the etructure it had before domes- tication ; yet it approximates to that structure. The Dingo, or wild dog of Australia, is one of the instances given of this ; and the wild horse of South America is another. Mankind, too, supplies us with instances. In the Austra- lian bush, and in the backwoods of America, the Anglo- Saxon race, in which civilization has developed the higher feelings to a considerable degree, rapidly lapses into compara- tive barbarism : adopting the moral code, and sometimes the habits, of savages.

§ 68. It is important to reach, if possible, some rationale of these general truths — especially of the last two. A right understanding of these laws of organic modification, underlies a right understanding of the great question of species. While, as before hinted (§ 40), the action of structure on function, is one of the factors in that process of differentiation by which unlike forms of plants and animals are produced, the re-action of function on structure, is another factor. Hence, it is well worth while inquiring how far these induc- tions are deductively interpretable.

The first of them is the most difficult to deal with. Why an organ exerted somewhat beyond its wont, should presently grow, and thus meet increase of demand by increase of sup- ply, is not obvious. We know, indeed, (First Principles, § 96, 133,) that of necessity, the rhythmical changes pro-

ADAPTATION.

duced by antagonist organic actions, cannot any of them be. carried to an excess in one direction, without there being produced an equivalent excess in the opposite direction. It is a corollary from the persistence offeree, that any deviation effected by a disturbing cause, acting on some member of a moving equilibrium, must (unless it altogether destroys the moving equilibrium) be eventually followed by a compensating deviation. Hence, that excess of repair should succeed ex- cess of waste, is to be expected. But how happens the mean state of the organ to be changed ? If daily extra waste naturally brings about daily extra repair, only to an equiva- lent extent, the mean state of the organ should remain con- stant. How then comes the organ to augment in size and power ?

Such answer to this question as we may hope to find, must be looked for in the effects wrought on the organism as a whole, by increased function in one of its parts. For since the discharge of its function by any part, is possible only on condition that those various other functions on which its own is immediately dependent, are also discharged ; it follows that excess in its function presupposes some excess in their functions. Additional work given to a muscle, implies ad- ditional work given to the branch arteries which bring it blood, and additional work, smaller in proportion, to the arteries from which these branch arteries come. Similarly, the smaller and larger veins which take away the blood, as well as the absorbents which carry off effete products, must have more to do. And yet further, on the nervous centres which excite the muscle, a certain extra duty must fall. But excess of waste will entail excess of repair, in these parts as well as in the muscle. The several appliances by which the nutrition and excitation of an organ are carried on, must also be influenced bv this rhythm of action and re-action : and

v &/

therefore, after losing more than usual by the destructive process, they must gain more than usual by the constructive process. But temporarily-increased efficiency in these ap-

192 THE INDUCTIONS OF BIOLOGY.

pliances by which blood and nervous force are brought to an organ, will cause extra assimilation in the organ, beyond that required to balance its extra expenditure. Regarding the functions as constituting a moving equilibrium, we may say, that divergence of any function in the direction of in- crease, causes the functions with which it is bound up to diverge in the same direction ; that these again cause the functions which they are bound up with, also to diverge in the same direction ; and that theso divergences of the con- nected functions, allow the specially-affected function to be carried further in this direction than it could otherwise be — further than the perturbing force could carry it if it had a fixed basis.

It must be admitted that this is but a vague explanation. Among actions so involved as these, we can scarcely expect to do more than dimly discern a harmony with first princi- ples. That the facts are to be interpreted in some such way, may, however, be inferred from the circumstance that an extra supply of blood continues for some time to be sent to an organ that has been unusually exercised ; and that when unusual exercise is long continued, a permanent increase of vascularity results.

§ 63. Answers to the questions — Why do these adaptive modifications in an individual animal, soon reach a limit? and why, in the descendants of such animal, similarly condi- tioned, is this limit very slowly extended ? — are to be found in the same direction as was the answer to the last question. And here the connexion of cause and consequence is much more manifest.

Since the function of any organ is dependent on the func- tions of the organs which supply it with materials and forces ; and since the functions of these subsidiary organs are de- pendent on the functions of organs which supply them wrt h materials and forces ; it follows that before any great extra power of discharging its function, can be gained by a

ADAPTATION. 193

specially- exercised organ, a considerable extra power must be gained by a series of immediately- subservient organs, and some extra power by a secondary series of remotely-sub- servient organs. Thus there are required numerous and wide-spread modifications. Before the artery which feeds a hard-worked muscle, can permanently furnish a large ad- ditional quantity of blood, it must increase in diameter and contractile power ; and that its increase of diameter and con- tractile power may be of use, the main artery from which it diverges, must also be so far modified as to bring this addi- tional quantity of blood to the branch artery. Similarly with the veins ; similarly with the absorbents ; similarly with the nerves. And when we ask what these subsidiary changes imply, we are forced to conclude that there must be an analogous group of more numerous changes, ramifying throughout the system. The growth of the arteries prima- rily and secondarily implicated, cannot go to any extent, without growth in the minor blood-vessels on which their nutrition depends ; while their greater contractile power in- volves enlargement of the nerves which excite them, and some modification of that part of the spinal cord whence these nerves proceed. Thus, without tracing the like remote alterations implied by extra growth of the veins, absorbents, and other agencies, it is manifest that a large amount of re- building must be done throughout the organism, before any organ of importance can be permanently increased in size and power to a great extent. Hence, though such extra growth in any part as does not necessitate considerable changes throughout the rest of the organism, may rapidlv take place ; a further growth in this part, requiring a re- modelling of numerous parts remotely and slightly affected, must take place but slowly.

We have before found our conceptions of vital processes made clearer by studying analogous social processes. In societies there is a mutual dependence of functions, essentially like that which exists in organisms; and there is also an

191 THE INDUCTIONS OF BIOLOGY.

essentially like re-action of functions on structures. From tL<? laws of adaptive modification in societies, we may therefore hope to get a clue to the laws of adaptive modification in organisms. Let us suppose, then, that a society has arrived at a state of equilibrium like that of a mature animal — a state not like our own, in which growth and structural de- velopment are rapidly going on ; but a state of settled balance among the functional powers of the various classes and industrial bodies, and a consequent fixity in the relative sizes of such classes and bodies. Further, let us suppose that in a society thus balanced, there occurs something which throws an unusual demand on some one industry — say an unusual demand for ships (which we will assume to be built of iron) in consequence of a competing mercantile nation having been prostrated by famine or pestilence. The imme- diate result of this additional demand for iron ships, is the employment of more workmen, and the purchase of more iron, by the ship-builders ; and when, presently, the demand con- tinuing, the builders find their premises and machinery in- sufficient, they enlarge them. If the extra requirement persists, the high interest and high wages bring such extra capital and labour into the business, as are needed for new ship-building establishments. But such extra capital and labour do not come quickly ; since, in a balanced community, not increasing in population and wealth, labour and capital have to be drawn from other industries, where they are already yielding the ordinary returns. Let us now go a step further. Suppose that this iron-ship-building industry, having enlarged as much as the available capital and labour permit, is still unequal to the demand ; what limits its im- mediate further growth ? The lack of iron. By the hypo- thesis, the iron-producing industry, like all the other indus- tries throughout the community, yields only as much iron as is habitually required for all the purposes to which iron is applied : ship-building being only one. If, then, extra iron is required for ship-building, the first effect is to withdraw

ADAPTATION 195

part of the iron habitually consumed for other purposes, and to raise the price of iron. Presently, the iron-makers feol this change, and their stocks dwindle. As, however, the. quantity of iron required for ship-building, forms but a small part of the total quantity required for all purposes ; the ex- tra demand on the iron-makers, can be nothing like so great in proportion as is the extra demand on the ship- builders. "Whence it follows, that there will be much less tendency to an immediate enlargement of the iron-producing industry — the extra quantity will for some time be obtained by working extra hours. Nevertheless, if, as fast as more iron can be thus supplied, the ship- building industry goes on growing — if, consequently, the iron-makers experience a permanently- increased demand, and out of their greater profits get higher interest on capital, as well as pay higher wages ; there will eventually be an abstraction of capital and labour from other industries, to enlarge the iron-producing industry : new blast- furnaces, new rolling-mills, new cottages for workmen, will be erected. But obviously, the inertia of capital and labour to be overcome, before the iron-producing industry can grow by a decrease of some other industries, will prevent its growth from taking place until long after the increased ship-build- ing industry has demanded it ; and meanwhile, the growth of the ship-building industry must be limited by the deficiency of iron. A remoter restraint of the same nature, meets us if we go a step further — a restraint which can be overcome, only in a still longer time. For the manu- facture of iron depends on the supply of coal. The pro- duction of coal being previously in equilibrium with the consumption ; and the consumption of coal for the manu- facture of iron, being but a small part of the total con- sumption ; it follows that a considerable extension of the iron manufacture, when it at length takes place, will cause but a comparatively small additional demand on the coal-owners and coal-miners— a demand which will not, for a long period, suf- fice to cause enlargement of the coal- trade, by drawing capita]

196 THE INDUCTIONS OF BIOLOGY.

and labour from other investments and occupations. And until the permanent extra demand for coal, has become great enough to draw from other investments and occupations, suf- ficient capital and labour to sink new mines, the increasing production of iron must be restricted by the scarcity of coal ; and the multiplication of ship-yards and ship-builders, must be checked by the want of iron. Thus, in a com- munity which has reached a state of moving equilibrium, though any one industry directly affected by an additional demand, may rapidly undergo a small extra growth ; yet a growth beyond this, requiring, as it does, the build- ing-up of subservient industries, less directly and strongly affected, as well as the partial zinbuildmg of other industries, can take place only with comparative slowness. And a still further growth, requiring structural modifications of industries still more distantly affected, must take place still more slowly.

Returning from this analogy, we realize more clearly the truth, that any considerable member of an animal organism, cannot be greatly enlarged without some general re- organiza- tion. Besides a building-up of the primary, secondary, and tertiary groups of subservient parts, there must be an un- building of sundry non-subservient parts ; — or at any rate, there must be permanently established, a lower nutrition of such non- subservient parts. For it must be remembered that in a mature animal, or one which has reached a balance between assimilation and expenditure, there cannot be an in- crease in the nutrition of some organs, without a decrease in the nutrition of others ; and an organic establishment of the increase, implies an organic establishment of the decrease — implies more or less change in the processes and structures throughout the entire system. And here, in-

deed, is disclosed one reason why growing animals under- go adaptations so much more readily than adult ones. For while there is surplus nutrition, it is possible for specially- ex- ercised parts to be specially enlarged, without any positive

ADAPTATION. 197

deduction from other parts. There is required only that negative deduction, shown in the diminished growth, of other parts.

§ 70. Pursuing the argument further, we reach an ex- planation of the third general truth ; namely, that organisms, and species of organisms, which, under new conditions, have undergone adaptive modifications, soon return to something like their original structures, when restored to their original conditions. Seeing, as we have done, how excess of action and excess of nutrition in any part of an organism, must affect action and nutrition in subservient parts, and these again in other parts, until the re-action has divided and subdivided itself throughout the organism, affecting in decreasing degrees the more and more numerous parts more and more remotely implicated ; we see that the consequent changes in the parts remotely implicated, consti- tuting the great mass of the organism, must be extremely slow. Hence, if the need for the adaptive modification ceases, before the great mass of the organism has been much altered in its structure by these ramified but minute re-ac- tions ; we shall have a condition in which the specially- modified part, is not in equilibrium with the rest. All the remotely-affected organs, as yet but little changed, will, in the absence of the perturbing cause, resume very nearly their previous actions. The parts that depend on them, will consequently by and by do the same. Until at length, by a reversal of the adaptive process, the organ at first affected will be brought back almost to its original state. Recon-

sidering the above- drawn analogy between an organism and society, will enable us better to realize this necessity. If, in the case supposed, the extra demand for iron ships, after causing the erection of some additional ship-yards and the drawing of iron from other manufactures, were to cease ; the old dimensions of the ship-building trade would be quickly returned to : discharged workmen would seek fresh

198 THE INDUCTIONS OF BIOLOGY.

occupations, and the new yards would be devoted to othui uses. But if the increased need for ships lasted long enough, and became great enough, to cause a flow of capital and labour from other industries into the iron-manufacture, a falling off in the demand for ships, would much less rapidly entail a dwindling of the ship-building industry. For iron being now produced in greater quantity, a diminished con- sumption of it for ships, would cause a fall in its price, and a consequent fall in the cost of ships : thus enabling the ship-builders to meet the competition which we may sup- pose led to a decrease in the orders they received. And since, when new blast-furnaces and rolling-mills, &c., had been built with capital drawn from other industries, its transference back into other industries, would involve great loss ; the owners, rather than transfer it, would accept unusually low in- terest ; and an excess of iron would continue to be produced ; resulting in an undue cheapness of ships, and a maintenance of the ship-building industry at a size beyond the need. Eventually, however, if the number of ships required still diminished, the production of iron in excess would become very unremunerative : some of the blast-furnaces would bo

b

blown out ; and as much of the capital and labour as remained available, would be re-distributed among other occupations. "Without repeating the steps of the argument, it will be clear that were the enlargement of the ship-building industry great enough, and did it last long enough, to cause an in- crease in the number of coal-mines ; the ship-building in- dustry would be still better able to maintain itself under adverse circumstances ; but that it would, though at a more distant period, end by sinking down to the needful dimensions. Thus our conclusions are : — First, that if the extra activity and growth of a particular industry, has lasted long enough only to remodel the proximately-afiected industries ; it will dwindle away again after a moderate period, if the need for it disappears. Second, that an enormous period must be re- quired before the re-actions produced by an enlarged industry .

ADAPTATION. 199

can cause a re-construction of the whole society, and before the countless re-distributions of capital and labour, can again reach a state of equilibrium. And third, that only when such a new state of equilibrium is eventually reached, can the adaptive modification become a permanent one. How,

in animal organisms, the like argument will hold, needs not be pointed out. The reader will readily follow the parallel.

That organic types should be comparatively stables might be anticipated on the hypothesis of Evolution. If we assume, as we must according to this hypothesis, that the structure of any organism is a product of the almost infinite series of actions and re-actions to which all ancestral organisms have been exposed ; we shall see that any unusual actions and re- actions brought to bear on an individual, can have but an infinitesimal effect in permanently changing the structure of the organism as a whole. The new set of forces, com- pounded with all the antecedent sets of forces, can but inap- preciably modify that moving equilibrium of functions which all these antecedent sets of forces have established. Though there may result a considerable perturbation of certain func- tions— a considerable divergence from their ordinary rhythms; yet the general centre of equilibrium cannot be sensibly changed. On the removal of the perturbing cause, the pre- vious balance will be quickly restored : the effect of the new forces being almost obliterated by the enormous aggregate of forces which the previous balance expresses.

§ 71. As thus understood, the phenomena of adaptation fall into harmony with first principles. The inference that organic types are fixed, because the deviations from them which can be produced within assignable periods, are relatively small ; and because, when a force producing deviation ceases, there is a return to something like the original state ; proves to be an invalid inference. Without assuming fixity of species, we find good reasons for anticipating that kind and degree of stability which is observed. We find grounds for concluding,

200 THE INDUCTIONS OF BIOLOGY.

a priori, that an adaptive change of structure, will soon reach a point beyond which further adaptation will be slow ; for concluding that when the modifying cause has been but a short time in action, the modification generated, will be evanescent ; for concluding that a modifying cause acting even for many generations, will do but little towards per- manently altering the organic equilibrium of a race ; and for concluding that on the cessations of such cause, its effects will become unapparent in the course of a few gener- ations.

CHAPTER VI.

INDIVIDUALITY.

§ 72. WHAT is an individual ? is a question which many readers will think it easy to answer. Yet it is a question that has led to much controversy among Zoologists and Botanists ; and no quite satisfactory reply to it seems possi- ble. As applied to a man, or to any one of the higher animals, which are all sharply-defined and independent, the word individual has a clear meaning ; though even here, when we turn from average cases to exceptional cases — as a calf with two heads and two pairs of fore-limbs — we find ourselves in doubt whether to predicate one individuality or two. But when we extend our range of observation to the organic world at large, we find that difficulties allied to this exceptional one, meet us everywhere under every variety of form.

Each uniaxial plant may perhaps fairly be regarded as a distinct individual ; though there are botanists who do not make even this admission. TVhat, however, are we to say of a multiaxial plant ? It is, indeed, usual to speak of a tree with its many branches and shoots, as singular ; but strong reasons may be urged for considering it as plural. Every one of its axes has a more or less independent life, and when cut off and planted, may grow into the likeness of its parent ; or by grafting and budding, parts of this tree may be developed upon another tree, and there manifest their

202 THE INDUCTIONS OF BIOLOGY.

specific peculiarities. Shall we regard all the growing axes thus resulting from slips and grafts and buds, as parts of one individual, or as distinct individuals ? If a strawberry- plant sends out runners carrying buds at their ends, which strike root and grow into independent plants, that separate from the original one by decay of the runners, must we not say that they possess separate individualities ; and yet if we do this, are we not at a loss to say when their separate individu- alities were established, unless we admit that each bud was from the beginning an individual ? Commenting on such perplexities, Schleiden says — " Much has been written and disputed concerning the conception of the individual, with- out, however, elucidating the subject, principally owing to the misconception that still exists as to the origin of the con- ception. Now the individual is no conception, but the mere subjective comprehension of ail actual object, presented to us under some given specific conception, and on this latter it alone depends whether the object is or is not an individual. Under the specific conception of the solar system, ours is an individual : in relation to the specific conception of a planet- ary body, it is an aggregate of many individuals." * * * "I think, however, that looking at the indubitable facts already mentioned, and the relations treated of in the course of these considerations, it will appear most advantageous and most useful, in a scientific point of view, to consider the vegetable cell as the general type of the plant (simple plant of the first order). Under this conception, Protococcus and other plants consisting of only one cell, and the spore and pollen- granule, will appear as individuals. Such individuals may, however, again, with a partial renunciation of their in- dividual independence, combine under definite laws into definite forms (somewhat as the individual animals do in the globe of the Vohox ylobator*}. These again appear empiri- cally as individual beings, under a conception of a species

* It is now generally agreed that the Volvox globaior is a plant.

INDIVIDUALITY. 203

(simple plants of tlie second order) derived from the form of the normal connexion of the elementary individuals. But we cannot stop here, since nature herself combines these in- dividuals, under a definite form, into larger associations, whence we draw the third conception of the plant, from a connexion, as it were, of the second power (compound plants — plants of the third order). The simple plant proceeding from the combination of the elementary individuals is then termed a bud (gemma], in the composition of plants of the third order."

The animal kingdom presents still greater difficulties. When, from sundry points on the body of a common polype, there bud-out young polypes, which, after acquiring mouths and tentacles and closing up the communications between their stomachs and the stomach of the parent, finally separate from the parent ; we may with propriety regard them as dis- tinct individuals. But when, in the allied compound Hydro- zoa, we find that these young polypes continue permanently connected with the parent ; and when, by this continuous budding-out, there is presently produced a tree-like aggre- gation, having a common alimentary canal into which, the digestive cavity of each polype opens; it is no longer so clear that these little sacs furnished with mouths and tenta- cles, are severally to be regarded as distinct individuals. AVe cannot deny a certain individuality to the polypedom. And on discovering that some of the buds, instead of unfolding in the same manner as the rest, are transformed into capsules in which eggs are developed — on discovering that certain of the incipient polypes thus become wholly dependent on the aggregate for their nutrition, and discharge functions which have nothing to do with their own maintenance, we have still clearer proof that the individualities of the members are partially merged in the individuality of the group. Other organisms belonging to the same order, display still more decidedly this transition from simple individualities to a com- plex individuality. In the Uiphyes there is a special modifi-

204 THE INDUCTIONS OF BIOLOGY.

cation of one or more members of the polypedom. into a swimming apparatus, which, by its rhythmical contractions, propels itself through the water, drawing the polypedom after it. And in the more differentiated Physalia, various organs result from the metamorphosis of parts that are the homo- logues of individual polypes. In this last instance, the in- dividuality of the aggregate is so predominant, that the individualities of the members are practically lost. This

combination of individualities in such way as to produce a composite individual, meets us in other forms among the ascidian molluscs. While in some of these, as in the Clavelina, the animals associated are but little subordinated to the community they form ; in others, as in the Botry Hides, they are so fused into a rounded mass, as to present the appearance of a single animal with several mouths and stomachs.

On the hypothesis of Evolution, perplexities of this nature are just such as we might anticipate. If Life in general, com- menced with minute and simple forms, like those out of which all individual organisms, however complex, now originate ; and if the transitions from these primordial units to organisms made up of groups of such units, and to higher organisms made up of groups of such groups, took place by degrees ; it is clear that individualities of the first and simplest order, would merge gradually in those of a larger and more complex order, and these again in others of an order having still greater bulk and organization ; and that hence it would be impossible to say where the lower indivi- dualities ceased, and the higher individualities commenced.

v

§ 73. To meet these difficulties, it has been proposed that the whole product of a single fertilized germ, shall be re- garded as a single individual : whether such whole product be organized into one mass, or whether it be organized into many masses, that are partially or completely separate. It is urged that whether the development of the fertilized germ

INDIVIDUALITY. 205

be continuous or discontinuous (§ 50) is a matter of secondary importance ; that the totality of living tissue to which the fertilized germ gives rise in any one case, is the equivalent of the totality to which it gives rise in any other case ; and that we must recognize this equivalence, whether such totality of living tissue takes a concrete or a discrete arrangement. In pursuance of this view, a zoological individual is consti- tuted either by any such single animal as a mammal or bird, which may properly claim the title of a zoon, or by any such group of animals as the numerous Mcdusce that have been developed from the same egg, which are to be severally dis- tinguished as zooids.

Admitting it to be very desirable that there should be words for expressing these relations and this equivalence, it may still be objected, that to apply the word individual to a number of separate living bodies, is inconvenient: conflicting so much, as it does, with the ordinary conception which this word suggests. It seems a questionable use of language to say that the countless masses of Anacharis Alsinastrum, which, within these few years, have grown up in our rivers, canals, and ponds, are all parts of one individual ; and yet as this plant does not seed in England, these countless masses, having arisen by discontinuous development, must be so regarded, if we accept the above definition.

It may be contended, too, that while it does violence to our established way of thinking, this mode of interpreting the facts is not without its difficulties — smaller, perhaps, than those it escapes, but still considerable. Something seems to be gained by restricting the application of the title individual, to organisms which, being in all respects fully developed, possess the power of producing their kind after the ordinary sexual method ; and denying this title to those incomplete organisms which have not this power. But the definition does not really establish this distinction for us. On the one hand, we have cases in which, as in the working bee, the whole of the germ-product is aggregated into a single

206 THE INDUCTIONS OF BIOLOGY.

organism ; and yet, though an individual according to the definition, this organism has no power of reproducing its kind. On the other hand, we have cases like that of the perfect Aphides, where the organism is but an infinitesimal part of the germ-product ; and yet has that completeness required for sexual reproduction. Moreover, if we

adopt the proposed view, we find ourselves committed to the anomalous position, that among many orders of animals, there are no concrete individuals at all. If the individual is consti- tuted by the whole germ-product, whether continuously or discontinuously developed, then, not only must individuality be denied to each of the imperfect Aphides, but also to each of the perfect males and females ; since no one of them is more than a minute fraction of the total germ- product. And yet further, it might be urged with some show of reason, that if the conception of individuality involves the conception of completeness ; then, an organism which possesses an independent power of reproducing itself, being more complete than an organism in which this power is dependent on the aid of another organism, is more in- dividual.

§ 74. There is, indeed, as already implied, no definition of individuality that is unobjectionable. All we can do is to make the best practicable compromise.

As applied either to an animate or an inanimate object, the word individual ordinarily connotes union among the parts of the object, and separateness from other objects. This fundamental element in the conception of individuality, we cannot with propriety ignore in the biological application of the word. That which we call an individual plant or animal, must, therefore, be some concrete whole, and not a discrete whole. If, however, we say that each concrete

living whole is to be regarded as an individual, we are still met by the question — What constitutes a concrete living whole ? A young organism arising by internal or external

INDIVIDUALITY. 207

gemmation from a parent organism, passes gradually from a state in which it is an indistinguishable part of the parent organism, to a state in which it is a separate organism of like structure with the parent. At what stage does it become an individual ? And if its individuality be conceded only when it completely separates from the parent, must we deny in- dividuality to all organisms thus produced, which permanently retain their connexions with their parents ? Or again, what must we say of the Hectocotylus, which is an arm of the Cuttle-fish that undergoes a special development, and then detaching itself, lives independently for a considerable period? And what must we say of that larval Echinus, which is left to move about awhile after being robbed of its viscera by the young Echinus developed within it ?

To answer such questions, we must revert to the definition of Life. The distinction between individual in its biological sense, and individual in its more general sense, must consist in the manifestation of Life, properly so called. Life we have seen to be, " the definite combination of heterogeneous changes, both simultaneous and successive, in correspondence with external co-existences and sequences/' Hence, a biolo- gical individual is any concrete whole having a structure which enables it, when placed in appropriate conditions, to continuously adjust its internal relations to external relations, so as to maintain the equilibrium of its functions. In

pursuance of this conception, we must consider as individuals, all those wholly or partially independent organized masses, which arise by multicentral and multiaxial development that is either continuous or discontinuous (§ 50). "\Ve must jH'-jord the title to each separate aphis, each polype of a ]i Hvpedom, each bud or shoot of a flowering plant, whether ir detaches itself as a bulbil or remains attached as a branch.

l\\ thus interpreting the facts, we do not, indeed, avoid all tin jmalies. While, among flowering plants, the power of in- dependent growth and development, is usually possessed only by shoots or axes ; yet, in some cases, as in that of the Begonia'

208 THE INDUCTIONS OF BIOLOGY.

leaf awliile since mentioned, the appendage of an axis, or even a small fragment of such appendage, is capable of initiating and carrying on the functions of life ; and in other cases, as shown by M. ISTaudin in the Droscra intermedia, young

•/ * «/ O

plants are occasionally developed from the surfaces of leaves, while still connected with the parent plant. Nor among forms like the compound Hydrozoa, does the definition enable us to decide where the line is to be drawn between the individuality of the group and the individualities of the members — merging into each other, as these do, in different degrees. But, as before said, such difficulties must necessa- rily present themselves, if organic forms have arisen by in- sensible gradations. We must be content with a course which commits us to the smallest number of incongruities ; and this course is, to consider as an individual, any centre or axis that is capable of independently carrying on that con- tinuous adjustment of inner to outer relations which consti- tutes Life.

CHAPTER YIL

GENESIS.

§75. HAMNG concluded what constitutes an individual, we are in a position to deal with the multiplication of in- dividuals. For this, the title Genesis is here chosen, as being the most comprehensive title — the least specialized in its meaning. By some biologists, Generation has been used to signify one method of multiplication, and Reproduction to signify another method ; and each of these words has been

«/

thus rendered in some degree unfit to signify multiplication in general.

Here the reader is indirectly introduced to the fact, that the production of new organisms is carried on in fundament- ally unlike ways. Up to quite recent times, it was believed, even by naturalists, that all the various processes of multipli- cation observable in different kinds of organisms, have one essential character in common : it was supposed that in every species, the successive generations are alike. It has now been proved, however, that in plants, and in numerous animals, the successive generations are not alike ; that from one generation there proceeds another whose members differ more or less in structure from their parents ; that these produce others like themselves, or like their parents, or like neither ; but that eventually, the original form re- appears. Instead of there being, as in the cases most familiar to us, a constant

recurrence of the eame form, there is a cyclical recurrence of 10

210 THE INDUCTIONS OF BIOLOGY.

the same form. These two distinct processes of multiplication, may be aptly termed homogcnesis and hetcrogcncsis* Under these heads let us consider them more closely.

The kind of genesis, once supposed to be universal, in which the successive generations are alike, is always sexual genesis ; or, as it has been otherwise called — gamogenesis. In every species of organism, which multiplies by homogenesis, each generation consists of males and females ; and from the fertilized germs they produce, the next generation of similar males and females arises. This method of propagation is further distinguished by the peculiarity, that each fertilized germ gives rise to but one individual — the product of de- velopment is always organized round one axis, and not round several axes. Between the different kinds of homo-

genesis, the most marked contrast, and the only one which need here detain us, is that between the oviparous and the viviparous. The oviparous kind is that in which the fertil- ized germ is detached from the parent, before it has undergone any considerable development. The viviparous kind is that in which development is considerably advanced, or almost completed, before final detachment takes place. This distinction is, however, not a sharply- denned one : there are transitions between the oviparous and the viviparous processes. In ovo-viviparous genesis, there is an internal incubation ; and though the young are in this case finally detached from the parent in the shape of eggs, they do not leave the parent's body until after they have assumed something like the parental form. Looking

around, we find that homogenesis is universal among the Vertebrata : there is no known vertebrate animal but what arises from a fertilized germ, and unites into its single indi- viduality the whole products of this fertilized germ. In

* Unfortunately the word heterogenesis, has been already used as a synoiivme for " spontaneous generation." Save by those few who believe in " spontaneous generation," however, little objection will be felt to using the word iu a sense that seems much more appropriate.

GENESIS. 211

the mammals or highest Vertebrata, this homogenesis is in every case viviparous ; in birds it is uniformly oviparous ; and in reptiles and fishes, it is always essentially oviparous, though there are cases, of the kind above referred to, in which viviparity is simulated. Passing to the Invertebratat we find oviparous homogenesis universal among the Arach- r-ida (except the Scorpions, which are ovo- viviparous) ; universal among the higher Crustacea, but not among the lower ; extremely general, though not universal, among Insects ; and universal among the higher Mollusca, though not among the lower. Along with extreme inferiority among animals, we find homogenesis to be the exception rather than the rule ; and in the vegetal kingdom, there appear to be no cases, save those of a few aberrant parasites like the Rafflesiacece, in which the centre or axis which, arises from a fertilized germ, becomes the immediate producer of fertilized germs.

Where propagation is carried on by heterogenesis, or is characterized by unlikeness of the successive generations, there is always asexual genesis with occasionally-recurring sexual genesis ; in other wrords — ayamogenesis interrupted more or less frequently by gamogenesis. If we set out with a generation of perfect males and females ; then, from their ova or seeds, there arise individuals that are neither males nor females, but that produce the next generation from buds. By this method of multiplication, many individuals originate from a single fertilized germ : the product of development is organized round more than one centre or axis. The simplest form of heterogenesis is that

seen in uniaxial plants. If, as we find ourselves obliged to do, we regard each separate shoot or axis of growth, as a dis- tinct individual ; then, in uniaxial plants, the successive in- dividuals are not represented by the series A, A, A, A,. &c., like those resulting from homogenesis ; but they are repre- sented by the series A, B, A, B, A, B, &c.- For in plants which were before classed as uniaxial (§ 50), and w'hicb may

212 THE INDUCTIONS OF BIOLOGY.

be conveniently so distinguished from other plants, the axis which shoots up from the seed, and substantially constitutes the plant, does not itself flower and bear seed; but gives lateral origin to flowering, or seed -bearing, axes. Though in uni-

o o 7 o * o

axial plants, the fructifying apparatus appears to be at the end of the primary, vertical axis ; yet dissection shows that, morphologically considered, each fructifying axis is usually an offspring from the primary axis. There arises from the seed, a sexless individual, from which spring by gemmation, in- dividuals having reproductive organs ; and from these there result fertilized germs or seeds, that give rise to sexless individuals. That is to say, gamogenesis and agamogenesis alternate : the peculiarity being, that the sexual individu- als arise from the sexless ones by continuous development. The Salpce show us an allied form of heterogenesis in the animal kingdom. Individuals developed from fertilized ova, instead of themselves producing fertilized ova, produce, by gemmation, strings of individuals ; from which fertilized ova again originate. In multiaxial plants, we have

a succession of generations represented by the series A, B, B, B, &c., A, B, B, B, &c. Supposing A to be a flowering axis, or sexual individual ; then, from any fertilized germ it casts off, there grows up a sexless individual, B ; from this there bud-out other sexless individuals, B ; and so on for generations more or less numerous ; until at length, from some of these sexless individuals, there bud-out seed-bearing individuals of the original form A. Branched herbs, shrubs, and trees, exhibit this form of heterogenesis : the successive generations of sexless individuals thus produced, being in most cases continuously developed, or aggregated into a compound individual ; but being in some cases dis- continuously developed. Among animals, a kind of hetero- genesis represented by the same succession of letters, occurs in such compound polypes as the Sertularia; and in those of the Hydrozoa which assume alternately the poly- poid form, and the form of the Medusa : the chief difference

GENESIS. 213

presented by these groups, arising from the fact that the successive generations of sexless individuals produced by budding, are in some cases continuously developed, arid in others discontinuously developed ; and from the fact that, in some cases, the sexual individuals give off their fertilized germs while still growing on the parent-polypedom, but in other cases, not until after leaving the parent-polypedom and undergoing further development. Where, as in all

the foregoing kinds of agamogenesis, the new individuals bud-out, not from any specialized reproductive organs, but from unspecialized parts of the parent ; the process has been named, by Prof. Owen, metagenesis. In most instances, the individuals thus produced, grow from the outsides of the parents — the metagenesis is external. But there is also a kind of metagenesis which we may distinguish as internal. Certain entozoa of the genus Distonw, exhibit it. From the egg of a Disfoma, there results a rudely-formed creature known to naturalists as the " King's-yellow worm." Gradu- ally as this increases in size, the greater part of its inner substance is transformed into young animals called Cercarice. (which are the Iarva3 of Distomata] ; until at length, it becomes little more than a living sac, full of living offspring. In the Dlstoma pacifica, the brood of young animals thus arising by internal gemmation, are not Cercarice, but are of the same form as their parent : themselves becoming the producers of Cercarice after the same manner, at a subsequent period. So that sometimes the succession of forms is repre- sented by the series A, B, A, B, &c. ; and sometimes by the series A, B, B, A, B, B, &c. Both cases, however, exemplify internal metagenesis, in contrast with the several kinds of external metagenesis described above. That agamo-

genesis which is carried on in a reproductive organ — either a true ovarium or the homologue of one — has been called, by Prof. Owen, parthenogenesis. In his work published under this title, he embraced those cases in which the buds arising in the pseud- ovarium, are not ova in the full sense of the

214 THE INDUCTIONS OF BIOLOGY.

but rather, as they have since been called by Prof. Huxley, pseucl-ova. Yon Siebold and other naturalists, have hence applied the term parthenogenesis to a narrower class of cases. Perhaps it would be best to distinguish this process, which is intermediate between metagenesis and parthenogenesis, by the term pseudo-parthenogenesis. It is the process familiarly exemplified in the Aphides. Here, from the fertilized eggs laid by perfect females, there grow up imperfect females, in the pseud-ovaria of which there are developed pseud-ova ; and these, rapidly assuming the organization of other imperfect females, are born vivi- parously. From this second generation of imperfect females, there by and by arises, in the same manner, a third genera- tion, of the same kind ; and so on for many generations : the series being thus symbolized by the letters A, B, B, B, B, B, &c., A. Respecting this kind of heterogenesis, it should be added, that in animals, as in plants, the number of genera- tions of sexless individuals produced before the re-appearance of sexual ones, is indefinite ; both in the sense that in the same species it may go on to a greater or less extent accord- ing to circumstances, and in the sense that among the genera- tions of individuals proceeding from the same fertilized germ, a recurrence of sexual individuals takes place earlier in some of the diverging lines of multiplication than in others. In trees we see that on some branches, flower-bearing axes arise while other branches are still producing only leaf-bearing axes ; and in the successive generations of Aphides, a parallel truth has been observed. Lastly has to be set down,

that form of heterogenesis in which, along with gamogenesis, there occurs a form of agamogenesis exactly like it, save in the absence of fecundation. This is called true parthenogenesis- reproduction carried on by virgin mothers, which are in all respects like other mothers. In. the silk-worm-motha this parthenogenesis is exceptional, rather than ordinary : usually the eggs of these insects are fertilized ; but if they are not, they are still laid, and some of them produce larvae. In certain Lepidoptera, however, of the groups Psychidcp and

GENESIS.

215

Tiiieidcc, parthenogenesis appears to be a normal process — indeed, so far as is known, the only process ; for of some species the males have never been found.

A general conception of the relations among the different modes of Genesis, thus briefly described, will be best given by the following tabular statement.

GEXESISIS-

f Oviparous

or Homogenesis, which is Gamogenesis J Ovo-viviparous

or

Viviparous

or

.Heterogenesis, which is-

' Gamogenesis alternating

with

f Parthenogenesis

or Agamogenesis ^ Pseudo-parthenogenesis

or r Internal

Metagenesis -j or

[ External

This, like all other classifications of such phenomena, pre- sents anomalies. It may be justly objected, that the processes here grouped under the head agamogenesis, are the same as those before grouped under the head of discontinuous develop- ment (§ 50) : thus making development and genesis partially coincident. Doubtless it seems awkward that what are from one point of view considered as structural changes, are from another point of view considered as modes of multiplication.*

* Prof. Huxley avoids this difficulty by making every kind of Genesis a mode of development. His classification, which suggested the one given above, is as follows : — •

'Continuous

Growth

( Metamorphosis

Development-; ,, .

/ Metagenesis

/ Agamogenesis < [Discontinuous ( Parthenogenesis

' Gamogenesis

216 THE INDUCTIONS OF BIOLOGY.

There is, however, nothing for us but a choice of imperfec- tions. We cannot by any logical dichotomies, accurately express relations which, in Nature, graduate into each other insensibly. Neither the above, nor any other scheme, can do

wl

more than give an approximate idea of the truth.

§ 76. Genesis under every form, is a process of negative or positive disintegration ; and is thus essentially opposed to that process of integration, which is one element of individual evolution. Negative disintegration occurs in those cases where, as among the compound Hydrozoa, there is a con- tinuous development of new individuals by budding from the bodies of older individuals ; and where the older individuals are thus prevented from growing to a greater size, or reach- ing: a higher degree of integration. Positive disintegration

Cj O O O <-/

occurs in those cases of agamogenesis where the formation of new individuals is discontinuous, and in all cases of gamo- genesis. The degrees of disintegration are various. At the one extreme, the parent organism is completely broken up, or dissolved into new individuals ; and at the other extreme, the new individual forms but a small deduction from the parent organism. Protozoa and Protophyta, show us that form of disintegration called spontar.ro :;« fission: two or four individuals being produced by the spiitting-up of the original one. The Volcox and the Hydrodictyon, are plants which, having developed broods of young plants within themselves, give them exit by bursting ; and among animals, the one lately referred to, which arises from the Distoma egg, entirely loses its individuality in the individ- ualities of the numerous Distoma-l&T\2Q with which it be- comes filled. Speaking generally, the degree ot disintegration becomes less marked, as we approach the higher organic forms. Plants of advanced types throw off from themselves, whether by gamogenesis or agamogenesis, parta that are relatively small ; and among the higher animals, there is no case in which the parent individuality is habitually

GENESIS. 217

fost, in the production of new individualities. To the

last, however, there is of necessity a greater or less disinte- gration. The seeds and pollen-grains of a flowering plant, are disintegrated portions of tissue ; as are also the ova and spermatozoa of animals. And whether the fertilized germs carry away from their parents small or large quantities of nutriment, these quantities of nutriment in all cases involve further negative or positive disintegrations of the parents.

New individuals that result from agamogenesis, usually do not separate from the parent-individuals, until they have undergone considerable development, if not complete develop- ment. The agamogenetic offspring of those lowest organisms which develop centrally, do not, of course, pass beyond cen- tral structure ; but the agamogenetic offspring of organisms that develop axially, commonly assume an axial structure before they become independent. The vegetal kingdom shows us this in the advanced organization of detached bulbils, and of buds that root themselves before separating. Of animals, the Hydrozoa, the Trematoda, the Salpce, and the Aphides, present us with different kinds of agamogenesis, in all of which the new individuals are organized to a considerable extent before being cast off. This rule is not without excep- tions, however. The winter-eggs of the Phimatella, developed in an unspecialized part of the body, present us with a case of metagenesis, in which centres of development, instead oi axes, are detached ; and in the above-described parthenogene- sis of moths and bees, such centres are detached from an ovarium.

When produced by gamogenesis, the new individuals be- come independent of the parents while in the shape of centres of development, rather than axes of development ; and this even where the reverse is apparently the case. The fertilized germs of those inferior plants which are central, or multicen- tral, in their development, are of course thrown off as centres. In the higher plants, of the two elements that go to the form- ation of the fertilized germ, the pollen-cell is absolutely

THE INDUCTIONS OF BIOLOGY.

separated from the parent-plant under the shape of a centre ; and the embryo-cell, though not absolutely separated from the parent, is still no longer subordinate to the organizing forces of the parent. So that when, the embryo- cell having been fertilized by matter from the pollen-tube, the develop- ment commences, it proceeds without parental control : the new individual, though remaining physically united with the old individual, becomes structurally and functionally separate while still only a centre of development ; and takes on its axial form by processes of its own — the old individual doing no more than supply materials. Through-

out the animal kingdom, the new individuals produced by gamogenesis, are obviously separated in the shape of centres of development wherever the reproduction is oviparous . the only conspicuous variation being in the quantity of nutritive matter bequeathed by the parent to the new centre of de- velopment, at the time of its separation. And though, where the reproduction is viviparous, the process appears to be different, and in- one sense is so; yet, intrinsically, it is the same. For in these cases, the new individual really detaches itself from the parent while still only a centre of develop- ment ; but instead of being finally cast off in this state, it is re-attached, and supplied with nutriment until it assumes a more or less complete axial structure.

§ 77. Under all its various forms, the essential act in gamo- genesis, is the union of two centres or cells, produced by different parent organisms : the sperm-cell being the male product, and the germ -cell the female. There are very many modes and modifications of modes in which these cells are produced ; very many modes and modifications of modes by which they are brought into contact ; and very manv modes and modifications of modes bv which the result-

V U

ing fertilized germs have secured to them the fit conditions for their development. But passing over these many diver- gent and re-divergent kinds of sexual multiplication, which

GENESIS. 219

t would take too much space here to specify, the one uni- versal peculiarity which it concerns us to remark, is, this co- alescence of a detached portion of one organism, with a more or less detached portion of another.

Such protophytes as the Palmellce and the Desmidieoss which are sometimes distinguished as unicellular plants, show us a coalescence, not of detached portions of two organisms, but of two entire organisms : in the Palmelke, conjugation is a complete fusion of the individuals ; and in the Desmidiece, the entire contents of the individuals unite to form the germ- mass. "Where, as among the Conferva, we have aggregated cells whose individualities are scarcely at all subordinate to that of the aggregate, the gamogenetic act is effected by tho union of the contained granules of two adjacent cells. In Spirogyra, it is not adjacent cells in the same thread which thus combine ; but cells of one thread with those of another. As we ascend to plants of high organization, we find that the two reproductive elements become quite distinct in

f,heir characters ; and further, that they arise in different

*

organs set apart for their production : the arrangements being such, that the sperm-cells of one plant combine with the germ -cells of another.

There is reason to think that, among the lowest Protozoa, a fusion of two individualities, analogous to that which occurs in the conjugation of certain Alyce, is the process from which results the germ" of a new series of individuals. But in animals formed by the aggregation of units that are homolo- gous with Protozoa, the sperm-cells and germ-cells are differ- entiated. And even in these humble forms, where there is no differentiation of sexes, we have good evidence that, as in all higher forms, the union is not between sperm-cells and germ- cells that have arisen in the same individual ; but between those that have arisen in different individuals.

The marvellous phenomena initiated by the meeting of sperm-cell and germ-cell, naturally suggest the conception ot' gome quite special and peculiar properties possessed by these

220 THE INDUCTIONS OF BIOLOGY.

cells. It seems obvious that this mysterious power \vliich they display, of originating a new and complex organism, distinguishes them in the broadest way from portions of organic substance in general. Nevertheless, the more we study the evidence, the more is this assumption shaken — the more are we led towards the conclusion, that these cells have not . | been made by some unusual elaboration, fundamentally different from all other cells. The first fact which

points to this modified conclusion, is the fact recently dwelt upon (§ 63), that in many plants and inferior animals, a small fragment of tissue that is but little differentiated, is capable of developing into the form of the organism from which it was taken. Conclusive proof obliged us to admit, that the component units of organisms, have inherent powers of arranging themselves into the forms of the organisms to which they belong. And if to these component units, which we distinguished as physiological, such powers must be con- ceded— if, under fit conditions, and when not much specialized, they manifest such powers in a way as marked as that in which the contents of sperm-cells and germ-cells manifest them ; then, it becomes clear that the properties of sperm- cells and germ-cells are not so peculiar as we are apt to assume. Again, the organs for preparing sperm-

cells and germ-cells, have none of the speciality of struc- ture which might be looked for, did sperm-cells and germ- cells need endowing with properties essentially unlike those of all other organic agents. On the contrary, these reproductive centres proceed from tissues that are character- ized by their low organization. In plants, for example, it is not appendages that have acquired considerable structure, which produce the fructifying particles : these arise at the extremities of the axes, where the degree of structure is the least. The embryo-cells are formed in the undifferen dated part of the cambium-layer ; the pollen-grains are formed at the little-differentiated extremities of the stamens ; and both are homologous with simple epithelium-cells. Among many

GENESIS. 221

inferior animals devoid of special reproductive organs, such as the Hydra, the ova and spermatozoa originate in the layer of indifferent tissue that lies between the endoderm and the ectoderm ; that is, they consist of portions of the least specialized substance. And in the higher animals, these same generative agents appear to be merely modified epithelium- cells — cells not remarkable for their complexity of structure, but rather for their simplicity. If, by

way of demurrer to this view, it is asked why other epithe- lium-cells do not exhibit like properties ; there are two replies. The first is, that other epithelium-cells are usually so far changed to fit them to their special functions, that they are unfitted for assuming the reproductive function. The second reply is, that in some cases, where the epithelium-cells are but very little specialized, they do exhibit the like properties: not, indeed, by uniting with, other epithelium-cells to produce new germs, but by producing new germs without such union. I learn from Dr Hooker, that the Begonia pJiyUomaniaca habitually develops young plants from the scales of its srern and leaves — nay, that many young plants are developed by a single scale. The epithelium-cells composing one of these scales, swell, here and there, into large globular cells ; form chlorophyll in their interiors ; shoot out rudimentary axes ; and then, by spontaneous constrictions, cut themselves off; drop to the ground ; and grow into Begonias. It appears, too, that in a succulent English plant, the Malaxis paludosa, a like process occurs : the self-detached cells being, in this case, produced by the surfaces of the leaves. Thus,

there is no warrant for the assumption that sperm-cells and germ-cells possess powers fundamentally unlike those of other cells. The inference to which the facts point, is, that they differ from the rest, mainly in not having undergone modifications such as those by which the rest are adapted to particular functions. They are cells that have departed but little from the original and most general type. Or, in the words suggested by a friend, it is not that they are peculiarly

222 THE INDUCTIONS OF BIOLOGY.

specialized, but rather that they are unspecialized : such specializations as some of them exhibit in the shape of loco- motive appliances, &c., being interpretable not as intrinsic, but as extrinsic, modifications, that have reference to nothing beyond certain mechanical requirements. Sundry

facts tend likewise to show, that there does not exist the pro- found distinction which we are apt to assume, between the male and female reproductive elements. In the common polype, sperm-cells and germ-cells are developed in the same layer of indifferent tissue ; and in TetJnja, one of the sponges, Prof. Huxley has observed that they occur mingled together in the general parenchyma. The pollen-grains and embryo- cells of plants, arise in adjacent parts of the cambium-layer ; and from a description of a monstrosity in the Passion-flower, recently given by Mr Salter to the Linnsean Society, it ap- pears, both that ovules may, in their general structure, graduate into anthers, and that they may produce pollen in their interiors. All which evidence is in perfect harmony with the foregoing conclusion ; since, if sperm-cells and germ-cells have natures not essentially unlike those of un- specialized cells in general, their natures cannot be essen- tially unlike each other.

The next general fact to be noted, is, that these cells whose union constitutes the essential act of gamogenesis, are cells in which the developmental changes have come to a close — cells which, however favourably circumstanced in respect of nutrition, are incapable of further evolution. Though they are not, as many cells are, unfitted for growth and metamorphosis by being highly specialized ; yet they have lost the power of growth and metamorphosis. They have severally reached- a state of equilibrium. And while the internal balance of forces prevents a continuance of con- structive changes, it is readily overthrown by external destructive forces. For it uniformly happens that sperm- cells and germ-cells which are not brought in contact, disap- pear. In a plant, the embryo-cell, if not fertilized, is

GENESIS. 223

absorbed or dissipated, while the ovule aborts ; and the un- impregnated ovum eventually decomposes.

Such being the characters of these cells, and such being their fates if kept apart, we have now to observe what hap- pens when they are united. For a long time, the immediate sequence of their contact was not ascertained. This is at length, however, decided. It has been shown that in plants, the extremity of the elongated pollen-cell applies itself to the surface of the embryo-sac, but does not enter the embryo- sac. In animals, however, the process is different. Careful observers agree, that the spermatozoon passes through the limiting membrane of the ovum. The result in both cases is presumed to be a mixture of the contents of the two cells. The evidence goes to show that in plants, matter passes by osmose from, the pollen-cell into the embryo- cell ; and that in animals, the substance contained in the spermatozoon becomes mingled with the substance contained in the ovum, either by simple diffusion or by cell-multiplica- tion. But the important fact which it chiefly con- cerns us to notice, is, that on the union of these reproductive elements, there begins, either at once or on the return of favourable conditions, a new series of developmental changes. The state of equilibrium at which each of them had arrived, is destroyed by their mutual influence ; and the constructive

V V '

changes which had come to a close, recommence : a process of cell-multiplication is set up ; and the resulting cells pre- sently begin to aggregate into the rudiment of a new

organism.

Thus, passing over the variable concomitants of gamo- genesis, and confining our attention to what is constant in it, we see : — that there is habitually, if not universally, a fusion of two portions of organic substance, which are either them- selves distinct individuals, or are thrown off by distinct individuals ; that these portions of organic substance, which are severally distinguished by their low degree of special- ization, have arrived at states of structural quiescence or

224 THE INDUCTIONS OF BIOLOGY.

equilibrium ; that if they are not united, this equilibrium ends in dissolution ; but that by the mixture of them, this equilibrium is destroyed, and a new evolution initiated.

§ 78. What are the conditions under which Genesis takes place ? How does it happen that some organisms multiply by homogenesis, and others by heterogenesis ? Why is it that where agamogenesis prevails, it is usually from time to time interrupted by gamogenesis ? These are questions of extreme interest ; but questions to which decisive answers cannot yet be given. In the existing state of Biology, we must be content if we can learn the direction in which answers lie. A survey of the facts, discloses certain correla- tions which, if not universal, are too general to be without significance.

Where the multiplication of individuals is carried on by heterogenesis, we find, in numerous cases, that agamogenesis continues as long as the forces which result in growth, are greatly in excess of the antagonistic forces. While conversely, we find that the recurrence of gamogenesis, takes place when (be conditions are no longer so favourable to growth. In like manner, where there is homogenetic multiplication, new individuals are usually not formed while the preceding in- dividuals are still rapidly growing — that is, while the forces producing growth exceed the opposing forces to a great extent; but the formation of new individuals begins when nutrition is nearly equalled by expenditure. To specify all the facts ihat seem to warrant these inductions, would take more space than can be here spared. A few of them must suffice.

The relation between fructification and innutrition, among plants, was long ago asserted by a German biologist — by Wolff, I am told. When, some years ago, I met with the assertion, I was not acquainted with the evidence on which it rested. Since that time, however, I have, when occasion favoured, examined into the facts for myself. The result has been a conviction, strengthened by every further inquiry,

GENESIS. 225

tli at such, a relation exists. Uniaxial plants begin to

produce their lateral, flowering axes, only after the main axis has developed the great mass of its leaves, and is show- ing its diminished nutrition by smaller leaves, or shorter internodes, or both. In multiaxial plants, two, three, or more generations of leaf-bearing axes, or sexless individuals, are produced before any seed-bearing individuals show them- selves. When, after this first stage of rapid growth and agamogenetic multiplication, some gamogenetic individual? arise, thev do so where the nutrition is least ; — not on the

•/

main axis, or on the secondary axes, or even on the tertiary axes ; but on axes that are the most removed from the channels which supply nutriment. Again, a flowering axis is commonly less bulky than the others : either much shorter, or, if long, much thinner. And further, it is an axis of which the terminal internodes are undeveloped : the foliar organs, which instead of becoming leaves become sepals, and petals, and stamens, follow each other in close succession, instead of being separated by portions of the still-growing axis. Another group of evidences meets us, when

we observe the variations of fruit-bearing that accompany variations of nutrition, in the plant regarded as a whole. Besides finding, as above, that gamogenesis commences only when the luxuriance of early growth has been somewhat checked, by the extension of the remoter parts of the plant to some distance from the roots ; we find that gamogenesis is induced at an earlier stage than usual, by checking the nutri- tion Trees are made to fruit while still quite small, by cutting their roots, or putting them in pots ; and luxuriant branches which have had the flow of sap into them diminished, oy what gardeners call " ringing," begin to produce flower- shoots instead of leaf- shoots. Moreover, it is to be remarked that trees which, by flowering early in the year, seem to show a direct relation between gamogenesis and increasing nutrition, really do the reverse ; for in such trees, the flower - buds are formed in the autumn — that structure which deter-

226 THE INDUCTIONS OF BIOLOGY.

mines these buds into sexual individuals, is given when the nutrition is declining. Conversely, very high nutri-

tion in plants, prevents, or arrests, gamogenesis. It is notorious that unusual richness of soil, or too large a quantity of manure, results in a continuous production of leaf-bearing, or sexless, shoots. Besides being prevented from producing sexual individuals, by excessive nutrition, plants are, by excessive nutrition, made to change the sexual individuals they were about to produce, into sexless ones. This arrest of gamogenesis may be seen in various stages. The familiar instance of flowers made barren by the trans- formation of their stamens into petals, shows us the lowest degree of this reversed metamorphosis. Where the petals and stamens are partially changed into green leaves, the return from the gamogenetic structure towards the agamo- genetic structure, is more marked ; and it is still more marked when, as occasionally happens in luxuriantly-growing plants, new flowering axes, and even leaf- bearing axes, grow out of the centres of flowers.* The anatomical

* Among various examples of tins which I have ohserved, some of the most remarkable were among Foxgloves, growing in great numbers and of large size, in a wood between Whatstandwell Bridge and Crich, in Derbyshire. In one case, the lowest flower on the stem, contained, in place of a pistil, a shoot or spike of flower-buds, similar in structure to the embryo-buds of the main spike. I counted seventeen buds on it ; of which the first had three stamens, but was other- wise normal ; the second had three ; the third, four ; the fourth, four ; Sec. Another plant, having more varied monstrosities, evinced excess of nutrition with equal clearness. The following are the notes I took of its structure : — 1st, or lowest flower on the stem, very large ; calyx containing eight divisions, one partly transformed into a corolla, and another transformed into a small bud with bract (this bud consisted of a five-cleft calyx, four sessile anthers, a pistil, and a rudimentary corolla) ; the corolla of the main flower, which was complete, con- tained six stamens, three of them bearing anthers, two others being flattened and coloured, and one rudimentary ; there was no pistil, but, in place of it, a large bud, consisting of a three-cleft calyx, of which two divisions were tinted at the ends, an imperfect corolla, marked internally with the usual purple spots and hairs, three anthers sessile on this mal-formed corolla, a pistil, a seed-vessei with ovules, and, growing to it, another bud of which the structure was indistinct. 2nd flower, large ; calyx of seven divisions, one being transformed into a bud

GENESIS. 22?

Structure of the sexual axis, affords corroborative evidence : giving very much the impression, as it does, of an aborted sexless axis. Besides lacking those internodes which the leaf-bearing axis commonly possesses, the flowering axis differs by the absence of rudimentary lateral axes. In a leaf- bearing axis, the axil of every leaf usually contains a small bud, which may or may not develop into a lateral axis ; but though the petals of a flower are homologous with leaves, they do not bear homologous buds at their bases. Ordinarily, too, the foliar appendages of sexual axes, are much smaller than those of sexless ones — the stamens and pistils especially, which are the last formed, being extremely dwarfed ; and there is even reason for thinking that the absence of chloro- phyll from the parts of fructification, is a fact of like mean- ing. Moreover, the formation of the seed-vessel appears to be a direct consequence of arrested nutrition. If a glovecl-finger be taken to represent a growing shoot, (the finger standing for the core of the shoot, and the glove for the cambium-layer, in which the process of growth takes place) ; and if it be supposed that there is a diminished supply of material for growth ; then, it seems a fair inference, that growth will first cease at the apex of the cambium-layer, represented by the end of the glove- finger ; and supposing growth to continue in those parts of the cambium-layer that are nearer to the supply of nutri- ment, their further longitudinal extension will lead to the formation of a cavity at the extremity of the shoot, like that which results in a glove-finger when the finger is partially withdrawn and the glove sticks to its end. Whence it seems,

with bract, but much smaller than the other ; corolla large but cleft along the top ; six stamens with anthers, pistil, and seed-vessel. 3rd flower, large; six-cleft calyx, cleft corolla, with six stamens, pistil, and seed-vessel, with a second pistil halt unfolded at its apex. 4th flower, large ; divided along the top, six stamens. 5th flower, large; corolla divided into three parts, six stamens. 6th flower, large ; corolla cleft, calyx six cleft, the rest of the flower normal. 7th, and all sue* Beetling flowers, normal.

228 THE INDUCTIONS OF BIOLOGY.

both, that this introversion of the cambium-layer may be considered as due to failing nutrition, and that the ovules growing from its introverted surface (which would have been its outer surface but for the defective nutrition) are extremely aborted homologues of external appendages — either leaves or lateral axes : the essential organs of fructification thus

o

arising where the defective nutrition has reached its extreme.*

o

To all which let us not forget to add, that the sperm- cells and germ-cells are formed at the very ends of the organs of fruc- tification.

Those kinds of animals which multiply by heterogenesis, present us with a parallel relation between the recurrence of gamogenesis and the recurrence of conditions unfavourable to growth — at least, this is shown where experiments have thrown light on the connexion of cause and effect ; namely, among the Aphides. These creatures, hatched from eggs in the spring, multiply by agamogenesis throughout the summer. When the weather becomes cold, and plants no longer afford abundant sap, perfect males and females are produced ; and from gamogenesis there result fertilized ova. But now observe that beyond this evidence, we have much more conclusive evidence. For it has been shown* both that the rapidity of the agamogenesis is proportionate to the warmth and nutrition.,, and that if the temperature and

* It appears that botanists do not agree respecting the homologies of the crvules: some thinking that they are rudimentary foliar organs, and others that they are rudimentary axial organs. Possibly the dispute will prove a bootless one ; since there seems evidence that ovules may be transformed into either one or the other. Mr Salter's paper, lately referred to, shows that they niay graduate into stamens, which are foliar organs ; and the case of the Foxglove, which I have described above, shows that they may develop into flower- oucls, which are axial organs. I would venture to suggest, that the conflicting evidence can be reconciled, only by regarding ovules as the homologues of lateral append- ages ; and considering a lateral appendage is composed of a leaf, plus a rudiment- ary axis, either of which may abort. This is the view which seems countenanced by development; since, in its first stage, a lateral bud, whence a lateral append- age arises, shows no division into rudimentary leaf and rudimentary axis ; and it is to the lateral bud in. this first stage, that the seed-bud or ovule is homo* logons.

GENESIS. 229

supply of food be artificially maintained, the agamogenesia continues through the winter. JN"ay more — it not only, under these conditions, continues through one winter, but it has been known to continue for four successive years : some forty or tifty sexless generations being thus produced. And those who have investigated the matter, see no reason to doubt the indefinite continuance of this asramoorenetic mul-

o o

tiplication, so long as the external requirements are duly met. Evidence of another kind, which points very

distinctly to the same conclusion, is furnished by the hetero- genesis of the Daphnia — a small crustacean commonly known as the Water- flea, which inhabits ponds and ditches. From the nature of its habitat, this little creature is exposed to very variable conditions. Besides being frozen up in winter, the small bodies of water in which it lives, are often unduly heated by the summer sun, or dried up by continued drought. The circumstances favourable to the Daphnia s life and growth, being thus liable to interruptions which, in our cli- mate, have a regular irregularity of recurrence ; we may, in conformity with the hypothesis, expect to find both that the gainogenesis recurs along with evidence of declining nutri- tion, and that its recurrence is very variable. This we do find. From Mr Lubbock's paper on the Daplmia in the " Philosophical Transactions ' for 1857, and from further information which he has been good enough to furnish me, the following general facts are deducible : — First, that in each ovarium, alon^ with the rudiments of agamic e^srs, or

7 O O O O *

eggs which, if developed, produce young by true partheno- genesis, there usually, if not always, exists the rudiment of an ephippial egg ; which, from sundry evidences, is inferred to be a sexual or gamic egg. Second, that according to cir- cumstances, either agamogenesis or gamogenesis takes place ; but that if the agamic eggs develop, the rudimentary gamic egg disappears, or becomes absorbed ; and conversely, if the gamic egg develops, the agamic eggs disappear, or are ab- sorbed by it. Third, that the brood of agamic eggs contained

230 THE INDUCTIONS OF BIOLOGY.

in each ovarlum, amounts, under favourable circumstances, to as many as eight or nine ; while of the gamic eggs, only one at a time is produced in each ovarium, and occasionally one of the ovaria produces none : whence it follows, that as the gamic egg is not more than twice the bulk of the agamic egg, the quantity of matter contained in an agamic brood, is four times, and occasionally even eight times, as great as that contained in a gamic brood. Thus the quantity of nutriment expended in gamogenesis during a given period (making allowance for that which goes to the formation of the ephippium), is far less than that expended in agamogenesis during a like period. Seeing, then, this constant preparation for either gamic or agamic genesis, in a creature liable to such irregular variations of nutrition ; and seeing that the agamogenesis implies by its amount, a large excess of nutri- tion, while the gamogenesis implies by its amount, a small excess of nutrition ; we can scarcely doubt that the one or the other mode of multiplication occurs, according as the external conditions are or are not favourable to nutrition.

Passing now, to animals which multiply by homogenesis — animals in which the whole product of a fertilized germ ag- gregates round a single centre or axis, instead of round many centres or axes ; we see, as before, that so long as the con- ditions allow rapid increase in the mass of this germ-product, the formation of new individuals by gamogenesis does not take place. Speaking generally, we find that only when growth is declining in relative rapidity, do perfect sperm - cells and germ-cells begin to appear ; and that the fullest activity of the reproductive function, arises as growth ceases — speaking generally, we must say, because, though this relation is tolerably definite in the highest orders of animals which multiply by gamogenesis, it is less definite in the lower orders. This admission does not militate against the hypo- thesis, as it seems to do ; for the indefiniteness of the relation occurs where the limit of growth is comparatively indefinite, We saw (§ 46) that among active, hot-blooded creatures,

GENESIS.

euch as "mammals and birds, the inevitable balancing of

assimilation by expenditure, establishes, for each species, aii almost uniform adult size ; and among creatures of these kinds, (birds especially, in which this restrictive effect of expenditure is most conspicuous), the connexion between cassation of growth and commencement of reproduction, is distinct. But we also saw (§ 46) that where, as in the Cro- codile and the Pike, the conditions and habits of life are such, that expenditure does not overtake assimilation as the size increases, there is no precise limit of growth ; and in creatures thus circumstanced, we may naturally look for a compara- tively indeterminate relation between declining growth and commencing reproduction.* There is, indeed, among

fishes, at least one case which appears very anomalous. The male parr, or young of the male salmon, a fish of four or five inches in length, is said to produce rnilt. Having, at this early stage of its growth, not one hundredth of the weight of a full-grown salmon, how does its production of milt consist with the alleged general law ? The answer must be in a great measure hypothetical. If the salmon is (as it appears in its young state) a species of fresh-water trout, that has contracted the habit of annuallv migrating to the

«/

sea, where it finds a food on which it thrives — if the original size of this species was not much greater than that of the parr (which is nearly as large as some varieties of lake-trout and river- trout) — and if the limit of growth in the trout tribe is very indefinite, as we know it to be ; then we may reasonably infer, that the parr has nearly the adult form and size of this species of trout, before it acquired its migratory habit ; and that this production of milt, is,

* I owe to Mr Lubbock an important confirmation of this view. After stat- ing l.':s belief, that between Crustaceans and Insects, there exists a physiological relation analogous to that which exists between water-vertebrata and land-verte- brata ; he pointed out to me, that while among Insects, there is a definite limit of growth, and an accompanying definite commencerient of reproduction, amon* Crustaceans, where growth has no definite limit, there is no definite relation between the commencement of reproduction and the decrease or arrest of growth.

232 THE INDUCTIONS OF BIOLOGY.

in such, case, a concomitant of the incipient decline of growth naturally arising in the species, when living under the conditions of its remote ancestors. If this be admitted, the immense subsequent growth of the parr into the salmon, must be regarded as due to a suddenly -increased facility in obtaining food — a facility which removes to a great distance the limit at which assimilation is balanced by expenditure ; and which has the effect, analogous to that produced in plants, of arresting the incipient reproductive process, and causing a resumption of growth. A confirmation of this view may be drawn from, the fact, that when the parr, after its first migration to the sea, returns to fresh water, having increased in a few months from a couple of ounces to five or six pounds, it no longer shows any fitness for propagation : the grilse, or immature salmon, does not produce milt or spawn. But without citing further illustrations, or

attempting to meet further difficulties, it has, I think, been made sufficiently clear, that some such connexion as that al- leged, exists. Traversed, as is this relation between commence- ment of sexual reproduction and declining rate of growth, by various other relations, it is quite as manifest as we can expect it to be.

The general law to which both homogenesis and hetero- genesis conform, thus appears to be, that the products of a fertilized germ go on accumulating by simple growth, so long as the forces whence growth results are greatly in excess of the antagonist forces ; but that when diminution of the one set of forces, or increase of the other, causes a considerable decline in this excess, and an approach towards equilibrium, fertilized germs are again produced. Whether the germ- product be organized round one axis, or round the many axes that arise by agamogenesis — whether the development be continuous or discontinuous ; matters not. Whether, as in concrete organisms like the higher animals, this; approach to equilibrium results from that disproportionate increase of expenditure entailed by increase of size ; or whether, as in

GENESIS. 233

partially and wholly discrete organisms, like most plants and many inferior animals, this approach to equilibrium results from absolute or relative decline of nutrition ; matters not. In any case, the recurrence of gambgenesis is associated with a more or less marked decrease in the excess of tissue-pro- ducing power. We cannot say, indeed, that a de-

crease in this excess always results in gamogenesis ; for we have evidence to the contrary, in the fact that some organ- isms multiply for an indefinite period by agamogenesis only. Thus, the weeping willow, which has been propagated through- out Europe, does not seed in Europe ; and yet, as the weep- ing willow, by its large size and the multiplication of generation upon generation of lateral axes, presents the same causes of local innutrition as other trees, we cannot ascribe the absence of sexual axes to the continued predominance of nutrition. Among animals, too, the anomalous case of the Tiiieidce, a group of moths in which parthenogenetic mul- tiplication goes on for generation after generation, shows us that gamogenesis does not necessarily result from an approxi- mate balance of assimilation by expenditure. What we must say, is, that an approach towards equilibrium between the forces which cause growth and the forces which oppose growth, is the chief condition to the recurrence of gamo- genesis ; but that there are other unknown conditions, in the absence of which this approach to equilibrium is not followed by gamogenesis.

79. The above induction is an approximate answer to the question — When does gamogenesis recur ? but not to the question which was propounded — Why does gamogenesis re- cur ?--Why cannot multiplication be carried on in all cases, as it is in many cases, by agamogenesis ? As already said, biologic science is not yet advanced enough to reply. Mean- while, the evidence above brought together, suggests a cer- tain hypothetical answer, which it may be well to set down. Seeing as we do, on the one hand, that gamogenesis recur a

11

234 THE INDUCTIONS OF BIOLOGY.

only in individuals that are approaching towards a state of organic equilibrium ; and seeing, on the other hand, as we do, that the sperm-cells and germ-cells thrown off by such individuals, are cells in which developmental changes have ended in quiescence, but in which, after their union, there arises a process of active cell- formation ; we may suspect that the approach towards a state of general equilibrium in such gamogenetic individuals, is accompanied by an approach towards molecular equilibrium in them ; and that the need for this union of sperm-cell and germ-cell, is the need for overthrowing this equilibrium, and re-establishing active mole- 'cuiar change in the detached germ — a result which is pro- bably effected by mixing the slightly different physiological units of slightly different individuals. The several argu- ments that may be brought in support of this view, cannot be satisfactorily set forth until after the topics of Heredity and Variation have been dealt with. Leaving it for the pre- sent, I propose hereafter to reconsider this question, in con- nexion with sundry others that are raised by the phenomena of Genesis.

Before ending the chapter, however, it may be well to note the relations between these different modes of multiplication, and the conditions of existence under which they are respect- ively habitual. While the explanation of the teleologist is untrue, it is often an obverse to the truth ; for though, on the hypothesis of Evolution, it is clear that things are not arranged thus or thus for the securing of special ends, it is also clear, that arrangements which do secure these special ends, tend continually to establish themselves — are establish- ed by their fultilinent of these ends. Besides insuring a structural fitness between each kind of organism and its cir- cumstances, the working of " natural selection " also insures a fitness between the mode and rate of multiplication of each kind of organism and its circumstances. We may, therefore, without any teleological implication, consider the fitness of

GENESIS. 235

homogenesis and lieterogenesis to the needs of the different classes of organisms which exhibit them.

One of the facts to be observed, is, that heterogenesis pre- vails among organisms of which the food, though abundant compared with their expenditure, is dispersed in such a way that it cannot be appropriated in a wholesale manner. Pro- tophyta, subsisting on diffused gases and decaying organic matter in a state of minute subdivision ; and Protozoa, to which food comes in the shape of extremely small floating particles ; are enabled by their rapid agamogenetic multipli- cation, to obtain materials for growth, better, than they would do did they not thus continually divide and disperse in pur- suit of it. The higher plants, having for nutriment the car- bonic acid of the air and certain mineral components of the soil, show us modes of multiplication adapted to the fullest utilization of these substances. A herb, with but little power of forming the woody-fibre requisite to make a stern that can support wide-spreading branches, after producing a few sex- less axes, produces sexual ones ; and maintains its race better by the consequent early dispersion of seeds, than by a further production of sexless axes. But a tree, able to lift its suc- cessive generations of sexless axes high into the air, where each axis gets carbonic acid and light almost as freely as if it grew by itself, may with advantage go on budding-out sex- less axes year after year; since it thereby increases its sub- sequent power of budding- out sexual axes. Meanwhile, it may advantageously transform into seed-bearers, those axes which, in consequence of their less direct access to materials absorbed by the roots, are failing in their nutrition ; for in doing this, it is throwing- off from a point at which sus- tenance is deficient, a migrating group of germs that may find sustenance elsewhere. The heterogenesis displayed by animals of the Coelenterate type, has evidently a like utility. A polype, feeding on minute annelids and crustaceans, which, flitting through the water, come in contact with its tentacles;

236 THE INDUCTIONS OF BIOLOGY.

and limited to that quantity of prey which chance brings within its grasp ; buds out young polypes which, either as a colony or as dispersed individuals, spread their tentacles through a larger space of water than the parent alone can ; and by producing them, the parent better insures the continu- ance of its species, than it would do if it went on slowly grow- ing until its nutrition, was nearly balanced by its waste, and then multiplied by gamogenesis. Similarly with the Aphis. Living on sap sucked through its proboscis from tender shoots and leaves, and able thus to take in but a very small quan- tity in a given time, this creature's race is more likely to be preserved by a rapid asexual propagation of small indi- viduals, which disperse themselves over a wide but nowhere rich area of nutrition, than it would be did the individual growth continue so as to produce large individuals multiply- ing sexually. While at the same time we see, that when autumnal cold and diminishing supply of sap, put a check to growth, the recurrence of gamogenesis, and production of fertilized ova that remain dormant through the winter, is more favourable to the preservation of the race, than would be a further continuance of agamogenesis. On the

other hand, it is obvious that among the higher animals, living on food which, though dispersed, is more or less aggregated into large masses, this alternation of garnic and agamic reproduction ceases to be useful. The development of the germ-product into a single organism of considerable bulk, is in many cases a condition without which these large masses of nutriment could not be appropriated ; and here the formation of many individuals instead of one, would be fatal. But we still see the beneficial results of the general law — the postponement of gamogenesis until the rate of growth begins to decline. For so long as the rate of growth continues rapid, it is a proof that the organism gets food with great facility — that expenditure is not such as seriously to check accumulation ; and that the size reached is as }ret not disad- vantageous— or rather, indeed, that it is advantageous. But

GENESIS. 237

when the rate of growth is much decreased by the compara- tively rapid increase of expenditure— whan the excess of assimilative power is diminishing in such a way as to indi- cate its approaching disappearance ; it becomes needful for the maintenance of the species, that this excess shall bo turned to the production of new individuals ; since, did growth continue until this excess disappeared through the complete balancing of assimilation and expenditure, the pro- duction of new individuals would be either impossible or fatal to the parent. And it is clear that " natural selection ' ' will continually tend to determine the period at which gamo- genesis commences, in such a way as most favours the main- tenance of the race.

Here, too, may fitly be pointed out the fact, that, by 61 natural selection," there will in every case be produced, the most advantageous proportion of males and females. If the conditions of life are such as to render a greater or less in- equality of the sexes beneficial to the species, in respect either of the number of the offspring, or the character of the offspring ; then, those varieties of the species which, from any cause, approach more than other varieties towards this beneficial degree of inequalit}7, will be apt to supplant other varieties. And conversely, where equality in the number of males and females is beneficial, the equilibrium will be main- tained by the dying out of such varieties as produce offspring among which the sexes are not balanced.

CHAPTER Yin.

HEREDITY.

80. ALREADY, in the last two chapters, the law of heredi- tary transmission has heen tacitly assumed ; as, indeed, it unavoidably is in all such discussions. Understood in its entirety, the law is, that each plant or animal produces others of like kind with itself : the likeness of kind consist- ing, not so much in the repetition of individual traits, as in the assumption of the same general structure. This truth has been rendered so familiar by daily illustration, as almost to have lost its significance. That wheat produces wheat — that existing oxen have descended from ancestral oxen — that every unfolding organism eventually takes the form of the class, order, genus, and species from which it sprang ; is a fact which, by force of repetition, has acquired in our minds almost the aspect of a necessity. It is in this, however, that Heredity is principally displayed : the phenomena com- monly referred to it, being quite subordinate manifestations. And, as thus understood, Heredity is universal. The various instances of heterogenesis lately contemplated, seem, indeed, to be at variance with this assertion. But they are not really so. Though the recurrence of like forms, is, in these in- stances, not direct but cyclical, still, the like forms do recur ; and when taken together, the group of forms produced during one of the cycles, is as much like the groups produced in pre- ceding cycles, as the single individual arising by homo- genesis, is like ancestral individuals.

HEREDITY. 239

While, however, the general truth that organisms of a given type uniformly descend from organisms of the same type, is so well established by infinite illustrations, as to have assumed the character of an axiom ; it is not universally admitted that non-typical peculiarities are inherited. While the botanist would be so incredulous if told that a plant oi one class had produced a plant of another class, or that from seeds belonging to one order individuals belonging to another order had grown, that he would deem it needless to examine the evidence ; and while the zoologist would treat with con- tempt the assertion, that from the egg of a fish a reptile had arisen, or that an implacental mammal had borne a pla- cental mammal, or that an unguiculate quadruped had sprung from an ungulate quadruped, or even that from individuals of one species offspring of an allied species had proceeded ; yet there are botanists and zoologists who do not consider it certain, that the minor specialities of organization are trans- mitted from one generation to another. Some naturalists seem to entertain a vague belief, that the law of Heredity applies only to main characters of structure, and not to de- tails ; or, at any rate, that though it applies to such details as constitute differences of species, it does not apply to smaller details. The circumstance that the tendency to re- petition, is in a slight degree qualified by the tendency to variation (which, as we shall hereafter see, is but an indirect result of the tendency to repetition), leads some to doubt whether Heredity is unlimited. A careful weighing of the evidence, however, and a due allowance for the influences by which the minuter manifestations of Heredity are obscured

V

will remove the grounds for this scepticism.

First in order of importance, comes the fact, that not only are there uniformly transmitted from an organism to its offspring, those traits of structure which distinguish the class, order, genus, and species ; but also those which distinguish the variety. We have numerous cases, among both plants and animals, where, by natural or artificial conditions, there

240 THLE INDUCTIONS OF BIOLOGY.

have been produced divergent modifications of the same species ; and abundant proof exists that the members of any one sub-species, habitually transmit their distinctive pecu- liarities to their descendants. Agriculturists and gardeners can furnish unquestionable illustrations. Several varieties of wheat are known ; of which each reproduces itself. Since its introduction into England, there have been formed from the potato, a number of sub-species: some of them differing greatly in their forms, sizes, qualities, and periods of ripening. Of peas, also, the like may be said. And the case of the cabbage-tribe, is often cited as showing the per- manent establishment of races that have diverged widely from a common stock. Among fruits and flowers, the multi- plication of kinds, and the continuance of each kind with certainty by agamogenesis, and to some extent by gamo- genesis, might be exemplified without end. From all sides evidence may be gathered showing a like persistence of varieties in each species of animal. We have our distinct breeds of sheep, our distinct breeds of cattle, our distinct breeds of horses : each breed maintaining its characteristics. The several sorts of dogs, which, if we accept the physiolo gical test, we must consider as all of one species, show us in a marked manner the hereditary transmission of small diner- ences — each sort, when kept pure, reproducing itself not only in size, form, colour, and quality of hair, but also in disposition and speciality of intelligence. Rabbits, too, have their permanently- established races. And in the Isle of Man, we have a tail-less kind of cat. Even in the absence of other evidence, that which ethnologv furnishes would

7 O»/

suffice. Grant them to be derived from one stock, and the varieties of man yield proof upon proof that non-specific traits of structure are bequeathed from generation to gener- ation. Or grant only that there is evidence of their deriva- tion from several stocks, and we still have, between races de- scended from a common stock, distinctions which prove the inheritance of minor peculiarities. Besides seeing that

HEREDITY. 241

negroes continue to produce negroes, copper-coloured men to produce men of a copper colour, and the fair-skinned raceg to perpetuate their fair skins — besides seeing that the broad- faced and flat-nosed Calmuck begets children with broad faces and flat noses, while the Jew bequeaths to his offspring the features which have so long characterized Jews ; we see that those small unlikenesses which distinguish more nearly-allied varieties of men, are maintained from generation to generation. In Germany, the ordinary shape of skull is appreciably differ- ent from that common in Britain : near akin though the Germans are to the British. The average Italian face con- tinues to be unlike the faces of northern nations. The French character is now, as it was centuries ago, contrasted in sundry respects with the characters of neighbouring peoples. Nay, even between races so closely allied as the Scotch Celts, the Welch Celts, arid the Irish Celts, appreciable differences of form and nature have become established.

That sub-species and sub-sub-species, thus exemplify that same general law of inheritance which shows itself in the per- petuation of ordinal, generic, and specific peculiarities ; is strong reason for the belief that this general law is unlimited in its application. In addition to the warrant which this be- lief derives from evidence of this kind, it has also the support of still more special evidence. Numerous illustrations of He- redity are yielded by experiment, and by direct observation of successive generations. They are divisible into two classes. In the one class come cases where congenital peculiarities, not traceable to any obvious causes, are bequeathed to de- scendants. In the other class come cases where the peculiar- ities thus bequeathed are not congenital, but have resulted from changes of functions during the lives of the individuals bequeathing them. We will consider first the cases that come in the first class.

§81. Note at the outset the character of the chief testi- mony. Excluding those inductions that have been so fully

242 THE INDUCTIONS OF BIOLOGY.

verified as to rank with exact science, there are no inductions so trustworthy as those which have undergone the mercantile test. When we have thousands of men whose profit or loss depends on the truth of the inferences they draw from simple and perpetually-repeated observations ; and when we find that the inference arrived at, and handed down from genera- tion to generation of these deeply-interested observers, has become an unshakable conviction ; we may accept it without hesitation. In breeders of animals we have such a class, led by such experiences, and entertaining such a conviction — the conviction that minor peculiarities of organization are in- lierited as well as major peculiarities. Hence the immense prices given for successful racers, bulls of superior forms, sheep that have certain desired peculiarities. Hence the careful record of pedigrees of high-bred horses and sporting closrs. Hence the care taken to avoid intermixture with in-

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ferior stocks. Citing the highest authorities respecting the effects of breeding from animals having certain superiorities, with the view of propagating these superiorities, Mr Darwin writes : — " Youatt, who was probably better acquainted with the works of agriculturists than almost any other individual, and who was himself a very good judge of an animal, speaks of the principle of selection as ' that which enables the agri- culturist not only to modify the character of his flock, but to change it altogether. It is the magician's wand, by means of which he may summon into life whatever form and mould he pleases.' Lord Somerville, speaking of what breeders have done for sheep, says : — " It would seem that they had chalked upon a wall a form perfect in itself and then given it exist- ence." That most skilful breeder, Sir John Sebright, used to Bay, with respect to pigeons, that " he would produce any given feather in three years, but it would take him six years to obtain head and beak." In all which statements the tacit assertion is, that individual traits are bequeathed from generation to generation ; and that when they are not b -ought into conflict with opposite traits, they may ba

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eo perpetuated and increased as to become permanent dis- tinctions.

Of special instances, there are many besides that of the oft en-cited Otter-breed of sheep, descended from a single short legged lamb, and that of the six-fingered Gratio Kelleia, who transmitted his peculiarity in different degrees, to several of his children and to some of his grandchildren. In a paper con- tributed to the Edinburgh New Philosophical Journal for July 1863, Dr Strutters gives several cases of hereditary digital variations. Esther P — , who had six fingers on one hand, be- qusathed this malformation, along some lines of her descend- ants, for two, three, and four generations. A — S — inherited an extra digit on each hand and each foot from his father ; and C — Gr — , who also had six fingers and six toes, had an aunt and a grandmother similarly formed. A collection of evidence has been made by Mr Sedgwick, and published by him in the Medico- Chirurgical Review for April and for July 1863, in two articles on " The Influence of Sex in limiting Hereditary Transmission." From these articles are selected the following

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cases and authorities : — Augustin Duforet, a pastry-cook of Douai, who had but two instead of three phalanges to all his fingers and toes, inherited this malformation from his grand- father and father, and had it in common with an uncle and numerous cousins. An account has been given byDr Lepine, of a man with only three fingers on each hand and four toes on each foot, and whose grandfather and son exhibited the like anomaly. Bechet describes Victoire Barre as a woman who, like her father and sister, had but one developed finger on each hand, and but two toes on each foot, and whose mon- strosity re-appeared in two daughters. And there is a case where the absence of two distal phalanges on the hands was traced for two generations. The various recorded instances in which there has been transmission from one generation to

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another, of webbed-fingers, of webbed-toes, of hare-lip, of congenital luxation of the thigh, of absent patellae, of club-foot, &c., would occupy more space than can here be

244 THE INDUCTIONS OF BIOLOGY.

spared. Defects in the organs of sense are also not

unfrequently inherited. Four sisters, their mother, and grandmother, are described by Duval as similarly affected by cataract. Prosper Lucas details an example of hereditary am- aurosis affecting the females of a family for three generations. Duval, Graffe, Dufon, and others testify to like cases coming under their observation.* Deafness, too, is occasionally trans- mitted from parent to child. There are deaf-mutes whose imperfections have been derived from ancestors; and mal- formations of the external ears have also been perpetuated in offspring. Of transmitted peculiarities of the skin

and its appendages, many illustrations have been noted. One is that of a family remarkable for enormous black eyebrows ; another that of a family in which every member had a lock of hair of a lighter colour than the rest on the top of the head ; and there are also instances of congenital baldness being hereditary. Entire absence of teeth, absence of particular teeth, and anomalous arrangements of teeth, are recorded as traits that have descended to children. And we have evidence that soundness and unsoundness of teeth are transmissible.

The inheritance of such diseases as gout, consumption, and insanity, is universally admitted. Among the less-common diseases of which the descent from one generation to another has been observed, are, ichthyosis, leprosy, pityriasis, sebace- ous tumours, plica polonica, dipsomania, somnambulism, cata- lepsy, epilepsy, asthma, apoplexy, elephantiasis. General nervousness displayed by parents, almost always re-appears in their children. Even a bias towards suicide appears to be sometimes hereditary.

§ 82. To prove the transmission of those structural pecu- liarities that have resulted from functional peculiarities, is,

* "While this chapter is passing through the press, I learn from Mr White Cooper, that not only are near sight, long sight, dull sight, and squinting, here- ditary ; but that a peculiarity of vision confined to one eye, is frequently trans- mitted— re -appearing in the same eye in offspring.

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for several reasons, comparatively difficult. Changes pro- duced in the sizes of parts by changes in their amounts of action, are mostly unobtrusive. A muscle that has increased in bulk, is so obscured by natural or artificial clothing, that un- less the alteration is extreme it passes without remark. Such nervous developments as are possible in the course of a single life, cannot be seen externally. Yisceral modifications of a normal kind, are observable but obscurely, or not at all. And if the changes of structure worked in individuals by changes in their habits, are thus difficult to trace ; still more difficult to trace must be the transmission of them — further hidden, as this is, by the influence of other individuals that are often otherwise modified by other habits. Moreover, such special- ities of structure as are due to specialities of function, are usually entangled with specialities of structure that are, or may be, due to selection, natural or artificial. In the majority of cases, it is impossible to say that a structural peculiarity which seems to have arisen in offspring from a functional peculiarity in the parent, is wrholly independent of some congenital peculiarity of structure in the parent, which in- duced this functional peculiarity. We are restricted to cases with which natural or artificial selection can have had nothing to do ; and such cases are difficult to find. Some, however, may here be noted.

A species of plant that has been transferred from one soil or climate to another, frequently undergoes what botanists call " a change of habit " — a change which, without affecting its specific characters, is yet conspicuous. In its new locality, the species is distinguished by leaves that are much larger, or much smaller, or differently shaped, or more fleshy ; or instead of being, as before, comparatively smooth, it becomes hairy; or its stem becomes woody instead of being herbaceous; or its branches, no longer growing upwards, assume a droop- ing character. Now these " changes of habit" are clearly de- termined by functional changes. Occurring, as they do, in Eiany individuals that have undergone the same transportation,

246 THE INDUCTIONS OF BIOLOGY.

they cannot be classed as " spontaneous variations." They are modifications of structure, consequent on modifications of function, that have been produced by modifications in the actions of external forces. And as these modifications re-ap- pear in succeeding generations, we have, in them, examples of functionally-established variations that are hereditarily transmitted. Further evidence is supplied by what

are called " sports " in plants. These are of two kinds — the gamogenetic and the agamogenetic. The gamogenetic may be ascribed wholly to " spontaneous variations ;" or if they are partly due to the inheritance of structural changes that are produced by functional changes, this cannot be proved. But where the individuals displaying the variations arise by agamogenesis, the reverse is the case: spontaneous variation is out of the question ; and the only possible interpretation is deviation of structure caused by deviation of function. A new axis which buds out from a parent- axis, assumes an un- like character — gives off lobed leaves in place of single leaves, or has an otherwise different mode of growth. This change of structure implies change in the developmental actions which produced the new bud — change, that is, in the actions going on in the parent shoot — functional change. And since the modified structure thus impressed on the new shoot by modified function, is transmitted by it to all the shoots it bears ; we are obliged to regard the case as one of acquired modification that has become hereditary.

Evidence of analogous changes in animals, is difficult to disentangle. Only among domesticated animals, have we any opportunity of tracing the effects of altered habits ; and here, in nearly all cases, artificial selection has obscured the results. Still, there are some facts which seem to the point. Mr Darwin, while ascribing almost wholly to " natural selection ' the production of those modifications which eventuate in differences of species, nevertheless admits the effects of use and disuse. He says — " I find in the domestic duck that the bonea of the wing weigh less and the bones of the leg more, in pro-

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portion to the whole skeleton, than do the same bones in the wild duck ; and I presume that this change may be safely attributed to the domestic duck flying much less, and walking more, than its wild parent. The great and inherited develop- ment of the udders in cows and goats in countries where they are habitually milked, in comparison with the state of these organs in other countries, is another instance of the effect of

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use. Not a single domestic animal can be named which has not in some country drooping ears ; and the view suggested by some authors, that the drooping is due to the disuse of the muscles of the ear, from the animals not being much alarmed, by danger, seems probable." Again — " The eyes of moles and of some burrowing rodents are rudimentary in size, and in some cases are quite covered up by skin and fur. This state of the eyes is probably due to gradual reduction from disuse, but aided perhaps by natural selection. " * * " It is well

known that several animals, belonging to the most differ- ent classes, which inhabit the caves of Styria and of Kentucky, are blind. In some of the crabs the footstalk of the eye re- mains, though the eye is gone ; the stand for the telescope is there, though the telescope with its glasses has been lost. As it is difficult to imagine that eyes, though useless, could be in any way injurious to animals living in darkness, I attribute their loss wholly to disuse." The direct inheritance of an ac- quired peculiarity is sometimes observable. Mr Lewes gives a case. He " had a puppy taken from its mother at six weeks old, who, although never taught, ' to beg' (an accomplishment his mother had been taught), spontaneously took to begging for everything he wanted when about seven or eight months old : he would beg for food, beg to be let out of the room, and one day was found opposite a rabbit hutch begging for rabbits." Instances are on record, too, of sporting dogs which spontaneously adopted in the field, certain modes of behaviour which their parents had learnt.

But the best examples of inherited modifications produced by modifications of function, occur in the human race. To no

248 THE INDUCTIONS OF BIOLOGY.

other cause can be ascribed the rapid metamorphoses under- gone by the British races when placed in new conditions. It is notorious that, in the United States, the descendants of the immigrant Irish lose their Celtic aspect, and become Ameri- canized. This cannot be ascribed to intermarriage with Americans ; since the feeling with which Irish are regard- ed by Americans, prevents any considerable amount of inter- marriage. Equally marked is the case of the immigrant Germans, who, though they keep themselves very much apart, rapidly assume the prevailing type. To say that " spontaneous variation ' increased by natural selection, can have produced this effect, is going too far. Races so numer- ous, cannot have been supplanted in the course of two or three generations by varieties springing from them. Hence there is no escape from the conclusion, that physical and so- cial conditions have here wrought modifications of function and structure, which offspring have inherited and increased. Similarly with special cases. In the Cyclopcedia of Practical Medicine, Vol. II. p. 419, Dr Brown states that he "has in many instances observed in the case of individuals whose complexion and general appearance has been modified by re- sidence in hot climates, that children born to them subse- quently to such residence, have resembled them rather in their acquired than primary mien."

Some special modifications of organs caused by special changes in their functions, may also be noted. That large hands are inherited by men and women whose ancestors led laborious lives ; and that men and women whose descent, for many generations, has been from those unused to manual labour, commonly have small hands ; are established opinions. It seems very unlikely that in the absence of any such con- nexion, the size of the hand should thus have come to be generally regarded as some index of extraction. That there exists a like relation between habitual use of the feet and large- ness of the feet, we have strong evidence in the customs of the Chinese. The torturing practice of artificially arresting the

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growth of the feet, could never have become established among the ladies of China, had they not found abundant proof that a small foot was significant of superior rank — that is of a luxurious life — that is of a life without bodily labour. There is some evidence, too, that modifica-

tions of the eyes, caused by particular uses of the eyes, are inherited. Short sight appears to be uncommon in rural populations ; but it is frequent among classes of people who use their eyes much for reading and writing ; and in these classes, short sight is often congenital. Still more marked is this relation in Germany. There, the educated classes are no- toriously studious ; and judging from the numbers of young Germans who wear spectacles, there is reason to think that congenital myopia is very frequent among them.

Some of the best illustrations of functional heredity, are

»/ '

furnished by the mental characteristics of human races. Cer- tain powers which mankind have gained in the course of civil- ization, cannot, I think, be accounted for, without admitting the inheritance of acquired modifications. The musical faculty Is one of these. To say that " natural selection" has developed it, by preserving the most musically endowed, seems an in- adequate explanation. Even now that the development and prevalence of the faculty have made music an occupation by which the most musical can get sustenance and bring up families ; it is very questionable whether, taking the musical life as a whole, it has any advantage over others in the struggle for existence and multiplication. Still more if we look back to those early stages through which the faculty must have passed, before definite perception of melody was arrived at, we fail to see how those possessing the rudimental faculty in a somewhat greater degree than the rest, would thereby be enabled the better to maintain themselves and their children. If so, there is no explanation but that the habitual association of certain cadences of human speech with certain emotions, has slowly established in the race an organized and inherited connexion between such cadences and such emotions ; that the

250 THE INDUCTIONS OF BIOLOGY.

combination of such cadences, more or less idealized, which constitutes melody, has all along had a meaning in the average mind, only because of the meaning which cadences had acquired in the average mind ; and that by the continual hearing and practice of melody, there has been gained and transmitted an increasing musical sensibility. Confirmation of this

view may be drawn from individual cases. Grant that among a people endowed with musical faculty to a certain degree, spontaneous variation will occasionally produce men possessing it in a higher degree ; it cannot be granted that spontaneous variation accounts for the frequent production, by such highly- endowed men, of men still more highly endowed. On the average, the offspring of marriage with others not similarly endowed, will be less distinguished rather than more distin- guished. The most that can be expected is, that this unusual amount of faculty shall re-appear in the next generation undi- minished. How then shall we explain cases like those of Bach, Mozart, and Beethoven, who were ail sons of men having un- usual musical powers, but who greatly excelled their fathers in their musical powers ? What shall we say to the facts, that Haydn was the son of the organist, that Hummel was born to a music master, and that Weber's father was a dis- tinguished violinist ? The occurrence of so many cases in one nation, within a short period of time, cannot rationally be ascribed to the coincidence of " spontaneous variations." It can be ascribed to nothing but inherited developments of structure, caused by augmentations of function.

But the clearest proof that structural alterations caused by alterations of function, are inherited, occurs when the alter- ations are morbid. " Certain modes of living engender gout ; ' and gout is transmissible. It is well known that in persons pre- viously healthy, consumption may be produced by unfavourable conditions of life — by bad and insufficient food ; by foul, damp, unventilated habitations; and even by long-continued anxiety. It is still more notorious that the consumptive diathesis is conveyed from parent to child. Unless, then, a distinction

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be assumed between constitutional consumption and con- sumption induced by unwholesome conditions — unless it be asserted that consumption of unknown origin is transmiss- ible, while functionally-produced consumption is not ; it must be admitted that those changes of structure from which the consumptive diathesis results, may be caused in parents by changes of function, and may be inherited by their chil- dren. Most striking of all, however, is the fact lately brought to light, that functional disorders artificially estab- lished, may be conveyed to offspring. Some few years since M. Brown-Sequard, in the course of inquiries into the nature and causes of epilepsy, hit on a method by which epilepsy could be originated. Guinea-pigs were the creatures on which, chiefly, he experimented ; and eventually, he disco- vered the remarkable fact, that the young of these epileptic guinea-pigs were epileptic : the functionally-established epilepsy in the parents, became constitutional epilepsy in the offspring. Here we have an instance which, standing even alone, decides the question. We have a special form of nervous action, not caused by any natural variation of structure that had arisen spontaneously in the organism, but one caused by a certain incidence of external forces. We have this special form of nervous action becoming confirmed by re- petition : the fits are more and more easily induced — there is established the epileptic habit. That is to say, the connected nervous actions constituting a fit, produce in the nervous system such changes of structure, that subsequent connected nervous actions of like kind, follow one another with increased readiness. And that this epileptic habit is inherited, proves conclusively that these structural modifications worked by functional modifications, are impressed on the whole organism in such way as to affect the reproductive centres, and cause them to unfold into organisms that exhibit like modifications. Evidence nearly allied to this, and scarcely less significant, is furnished by that transmission of general nervousness, no- ticed in the last section. Nervousness is especially common

A V

252 THE INDUCTIONS OF BIOLOCJY.

among classes of people who tax their brains much. Among these classes, we daily see this constitutional modification produced by excess of function, in men whose progenitors were not nervous ; and the children of such men habitually inherit more or less of the modification.

§ 83. Two modified manifestations of Heredity remain to be noticed. The one is the re-appearance in offspring, of traits not borne by the parents, but borne by the grandparents or by remoter ancestors. The other is the limitation of Heredity by sex — the restriction of certain transmitted peculiarities to offspring of the same sex as the parent possessing these peculiarities.

Atavism, which is the name given to the recurrence of ancestral traits, is proved by many and varied facts. In the picture-galleries of old families, and on the monumental brasses in the adjacent churches, are often seen types of feature that are still, from time to time, repeated in members of these families. It is matter of common remark that some con- stitutional diseases, such as gout and insanity, after missing a generation, will show themselves in the next. Dr Struthers, in his above-quoted paper on " Variation in the Number of Fingers and Toes, and of the Phalanges, in Man," gives cases of malformations that were common to grandparent and grandchild, but of which the parent had no trace. M. Girou (as quoted by Mr Sedgwick) says — " One is often surprised to see lambs black, or spotted with black, born of ewes and rams with white wool, but if one takes the trouble to go back to the origin of this phenomenon, it is found in the an- cestors.'' Instances still more remarkable, in which the re- moteness of the ancestors copied is very great, are given by Mr Darwin. He points out that in crosses between varieties of the pigeon, there will sometimes re-appear the plumage of the original rock-pigeon, from which these varieties descend- ed; and he instances the faint zebra-like markings occasion- ally traceable in horses, as having probably a like meaning.

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The limitation of Heredity by sex, cannot yet be regarded as established. While in many cases it seems clearly mani- fested ; it is in other cases manifested to a very small degree, if at all. In Mr Sedgwick's essays, already named, will be found evidence implying that there exists some such tendency to limitation, which does or does not show itself distinctly, according to the nature of the organic modification to be conveyed. But more facts must be collected before any positive conclusion can be reached.

§ 84. A positive explanation of Heredity is not to be expected in the present state of Biology. We can look for nothing beyond a simplification of the problem ; and a reduction of it to the same category with certain other problems which also admit of hypothetical solution only. If an hypothesis which certain other wide-spread phenomena have already thrust upon us, can be shown to render the phenomena of Heredity more in- telligible than they at present seem, we shall have reason to entertain it. The applicability of any method of interpreta- tion to two different but allied classes of facts, is evidence of its truth.

The power which organisms display of reproducing lost parts, we saw to be inexplicable except on the assumption that the units of which any organism is built have an innate tendency to arrange themselves into the shape of that organ- ism (§ 65). We inferred that these units must be the pos- sessors of special polarities, resulting from their special struc- tures ; and that by the mutual play of their polarities they are compelled to take the form of the species to which they belong. And the instance of the Begonia phyllomaniaca left us no escape from the admission that the ability thus to arrange themselves, is latent in the units contained in every undiffer- entiated cell. Quite in harmony with this conclusion,

are certain implications since noticed, respecting the characters of sperm-cells and germ-cells. We saw sundry reasons for rejecting the supposition that these are highly-specialized cells

254 THE INDUCTIONS OF BIOLOGY.

and for accepting the opposite supposition, that they are cells differing from others rather in being unspecialized. And here the assumption to which we seem driven by the ensemble of the evidence, is, that sperm-cells and germ-cells are essentially nothing more than vehicles, in which are contained small groups of the physiological units in a fit state for obeying their proclivity towards the structural arrangement of the species (hey belong to.

Thus the phenomena of Heredity are seen to assimilate with other phenomena ; and the assumption which these other phenomena thrust on us, appears to be equally thrust on us by the phenomena of Heredity. * We must con- clude that the likeness of any organism to either parent, is conveyed by the special tendencies of the physiological units derived from that parent. In the fertilized germ we have two groups of physiological units, slightly different in their structures. These slightly-different units, severally multiply at the expense of the nutriment supplied to the unfolding germ — each kind moulding this nutriment into units of its own type. Throughout the process of evolution, the two kinds of units, mainly agreeing in their polarities and in the form which they tend to build themselves into, but having mfuor differences, work in unison to produce an organism of the species from which they were derived, but work in antago asm to produce copies of their respective parent- organisms. And hence ultimately results, an organism in which traits or the one are mixed with traits of the other.

If the likeness of offspring to parents is thus determined, it becomes manifest, a priori, that besides the transmission of generic and specific peculiarities, there will be a transmis- sion of those individual peculiarities which, arising without assignable causes, are classed as " spontaneous.'* For if the assumption of a special arrangement or parts by an organism, is due to the proclivity of its physiological units towards that arrangement ; then the assumption of an arrangement of parts slightly different from that of the species, implies

HEREDITY. 255

physiological units slightly unlike those of the species ; and these slightly- unlike physiological units, communicated through the medium of sperm-cell or germ-cell, will tend, in the offspring, to build themselves into a structure similarly diverging from the average of the species.

It is not equally manifest, a priori, however, that on this hy- pothesis, alterations of structure caused by alterations of func- tion, must be transmitted to offspring. It is not obvious that change in the form of a part, caused by changed action, in- volves such change in the physiological units throughout the organism, that these, when groups of them are thrown off in the shape of reproductive centres, will unfold into organisms that have this part similarly changed in form. Indeed, when treating of Adaptation (§ 69), we saw that an organ modified bv increase or decrease of function, can but slowly so re-act on the system at large, as to bring about those correlative changes required to produce a new equilibrium ; and yet only when such new equilibrium has been established, can we ex- pect it to be fatty expressed in the modified physiological units of which the organism is built — only then can we count on a complete transfer of the modification to descendants. Nevertheless, that changes of structure caused by changes of action, must also be transmitted, however obscurely, from one generation to another, appears to be a deduction from first principles — or if not a specific deduction, still, a general implication. For if an organism A, has, by any peculiar habit or condition of life, been modified into the form A, it follows inevitably, that all the functions of A', reproductive function included, must be in some degree different from the functions of A. An organism being a combination of rhythmically-acting parts in moving equilibrium, it is im- possible to alter the action and structure of any one part, without causing alterations of action and structure in all the rest; just as no member of the Solar System could be modi- fied in motion or mass, without producing re-arrangements throughout the whole Solar System. And if the organism A

256 THE INDUCTIONS OF BIOLOGY.

when changed to A', must be changed in all its functions : then the offspring of A' cannot be the same as they would have been had it retained the form A. It involves a denial of the persistence of force to say that A may be changed into A, and may yet beget offspring exactly like those it would have begotten had it not been so changed. That the change in the offspring must, other things equal, be in the same direction as the change in the parent, we may dimly see is implied by the fact, that the change propagated throughout the parental system is a change towards a new state of equilibrium — a change tending to bring the actions of all organs, reproductive included, into harmony with these new actions. Or, bringing the question to its ultimate and simplest form, we may say that as, on the one hand, phy- siological units will, because of their special polarities, build themselves into an organism of a special structure ; so. on the other hand, if the structure of this organism is modified by modified function, it will impress some corresponding modification on the structures and polarities of its units. The units and the aggregate must act and re-act on each other. The forces exercised by each unit on the aggregate and by the aggregate on each unit, must ever tend towards a balance. If nothing prevents, the units will mould the aggregate into a form in equilibrium with their pre-existing polarities. If. contrariwise, the aggregate is made by incident actions to take a new form, its forces must tend to re-mould the units into harmony with this new form. And to say that the physiological units are in any degree so re- moulded as to bring their polar forces towards equilibrium with the forces of the modified aggregate, is to say that when separated in the shape of reproductive centres, these units will tend to bui)d themselves up into an aggregate modified in the same di- rection.

CHAPTEE IX.

VAKIATION.

§ 85. EQUALLY conspicuous with the truth that every organ* ism bears a general likeness to its parents, is the truth that no organism is exactly like either parent. Though similar to both in generic and specific traits, and usually, too, in those traits which distinguish the variety, it diverges in numer- ous traits of minor importance. JSTo two plants are indistin- guishable ; and no two animals are without differences. Variation is co-extensive with Heredity.

The degrees of variation have a wide range. There are deviations so small as to be not easily detected ; and there are deviations great enough to be callad monstrosities. In plants, we may pass from cases of slight alteration in the shape or texture of a leaf, to cases where, instead of a flower with its calyx above the seed-vessel, there is produced a flower with its calyx below the seed-vessel ; and while in one animal, there arises a scarcely noticeable unlikeness iu the length or colour of the hair, in another, an organ is absent, or a supernumerary organ appears. Though small variations are by far the most general, yet variations of considerable magnitude are not uncommon ; and even those variations constituted by additions or suppressions of parts, are not so rare as to be excluded from the list of causes by which organic forms are changed. Cattle without horns are fre- quent. Of sheep there are horned breeds and breeds that

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258 THE INDUCTIONS OF BIOLOGY.

have lost their horns. At one time, there existed in Scot- land a race of pigs with solid feet instead of cleft feet. In pigeons, according to Mr Darwin, " the number of the cau- dal and sacral vertebrce vary ; as does the number of the ribs, together with their relative breadth and the presence of processes."

That variations both small and large which arise without any specific assignable cause, tend to become hereditary, was shown in the last chapter. Indeed the evidence which proves Heredity in its smaller manifestations, is the same evidence which proves Variation ; since it is only when there occur vari- ations, that the inheritance of anything beyond the structural peculiarities of the species, can be proved. It remains here, however, to be observed, that the transmission of variations is itself variable ; and that it varies both in the direction of decrease and in the direction of increase. An individual trait of one parent, may be so counteracted by the influence of the other parent, that it may not appear in the offspring ; or not being so counteracted, the offspring may possess it, perhaps in an equal degree or perhaps in a less degree ; or the off- spring may exhibit the trait in even a still higher degree. Of the illustrations of this, one must suffice. I quote it from the essay by Dr Struthers, referred to in the last chapter.

" The great- great- grandmother, Esther P (who mar- ried A L ), had a sixth little finger on one hand. Of

their eighteen children (twelve daughters and six sons), only one (Charles) is known to have had digital variet}^. We have the history of the descendants of three of the sons, Andrew, Charles, and James.

" (1.) Andrew L had two sons, Thomas and Andrew ;

and Thomas had two sons all without digital variety. Here we have three successive generations without the variety possessed by the great -grandmother showing itself.

"(2.} James L , who was normal, had two sons and

seven daughters, also normal. One of the daughters became Mrs J (one of the informants), and had three daughters

VARIATION. 259

and five sons, all normal except one of the sons, James J — ~ , now cet. 17, who had six fingers on each hand. * * *

" In this branch of the descendants of Esther, we see it passing over two generations and reappearing in one member of the third generation, and now on both hands.

" (3.) Charles L , the only child of Esther who had

digital varietv, had six fingers on each hand. He had three

O »/ '

sons, James, Thomas, and John, all of whom were born with six fingers on each hand, while John has also a sixth toe on

O *

one foot. He had also five other sons and four daughters, all of whom were normal.

" (a.) Of the normal children of this, the third generation, the five sons had twelve sons and twelve daughters, and the four daughters have had four sons and four daughters, being the fourth generation, all of whom were normal. A fifth generation in this sub-group consists as yet of only two boys and two girls, who are also normal.

" In this sub-branch, we see the variety of the first gener- ation present in the second, passing over the third and fourth, and also the fifth as far as it has yet gone.

" (b.) James had three sons and two daughters, who are normal.

" (<?.) Thomas had four sons and five daughters, who are normal ; and has two grandsons, also normal.

" In this sub-branch of the descent, we sea the variety of the first generation, showing itself in the second and third, and passing over the fourth, and (as far as it yet exists) the fifth generation.

" (d.) John L — (one of the informants) had six fingers, the additional finger being attached on the outer side, as in the case of his brothers James and Thomas. All of them had the additional digits removed. John has also a sixth toe on one foot, situated on the outer side. The fifth and sixth toes have a common, proximal phalanx, and a common integument invests the middle and distal phalanges, each having a separate nail.

260 THE INDUCTIONS OF BIOLOGY.

" John L has a son who is normal, and a daughter

Jane, who was born with six fingers on each hand and six toes on each foot. The sixth fingers were removed. The sixth toes are not wrapped with the fifth as in her father's case, but are distinct from them. The son has a son and daughter, who, like himself, are normal.

" In this, the most interesting sub-branch of the descent, we see digital increase, which appeared in the first generation on one limb, appearing in the second on two limbs, the hands ; in the third on three limbs, the hands and one foot ; in the fourth on all the four limbs. There is as yet no fifth generation in uninterrupted transmission of the variety. The variety does not yet occur in any number of the fifth generation of Esther's descendants, which consists, as yet, only of three boys and one girl, whose parents were normal, and of two boys and two girls, whose grandparents were normal. It is not known whether in the case of the great-

great-grandmother, Esther P , the variety was original

or inherited."*

§ 86. Where there is great uniformity among the mem- bers of a species, the divergences of offspring from the average type, are usually small ; but where, among the members of a species, considerable unlikenesses have once been established, unlikenesses among the offspring are fre- quent and great. Wild plants growing in their natural habitats, are uniform over large areas, and maintain from generation to generation like structures ; but when cultiva- tion has caused appreciable differences among the members 3f any species of plant, extensive and numerous deviations are apt to arise. Similarly, between wild and domesticated

* This remarkable case appears to militate against the conclusion, drawn some few pages back, that the increase of a peculiarity by coincidence of " spontaneous variations " in successive generations, is very improbable ; and that the special superiorities of musical composers cannot have thus arisen. The reply is, that the extreme frequency of the occurrence among so narrow a class as that of musical composers, forbids the interpretation -hus suggested,

VARIATION. 261

animals of the same species, we see the contrast, that though the homogeneous wild race maintains its typo with great per- sistence, the comparatively heterogeneous domestic race fre- quently produces individuals more unlike the average type than the parents are.

Though unlikeness among progenitors is one antecedent of variation, it is by no means the sole antecedent. Were it so, the young ones successively born to the same parents would be alike. If any peculiarity in a new organism were a direct resultant of the structural differences between the two organisms which produced it ; then all subsequent new organisms produced by these two, would show the same pecu- liarity. But we know that the successive offspring have differ- ent peculiarities : no two of them are ever exactly alike.

One cause of such structural variation in progeny, is functional variation in parents. Proof of this is given by the fact that, among the progeny of the same parents, there is more difference between those begotten under different con- stitutional states, than between those begotten under the same constitutional state. It is notorious that twins are more nearly alike than children borne in succession. The functional conditions of the parents being the same for twins, but not the same for their brothers and sisters (all other ante- cedents being constant) ; we have no choice but to admit that variations in the functional conditions of the parents, are the antecedents of those greater unlikenesses which their brothers and sisters exhibit.

Some other antecedent remains, however. The parents being the same, and their constitutional states the same, va- riation, more or less marked, still manifests itself. Plants grown from seeds out of one pod, and animals produced at one birth, are not alike ; and sometimes differ considerably. In a litter of pigs or of kittens, we rarely see uniformity of markings ; and occasionally, there are important structural contrasts. I have myself recently been shown a litter of Newfoundland puppies, some of which had four digits to

262 THE INDUCTIONS OF BIOLOGY.

fcheir feet, while in others, there was present on each hind- foot, what is called the " dew-claw " — a rudimentary fifth digit.

Thus, induction points to three causes of variation, all in action together. We have heterogeneity among progenitors, which, did it act uniformly and alone in genera ting, by composi- tion of forces, new deviations, would impress such new devia- tions to the same extent on all offspring of the same parents ; which it does not. We have functional variation in the pa- rents, which, acting either alone or in combination with the pre- ceding cause, would entail like variations on all young ones simultaneously produced ; which it does not. And there is consequently some third cause of variation, yet to be found, which acts along with the structural and functional variations of ancestors and parents.

§ 87 Already, in the last section, there has been implied some relation between variation and the action of external conditions. The above-cited contrast, between the uniformity of wild species and the multiformity of the same species when cultivated or domesticated, thrusts this truth upon us. .Respecting the variations of plants, Mr Darwin remarks that " ' sports' are extremely rare under nature, but far from rare undei cultivation." Others who have studied the matter assert, that if a species of plant which, up to a certain time, has maintained great uniformity, once has its constitution thoroughly disturbed, it will go on varying indefinitely. Though, in consequence of the remoteness of the periods at which they were domesticated, there is a lack of positive proof that our extremely variable domestic animals have be- come variable under the changed conditions implied by do- mestication, having been previously constant ; yet competent judges do tot doubt that this has been the case.

Now the constitutional disturbance which precedes varia- tion, can be nothing else than an overthrowing of the pre- established equilibrium of functions. Transferring a plant from forest lands to a ploughed field or a manured garden, ia

VARIATION. 263

altering the balance of forces to which it has been hitherto subject ; by supplying it with different proportions of the assimilable matters it requires, and taking away some of the positive impediments to its growth which competing wild plants before offered. An animal taken from woods or plains, where it lived on wild food of its own procuring, and placed under restraint, while artificially supplied with food not quite like what it had before, is an animal subject to new outer ac- tions, to which its inner actions must be re-adjusted. From the general law of equilibration we found it to follow, thai " the maintenance of such a moving equilibrium ' as an or- ganism displays, " requires the habitual genesis of internal forces corresponding in number, directions, and amounts, to the external incident forces — as many inner functions, single or combined, as there are single or combined outer ae tions to be met " (First Principles, § 133) ; and more recently (§ 27), we have seen that Life itself is "the definite combin- ation of heterogeneous changes, both simultaneous and suc- cessive, in correspondence with external co-existences and sequences." Necessarily, therefore, an organism exposed to a permanent change in the arrangement of outer forces, must undergo a permanent change in the arrangement of inner forces. The old equilibrium must be destroyed ; and a ne\v equilibrium must be established. There must be func- tional perturbations, ending in a re-adjusted balance oi functions.

If, then, change of conditions is the only known cause by which the original homogeneity of a species is destroyed ; and if change of conditions can affect an organism only by altering its functions ; it follows that alteration of func- tions is the only known internal cause to which the com- mencement of variation can be ascribed. That such minoi functional changes as parents undergo from year to year, are influential on the offspring, we have seen to be proved by the greater unlikeness that exists between children born to the 'same parents at different times, than exists between

264 THE INDUCTIONS OF BIOLOGY.

twins. And here we seem forced to conclude, that the larger functional variations produced by greater external changes, are the initiators of those structural variations which, when once commenced in a species, lead by their combinations and antagonisms to multiform results. Whether they are or are not the direct initiators, they must still be the indirect initiators.

§ 88. That they are not in all cases, or even in most cases, the direct initiators, is clear. Were they so, those unlike- nesses which exist between plants that grow from seeds out of the same seed-vessel, or between animals belonging to the same litter, would be inexplicable. Here, all the antecedents, structural and functional, appear to be alike for each of the new organisms. Any deviations caused by structural con- trasts or functional disturbances in the parents, must be equally shared in by all simultaneously-produced offspring. Hence, an explanation of the variations arising under such conditions, has still to be sought.

These are the variations termed " spontaneous." Not that those who apply to them this word or some equivalent, mean to imply that they are uncaused. Mr Darwin expressly guards himself against such an interpretation. He says : — " I have hitherto sometimes spoken as if the variations — so common and multiform in organic beings under domestica- tion, and in a lesser degree in those in a state of nature — had been due to chance. This, of course, is a wholly incorrect expression, but it serves to acknowledge plainly our ignorance of the cause of each particular variation." Not only, how- ever, do I hold, in common with Mr Darwin, that there must be some cause for these apparently-spontaneous variations ; but it seems to me that a definite cause is assignable. I think it may be shown that unlikenesses must necessarily arise between the new individuals simultaneously produced by the same parents. Instead of the occurrence of such

VARIATION. 263

variations being inexplicable, we shall presently see that the absence of them would be inexplicable.

In any series of dependent changes, a small initial difference often works a marked difference in the results. The mode in which a particular breaker bursts on the beach, may determine whether the seed of some foreign plant which it bears, is or is not stranded — may cause the presence or absence of this plant from the Flora of the land; and may so affect, for millions of years, in countless ways, the living creatures throughout the land. A single touch, by introducing into the body some morbid mat- ter, may set up an immensely- involved set of functional dis- turbances and structural alterations. The whole tenor of a life may be changed by a word of advice ; or a glance may determine an action which alters thoughts, feelings, and deeds throughout a long series of years. In. those still more involved combinations of changes which societies exhibit, this truth is still more conspicuous. A hair's-breadth differ- ence in the direction of some soldier's musket at the battle of Arcola, by killing Napoleon, might have changed events throughout Europe : though the social organization in each European country, would have been now very much what it is, yet in countless details it would have been different.

Illustrations like these, with which pages might be filled, prepare us for the conclusion, that organisms produced by the same parents at the same time, must be more or less differentiated both by insensible initial differences, and by slight differences in the conditions to which they are subject during their evolution. "We need not, however, rest with assuming such initial differences : the necessity of them i& demonstrable. The individual germ- cells which, in succes- sion or simultaneously, are separated from the same parent, can never be exactly alike ; nor can the sperm-cells which fertilize them. When treating of the instability of the homogeneous (First Principles, § 109), we saw that no two parts of any aggregate, can be similarly conditioned with

2f)6 THE INDUCTIONS OF BIOLOGY.

respect to incident forces ; and that being subject to forces that are more or less unlike, they must become more or less unlike. Hence, no two ova in an ovarura or ovules in a saed-vessel — no two spermatozoa or pollen-cells, can be identical. Whether or not there arise other contrasts, there are certain to arise quantitative contrasts ; since the process of nutrition cannot be absolutely alike for all. The repro* ductive centres must begin to differentiate from the very outset. Such being the necessities of the case, what

will happen on any successive or simultaneous fertilizations ? There will inevitably result more or less unlikeness between the combined parental influences in every instance. Quan- titative differences among the sperm-cells and among the germ-cells, will insure this. Grant that the number of physiological units contained in any one reproductive cell, can rarely if ever be exactly equal to the number contained in any other, ripened at the same time or at a different time ; and it follows that among the fertilized germs produced by the same parents, the physiological units derived from each parent will bear a different numerical ratio to each other in every case. If now the parents are constitutionally alike, that is, alike in the polarities of their physiological units, the variation in the ratio between the physiological units they severally bequeath to the fertilized germs, cannot cause unlikenesses among the offspring. But if otherwise, no two of the offspring can be alike. In every case, the small initial difference in the proportions of the slightly- unlike units, will lead, during evolution, to a continual multiplication of differences : the insensible divergence at the outset, will gener- ate sensible divergences at the conclusion. Possi- bly some may hence infer, that though, in such case, the offspring must differ somewhat from each other and from both parents ; yet that in every one of them there must result a homogeneous mixture of the traits of the two parents. A little consideration shows that the reverse is inferable. If, throughout the process of development, the physiological

VARIATION. 267

tmits derived from each parent, preserved the same ratio to each other in all parts of the growing organism, each organ would show as much as every other, the influence of either parent. But we know, a priori, that no such uniform dis- tribution is possible. It has been shown (First Principles, § 123), that in any mixed aggregate of units, segregation must inevitably go on. Incident forces will tend ever to cause separation of the two orders of units from each other — will integrate groups of the one order in one place, and groups of the other order in another place. Hence there must arise, not a homogeneous mean between the two parents ; but a mixture of organs, some of which mainly follow the one parent and some the other. And this is the kind of mixture which observation shows us.

Still it may be fairly objected, that however the attributes of the two parents are variously mixed in their several offspring, they must in all the offspring fall between the extremes displayed in the parents. In no characteristic could one of the young exceed both parents, were there no cause of " spontaneous variation ' ' but the one alleged. Evi- dently, then, there is a cause yet unfound.

§ 89. Thus far we have contemplated the process under its simplest aspect. While we have assumed the two parents to be somewhat unlike, we have assumed that each parent has a homogeneous constitution — is built up of physiologi- cal units that are exactly alike. But in no case can such a homogeneity exist. Each parent had parents that were more or less contrasted — each parent inherited at least two orders of physiological units, not quite identical. Here then we have a further cause of variation. The sperm-cells or germ- cells which any organism produces, will differ from each other not quantitatively only, but qualitatively. Of tho sligbtly-unlike physiological units bequeathed to an organism, its reproductive cells cannot habitually contain the same pro- portions ; and we may expect the proportions to vary not

2G8 THE INDUCTIONS OF BIOLOGY.

slightly but greatly. Just as, during the evolution of an or- ganism, the physiological units derived from the two parents tend to segregate, and produce likeness to the male parent in this feature and to the female parent in that ; so, during the formation of reproductive cells by such organism, there will arise in one cell a predominance of the physiological units derived from one parent, and in another cell a predominance of the physiological units derived from the other parent. The instability of the homogeneous forbids us to assume an even distribution of the two orders of units in all the reproductive cells. And inequalities once arising among them, must tend ever to become more marked ; since, wherever units of a given order have begun to segregate, the process of differenti- ation and integration tends to segregate them more and more. Thus, then, every fertilized germ, besides containing different amounts of the two parental influences, will contain different kinds of influences — this having received a marked impress from one maternal or paternal ancestor, and that from an- other.

Here, then, we have a clue to the multiplied variations, and sometimes extreme variations, that arise in races which have once begun to vary. Amid countless different combinations of units derived from parents, and through them from ances- tors, immediate and remote — amid the various conflicts in their slightly-different polarities, opposing and conspiring with each other in all ways and degrees ; there will from time to time arise special proportions causing special devi- ations. From the general law of probabilities it is inferable, that while these involved influences, derived from many pro- genitors, must, on the average of cases, obscure and partially neutralize one another ; there must occasionally result such combinations of them as will produce considerable divergences from average structures ; and at rare intervals, such com- binations as will produce very marked divergences. There is thus a correspondence between the inferable results, and the results aa habitually witnessed.

VARIATION.

§ 90. Still there remains a difficulty. It may be said that admitting functional change to ba the initiator of variation — granting that the physiological units of an organism, modified by long subjection to new conditions, will tend to be- come modified in such way as to cause change of" structure in offspring ; yet there will still be no cause of ilie supposed heterogeneity among the physiological units of different in- dividuals. There seems validity in the objection, that as all the members of a species whose circumstances have been al- tered, will be affected in the same manner, the results, when they begin to show themselves in descendants, will show them- selves in the same manner : not multiform variations will arise, but deviations all in one direction.

The reply is simple. The members of a species thus cir- cumstanced, will not be similarly affected. In the absence of absolute uniformity among them, the functional changes caused in them will be more or less dissimilar. Just as men of slightly- unlike dispositions behave in quite opposite ways under the same circumstances ; or just as men of slightly- urilike constitutions get diverse disorders from the same cause, and are diversely acted on by the same medicine ; so, the insensibly-differentiated members of a species whose con- ditions have been changed, may at once begin to undergo various kinds of functional changes. As we have already seen, small initial contrasts may lead to large terminal con- trasts. The intenser cold of the climate into which a species has migrated, may cause in one individual increased con- sumption of food, to balance the greater loss of heat ; while in another individual, the new requirement may be met by a thicker growth of fur. Or, when meeting with the new foods which the new region furnishes, mere accident may deter- mine one member of the species to begin with one kind and another member with another kind ; and hence may arise established habits in these respective members and their descendants. Now when the functional divergences thus set up in sundry families of a species, have lasted long enough

270 THE INDUCTIONS OF BIOLOGY,

to affect their constitutions profoundly, and to modify some- what the physiological units thrown off in their reproductive cells, the divergences produced by these in offspring, will be of diverse kinds. And the original homogeneity of constitu- tion having been thus destroyed, valuation may go on with increasing facility. There will result a heterogeneous mix- ture of modifications of structure, caused by modifications of function ; and of still more numerous correlated modifica- tions, indirectly so caused. By natural selection of the most divergent forms, the unlikenesses of parents will grow more marked, and the limits of variation wider. Until at length the divergences of constitutions and modes of life, become great enough to lead to segregation of the varieties.

§ 91. That variations must occur, and that they must ever tend, both directly and indirectly, towards adaptive modifica- tions, are conclusions deducible from first principles ; apart from any detailed interpretations like the above. That the state of homogeneity is an unstable state, we have found to be a universal truth. • Each species must pass from the uni- form into the more or less multiform, unless the incidence of external forces is exactly the same for all its members ; which it never can. be. Through the process of differentiation and integration, which of necessity brings together, or keeps to- gether, like individuals, and separates unlike ones from them, there must nevertheless be maintained a tolerably uniform epecies ; so long as there continues a tolerably uniform set of conditions in which it may exist. But if the conditions change, either absolutely by some disturbance of the habitat, or relatively by spread of the species into other habitats, then the divergent individuals that result, must be segregated by the divergent sets of conditions into distinct varieties (First Principles, § 126). When, instead of contemplating a species in the aggregate, we confine our attention to a single member and its descendants, we see it to be a corollary from the general law of equilibration, that the moving equili-

VARIATION. 271

brium constituted by the vital actions in each member of this familv, must remain constant so lonq; as the external ac-

4/ * CJ

tions to which they correspond remain constant ; and that if the external actions are changed, the disturbed balance of internal changes, if not overthrown, cannot cease undergoing modification until the internal changes are again in equili- brium with the external actions : corresponding structural alterations having arisen.

Or passing from these derivative laws to the ultimate law, we see that Variation is necessitated by the persistence of force. The members of a species inhabiting any area,cannot be subject to like aggregates of forces over the whole of that area. And if, in different parts of the area, different kinds or amounts or combinations of forces act on them, they cannot but become different in themselves and in their progeny.. To say otherwise, is to say that differences in the forces will not produce differ- ences in the effects ; which is to deny the persistence of force.

Whence it is also manifest, that there can be no variation of structure, 'but what is directly or indirectly consequent on variation of function. On the one hand, organisms in com- plete equilibrium with their conditions, cannot be changed except by change in their conditions ; since, to assert other- wise, is to assert that there can be an effect without a cause ; which is to deny the persistence of force. Oji the other hand, any change of conditions can affect an organism only by changing the actions going on in it — only by altering its func- tions. The alterations of functions being necessarily towards a re-establishment of the equilibrium, (for if not, the equili- brium must be destroyed and the life cease, either in the in- dividual or in descendants,) it follows that the structural alter- ations directly caused, are adaptations ; and that the correlated structural alterations indirectly caused, are the concomitants of adaptations. Hence, though, by the intercourse of organisms that have been functionally and structurally modified in dif- ferent directions, there may result organisms that deviate in compound ways which appear unrelated to external condi-

5472 THE INDUCTIONS OF BIOLOGY

tions, the deviations of such organisms must still be regarded as indirect results of functional adaptations. We must say that in all cases, adaptive change of function is the primary and ever-acting cause of that change of structure which con- stitutes variation ; and that the variation which appears to be " spontaneous," is derivative and secondary.

CHAPTER X.

GENESIS. HEREDITY, AND VARIATION.

§ 92. A QUESTION raised, and hypothetically answered, in 78 and 79, was there postponed until we had dealt with the topics of Heredity and Variation. Let us now resume the consideration of this question, in connexion with sundry others Avhich the facts suggest.

After contemplating the several methods by which the multiplication of organisms is carried on — after ranging them under the two heads of Homogenesis, in which the suc- cessive generations are similarly produced, and Heterogenesis, in which they are dissimilarly produced — after observing that Homogenesis is always sexual genesis, while Heteroge- nesis is asexual genesis with occasionally-recurring sexual genesis ; we came to the questions — why is it that some or- ganisms multiply in the one way, and some in the other ? and why is it that where agamogenesis prevails, it is usually, from time to time, interrupted by gamogenesis ? In seeking an answer to this question, we inquired whether there are> common to both Homogenesis and Heterogenesis, any condi- tions under which alone sperm-cells and germ-cells arise and are united, for the production of new organisms ; and we reached the conclusion that, in all cases, they arise only when there is an approach to equilibrium between the forces which produce growth and the forces which oppose growth. This answer to the question — when does gamogenesis recur?

274 THE INDUCTIONS OF BIOLOGY.

still left unanswered the question — why does gamogenesifl recur ? And to this the reply suggested was, that the ap- proach towards general equilibrium in organisms, "is ac- companied by an approach towards molecular equilibrium in them ; and that the need for this union of sperm-cell and germ-cell, is the need for overthrowing this equilibrium, and re-establishing active molecular change in the detached germ — a result which is probably effected by mixing the slightly- different physiological units of slightly-different individuals."

L •/ O m/

This is the hypothesis which we have now to consider. Let us first look at the evidences which certain inorganic pheno- mena furnish.

The molecules of any aggregate which have not a balanced arrangement, inevitably tend towards a balanced arrangement. As before mentioned (First Principles, § 103) amorphous wrought iron, when subject to continuous jar, begins to arrange itself into crystals — its atoms assume a condition of polai equilibrium. The particles of unannealed glass, which are so unstably arranged that slight disturbing forces make them separate into small groups, take advantage of that greater freedom of movement given by a raised temperature, to ad- just themselves into a state of relative rest. During any such re-arrangement, the aggregate exercises a coercive force over its units. Just as in a growing crystal, the atoms suc- cessively assimilated from the solution, are made by the al-

*. *

ready-crystallized atoms to take a certain form, and even to re-complete that form when it is broken ; so in any mass of unstably-arranged atoms that passes into a stable arrangement, each atom conforms to the forces exercised on it by all the other atoms. This is a corollary from the general law of equilibration. "We saw (First Principles, § 130) that every change is towards equilibrium ; and that change can never cease until equilibrium is reached. Organisms, above

ill other aggregates, conspicuously display this progressive equilibration ; because their units are of such kinds, and so conditioned, as to admit of easy re-arrangement. Thoso

GENESIS, HEREDITY, AND VARIATION. 275

extremely active changes which go on during the early stages of evolution, imply an immense excess of the mole- cular forces over those antagonist forces which the aggregate exercises on the molecules, While this excess continues, it is expended in growth, development, and function — expendi- ture for any of these purposes, being proof that part of the force embodied in molecular tensions, remains unbalanced. Eventually, however, this excess diminishes. Either, as in organisms which do not expend much force, decrease of assi- milation leads to its decline ; or, as in organisms which ex- pend much force, it is counterbalanced by the rapidly-increas- ing re-actions of the aggregate (§ 46). The cessation of growth, when followed, as in some organisms, by death, im- plies the arrival at an equilibrium between the molecular forces, and those forces which the aggregate opposes to them. When, as in other organisms, growth ends in the establish- ment of a moving equilibrium, there is implied such a de- creased preponderance of the molecular forces, as leaves no surplus beyond that which is used up in functions. The de- clining functional activity, characteristic of advancing life, expresses a further decline in this surplus. And when all vital movements come to an end, the implication is, that the actions of the units on the aggregate and the re- actions of the aggregate on the units, are completely bal- anced. Hence, while a state of rapid growth indi- cates such a play of forces among the units of an aggregate, as will produce active re-distribution ; ths diminution and arrest of growth, shows that the units have fallen into such relative positions that re-distribution is no longer so facile. When, therefore, we see that gamogenesis recurs only when growth is decreasing, or has come to an end, we must say that it recurs only when the organic units are approxima- ting to equilibrium — only when their mutual restraints pre- vent them from readily changing their arrangements in obe- dience to incident forces.

That units of like forms can be built up into a more stablt?

276 THE INDUCTIONS OF BIOLOGY.

aggregate than units of slightly unlike forms, is tolerably manifest, a priori. And we have facts which prove that mixing allied but somewhat different units, docs lead to comparative in- stability. Most metallic alloys exemplify this truth. Com- mon solder, which is a mixture of lead and tin, melts at a much lower temperature than either lead or tin. The compound of lead, tin, and bismuth, called " fusible metal," becomes fluid at the temperature of boiling water ; while the temperatures at which lead, tin, and bismuth become fluid, are, respectively, G12D, 442°, and 497°, F. Still more remarkable is the illustra- tion furnished by potassium and sodium. These metals are very near akin in all respects — in their specific gravities, their atomic weights, their chemical affinities, and the properties of their compounds. That is to say, all the evidences unite to show that their units, though not identical, have a close resem- blance. What now happens when they are mixed ? Potassium alone melts at 136°, sodium alone melts at 190°, but the alloy of potassium and sodium, is liquid at the ordinary temperature of the air. Observe the meaning of these facts, expressed in general terms. The maintenance of a solid form by any group of units, implies among them an arrangement so stable, that it cannot be overthrown by the incident forces. Whereas the assumption of a liquid form, implies that the incident forces suffice to destroy the arrangement of the units. In the one case, the thermal undulations fail to dislocate the parts ; while in the other case, the parts are so dislocated by the thermal undulations, that they fall into total disorder — a disorder admitting of easy re- arrangement into any other order. For the liquid state is a state in which the units become so far free v from mutual restraints, that incident forces can change their relative positions very readily. Thus we have reason 'to conclude, that an aggregate of units which, though in the main similar to each other, have minor differences, must be more unstable than an aggregate of homogeneous units : the one will yield to disturbing forces which the other successfully resists.

OEXESIS, HEREDITY, AND VARIATION. 277

Now though, the colloidal atoms of which organisms are mainly built, are themselves highly composite ; and though the physiological units compounded out of these colloidal atoms, must have structures far more involved ; yet it must happen with such units, as with simple units, that those which have exactly like forms, will admit of arrangement into a more stable aggregate than those which have slightly- unlike forms. Among units of this order, as among units of a simpler order, imperfect similarity must entail imperfect polar balance, and consequent diminished ability to withstand disturbing forces. Hence, given two organisms which, by diminished nutrition or increased expenditure, are being ar- rested in their growths — given in each an approaching equilibrium between the forces of the units and the forces of the aggregate — given, that is, such a comparatively-balanced state among the units, that re-arrangement of them by inci- dent forces is no longer so easy ; and it will follow that by uniting a group of units from the one organism with a group of slightly-different units from the other, the tendency to- wards equilibrium will be diminished, and the mixed units will be rendered more modifiable in their arrangements by the forces acting on them : they will be so far freed as to be-

*

corne again capable of that re-distribution which constitutes evolution. This view of the matter is in harmony

»

with the results of observation on the initial stages of develop- ment. Some pages back, it was asserted that sperm-cell and germ-cell severally arrive, before their union, at a condition of equilibrium. Though approximately true, this is not liter- ally true. I learn from Dr "NY. H. Ransom, who has investi- gated the question with great care, that the unfertilized ovum continues, for a time, to undergo changes similar to those which the fertilized ovum undergoes; but that these changes, becoming

O ' D ' O

languid and incomplete, are finally arrested by decomposition. Here we find what might be expected. In the first place, an organism which develops germ-cells, is not ia a state of mole- cular equilibrium, but in a state of approach to such equiii-

278 TE1E INDUCTIONS OF BIOLOGY.

brium. Hence, a group of physiological units cast off from it, will not be wholly without a tendency to undergo the struc- tural re-arrangements which we call development ; but will have this tendency unduly restrained by partially-balanced polarities. In the second place, undue restraint of the phy- siological units, while it renders them as wholes less-easily altered in their relative positions by incident forces, thereby also renders them more liable to be individually decomposed by incident forces : the same thermal undulations which, if

•/

the physiological units are comparatively free, will aid their re-arrangement by giving them still greater freedom, will, if they are comparatively fixed, begin to change the arrange- ments of their components — will decompose them. In the third place, their decomposition will be prevented as well as their re-distribution facilitated, by such disturbance of their polarities as we have seen must result from mixing with them the slightly-unlike units of another organism.

And now let us test this hypothesis, by seeing what power it gives us of interpreting established inductions.

§ 93. The majority of plants being hermaphrodites, it has, until quite recently, been supposed that the ovules of each flower are fertilized by pollen from the anthers of the same flower. Mr Darwin, however, has shown that the arrange- ments are generally such as to prevent this : either the ovules and the pollen are not ripe simultaneously, or obstacles pre- vent access of the one to the other. At the same time, he has shown that there exist arrangements, often of a remarkable kind, which facilitate the transfer of pollen by insects from the stamens of one flower to the pistil of another. Simi-

kniy, it has been found that among the lower animals, hernia- phrodisrn does not usually involve the production of fertile germs, by the union of sperm-cells and germ-cells develouL'd in the same individual ; but that the reproductive centres of one individual are united with those of another, to produce fertile germs. Either, as in the Pyrosoma, the Perop/torat anO

GENESIS, HEREDITY, AND VARIATION. 279

m many higher molluscs, the ova and spermatozoa are ma- tured at different times ; or, as in annelids, they are prevented by their relative positions from coming in contact.

Remembering the fact that among the higher classes of organisms, fertilization is always effected by combining the sperm-cell of one individual with the germ-cell of another ; and joining with it the fact that among hermaphrodite organ- isms, the germ-cells developed in any individual, are usually not fertilized by sperm-cells developed in the same individual ; we see reason for thinking that the essential thing in fertiliz- ation, is the union of specially-fitted portions of different or- ganisms. If fertilization depended on the peculiar properties of sperm-cell and germ-cell, as such ; then, in hermaphrodite organisms, it would be a matter of indifference whether the united sperm-cells and germ-cells were those of the same in- dividual, or those of different individuals. But the circum- stance that there exist in such organisms, elaborate ap- pliances for mutual fertilization, shows that unlikeness of derivation in the united reproductive centres, is the deside- ratum. Now this is just what the foregoing hypothesis implies. If, as was concluded, fertilization has for its object the disturbance of that approximate equilibrium existing among the physiological units separated from an adult organ- ism. ; and if, as we saw reason to think, this object is effect-ed by mixture with the slightly-different physiological units of another organism ; then, we at the same time see reason to think, that this object will not be effected by mixture with physiological units belonging to the same organism. Thus, the hypothesis leads us to expect such provisions as we find exist.

§ 94. But here a difficulty presents itself. These proposi- tions seem to involve the conclusion, that self-fertilization is impossible. It apparently follows from them, that a group of physiological units from one part of an organism, ought to have no power of altering the state of approaching balance >J

280 THE INDUCTIONS OF BIOLOGY.

a group from another part of it. Yet self-fertilization does occur. Though the ovules of one plant, are generally fer- tilized by pollen from another plant of the same kind ; yet they may be, some of them, fertilized by the pollen of the same plant. And though, among hermaphrodite animals, self-fer- tilization is usually negatived by structural or functional ar- rangements ; yet in certain Entozoa, there appear to be special provisions by which the sperm-cells and germ-cells of the same individual may be united, when not previously united with those of another individual. Certainly, at first sight, these facts do not consist with the above supposition. Neverthe- less, there is a satisfactory solution of them.

In the last chapter, when considering the variations that may result in offspring from the combination of unlike parental constitutions, it was pointed out that in an unfolding organism, composed of slightly-different physiological units derived from slightly- different parents, there cannot be main- tained an even distribution of the two orders of units. We saw that the instability of the homogeneous, negatives the uniform blending of them ; and that, by the process of differ- entiation and integration, they must be more or less separated ; so that in one part of the body the influence of one parent will predominate, and in another part of the body the influence of the other parent : an inference which harmonizes with daily observation. And we also saw, that the sperm-cells or germ- cells produced by" such an organism, must, in virtue of these same laws, be more or less unlike one another. It was shown that through segregation, some of ir±e sperm- cells or germ- cells will get an excess of the physiological units derived from one side, and some of them an excess of those derived from the other side : a cause which accounts for the unlikenesses among offspring simultaneously produced. Now from this segregation of the different orders of physiological units, in- herited from different parents and lines of ancestry, there arises the possibility of self-fertilization in hermaphrodite organisms. If the physiological units contained in the sperm-

GENESIS, HEREDITY, AND VARIATION. 281

cells and germ- cells of the same flower, are not quite homo- geneous— if in some of the ovules the physiological units derived from the one parent greatly predominate, and in some of the ovules those derived from the other parent ; and if the like is true of the pollen-cells ; then, some of the ovules may be nearly as much contrasted with some of the pollen-cells, in the characters of their contained units, as were the ovules and pollen-cells of the parents from which the plant proceeded. Between part of the sperm-cells and part of the germ-cells, the community of nature will be such that fertilization will not result from their union ; but between some of them, the differences of constitution will be such that their union will produce the requisite molecular instability. The facts, so far as they are known, seem in harmony with this deduction. Self-fertilization in flowers, when it takes place, is not so efficient as mutual fertilization. Though some of the ovules produce seeds, yet more of them than usual are abortive. From which, indeed, results the establishment of varieties that have structures favourable to mutual fertilization ; since, being more prolific, these have, other things equal, greater chances in the " struggle for existence."

Further evidence is at hand in support of this interpreta- tion. There is reason to believe that self-fertilization, which at the best is comparatively inefficient, loses all efficiency in course of time. After giving an account of the provisions for an occasional, or a frequent, or a constant crossing between flowers ; and after quoting Prof. Huxley to the effect that among hermaphrodite animals, there is no case in which " the occasional influence of a distinct individual can be shown to be physically impossible ; ' Mr Darwin writes—" from these several considerations and from the many special facts which I have collected, but which I am not here able to give, I am strongly inclined to suspect that, both in the vegetable and animal kingdoms, an occasional intercross with a distinct in- dividual is a law of nature. * * * in none, as I suspect, can self-fertilization go on for perpetuity." This conclusion, 13

282 THE INDUCTIONS OF BIOLOGY.

based wholly on observed facts, is just the conclusion to the foregoing argument points. That necessary action and the re-action between the parts of an organism and the organism as a whole — that power of the aggregate to re-mould tne units, which is the correlative of the power of the units to build up into such an aggregate ; implies that any differences existing between the units inherited by an organism, must gradually diminish. Being subject in common to the total forces of the organism, they will in common be modified to- wards congruity with these forces; and therefore towards likeness with each other. If, then, in a self- fertilizing organism and its self-fertilizing descendants, such contrasts as origin- ally existed among the physiological units, are progressive- ly obliterated — if, consequently, there can no longer be a segregation of different physiological units in different sperm - cells and germ- cells ; self-fertilization will become impossible : step by step the fertility will diminish, and the series will finally die out.

And now observe, in confirmation of this view, that self- fertilization is limited to organisms in which an approximate equilibrium among the organic forces, is not long maintained. While growth is actively going on, and the physiological units are subject to a continually-changing distribution of forces, no decided assimilation of the units can be expected : like forces acting on the unlike units, will tend to segregate them, so long as continuance of evolution permits further segrega- tion ; and only when further segregation cannot go on, will the like forces tend to assimilate the units. Hence, where there is no prolonged maintenance of an approximate organic balance, self-fertilization may be possible for some gener- ations ; but it will be impossible in organisms distinguished by a sustairred moving equilibrium.

§ 95. The interpretation which it affords of sundry pheno- mena familiar to breeders of animals, adds probability to the Hypothesis. Mr Darwin has collected a large " body of

GENESIS, HEREDITY, AND VARIATION. 283

showing, in accordance with the almost universal belief oi breeders, that with animals and plants a cross between different varieties, or between individuals of the same variety but oi another strain, gives vigour and fertility to the offspring ; and on the other hand, that close interbreeding diminishes vigour and fertility," — a conclusion harmonizing with the current belief respecting family-intermarriages in the human race. Have we not here a solution of these facts ? Relations must, on the average of cases, be individuals whose physiological units are more nearly alike than usual. Animals of different varieties must be those whose physiological units are more unlike than usual. In the one case, the unlikeness of the units may frequently be insufficient to produce fertilization ; or, if sufficient to produce fertilization, not sufficient to produce that active molecular change required for vigorous develop- ment. In the other case, both fertilization and vigorous development will be made probable.

Nor are we without a cause for the irregular manifestation ot these general tendencies. The mixed physiological units com- posing any organism, being, as we have seen, more or less se- gregated in the reproductive centres it throws off; there may arise various results, according to the degrees of difference among the units, and the degrees in which the units are segre- gated. Of two cousins who have married, the common grand- parents may have had either similar or dissimilar constitu- tions; and if their constitutions were dissimilar, the probability that their married grandchildren will have offspring will be greater than if their constitutions were similar. Or the brothers and sisters from whom these cousins descended, in- stead of severally inheriting the constitutions of their parents in tolerably equal degrees, may have severally inherited them in very different degrees : in which last case, intermarriages among the grandchildren will be less likely to prove infertile. Or the brothers and sisters from whom these cousins de- scended, may severally have married persons very like, or very unlike, themselves ; and from this cause there may

284 THE INDUCTIONS OF BIOLOGY.

have resulted, either an undue likeness, or a due imlike- ness, between the married cousins. These several causes, conspiring and conflicting in endless ways and degrees, will work multiform effects. Moreover, differences of segrega- tion will make the reproductive centres produced by the same nearly-related organisms, vary considerably in their amounts of unlikeness ; and therefore, supposing their amounts of unlikeness great enough to cause fertilization, this fertiliza- tion will be effective in various degrees. Hence it may happen that among offspring of nearly- related parents, there may be some in which the want of vigour is not marked, and others in which there is decided want of vigour. So that we are alike shown why in-and-in breeding tends to diminish both fertility and vigour ; and why the effect cannot be a uniform effect, but only an average effect.

§ 96. While, if the foregoing arguments are valid, gamo- genesis has for its main end, the initiation of a new develop- ment by the over throw of that approximate equilibrium arrived at among the molecules of the parent-organisms ; a further end appears to be subserved by it. Those inferior organisms which habitually multiply by agamogenesis, have conditions of life that are simple and uniform ; while those organisms that have highly-complex and variable conditions of life, habitually multiply by gamogenesis. Now if a species has complex and variable conditions of life, its members must be severally exposed to sets of conditions that are slightly different : the aggregates of incident forces cannot be alike for all the scattered individuals. Hence, as functional deviation must ever be inducing structural deviation, each individual throughout the area occupied, tends to become fitted for the particular habits which its particular conditions necessitate ; and in so far, wzfitted for the average habits proper to the species. But these undue specializations are continually checked by gamogenesis. As Mr Darwin remarks — " intercrossing plays a very important part in nature in

GENESIS, HEREDITY, AND VARIATION.

keeping the individuals of the same species, or of the variety, true and uniform in character :" the idiosyncratic divergences obliterate each other. Gamogenesis, then, is a means of turning to positive advantage, the individual differentiations which, in its absence, would result in positive disadvantage. Were it not that individuals are ever being made unlike each other by their unlike conditions, there would not arise among them those contrasts of molecular constitution, which we have seen to be needful for producing the fertilized germs of new individuals. And were not these individual differentiations ever being mutually cancelled, they would end in a fatal narrowness of adaptation.

This truth will be most clearly seen if we reduce it to its purely abstract form, thus : — Suppose a quite homogeneous species, placed in quite homogeneous conditions ; and suppose the constitutions of all its members in complete concord with their absolutely -uniform and constant conditions; what must happen ? The species, individually and collectively, is in a state of perfect moving equilibrium. All disturbing forces have been eliminated. There remains no force which can, in any way, change the state of this moving equilibrium ; either in the species as a whole or in its members. But we have seen (First Principles, § 133) that a moving equilibrium is but a transition towards complete equilibration, or death. The absence of differential or un-equilibrated forces among the members of a species, is the absence of all forces that can cause changes in the conditions of its members— is the ab- sence of all forces which can initiate new organisms. To say, as above, that complete molecular homogeneitv existing among the members of a species, must render impossible that mutual molecular disturbance which constitutes fertilization, is but another way of saying, that the actions and re-actions of each organism, being in perfect balance with the actions and re-actions of the environment upon it, there remains in each organism, no force by which it differs from any other — no force which any other does not meet with an exactly

286 THE INDUCTIONS Of BIOLOGY.

equal force — no force which, can set up a new evofciiou

among the units of any other.

V

And so we reach the remarkable conclusion, that the life of a species, like the life of an individual, is maintained by the unequal and ever- varying actions of incident forces on its different parts. An individual homogeneous throughout, and having its substance everywhere continuously subject to like actions, could undergo none of those changes which life con- sists of; and similarly, an absolutely-uniform species, having all its members exposed to identical influences, would be deprived of that initiator of change which maintains its existence as a species. Just as, in each organism, incident forces constantly produce divergences from the mean state in various directions, which are constantly balanced by opposite divergences indi- rectly produced by other incident forces ; and just as the combination of rhythmical functions thus maintained, consti- tutes the life of the organism ; so, in a species, there is, through garnogenesis, a perpetual neutralization of those contrary de- viations from the mean state, which are caused in its different parts by different sets. of incident forces ; and it is similarly by the rhythmical production and compensation of these con- trary deviations, that the species continues to live. The moving equilibrium in a species, like the moving equilibrium in an individual, would rapidly end in complete equilibration, or death, were not its continually- dissipated forces continually re-supplied from without. Besides owing to the external world, those energies which, from moment to moment, keep up the lives of its individual members ; every species owes to certain more indirect actions of the external world, those energies which enable it to perpetuate itself in successive generations.

§ 97. What evidence still remains, may be conveniently woven up along with a recapitulation of the argument pursued through the last three chapters. Let us contemplate the facts in their synthetic order.

GENESIS, HEREDITY, AND VARIATION. 287

Tnat compounding and re-compounding through which we pass from the simplest inorganic substances to the most coin- pLex organic substances, has several concomitants. Each successive stage of composition, presents us with atoms that are severally larger or more integrated, that are severally morn heterogeneous, that are severally more unstable, and that aiv more numerous in their kinds (First Principles, § 111). And when we come to the substances of which living bodies are formed, we find ourselves among multiplied, divergent groups and sub-groups of compounds, the units of which are large, heterogeneous, and unstable, in high degrees. There is no reason to assume that this process ends with the formation of those complex colloids which characterize organic matter. A more probable assumption is, that out of the complex colloidal atoms, there are evolved, by a still further integration, atoms that are still more heterogeneous, and of kinds that are still more multitudinous. What must be their properties ? Al- ready the colloidal atoms are extremely unstable — capable of being variously modified in their characters by very slight incident forces ; and already the complexity of their polarities prevents them from readily falling into those positions of polar equilibrium which result in crystallization. Now the organic atoms composed of these colloidal atoms, must be simi- larly characterized in far higher degrees. Far more numerous must be the minute changes that can be wrought in them by minute external forces ; far more free must they remain for a long time to obey forces tending to re-distribute them ; and far greater must be the number of their kinds.

Setting out with these physiological units, the existence of which various organic phenomena compel us to recognize, and the production of which the general law of Evolution thus leads us to anticipate ; we get an insight into the phenomena of Genesis, Heredity, and Variation. If each organism is built of certain of these highly- plastic units peculiar to its species — units which slowly work towards an equilibrium of their complex polarities, in producing an aggregate of the specific

288 HIE INDUCTIONS OF BIOLOGY.

structure, and which, are at the same time slowly modifiable

' V

by the re-actions of this aggregate — we see why the mul- tiplication of organisms proceeds in the several ways, and with the various results, which naturalists have observed.

Heredity, as shown not only in the repetition of the specific structure, but in the repetition of ancestral deviations from it, becomes a matter of course ; and it falls into unison with the fact that, in various simple organisms, lost parts can be re- placed, and that, in still simpler organisms, a fragment can develop into a whole.

While an aggregate of physiological units continues to grow, by the assimilation of matter which it moulds into other units of like type ; and while it continues to undergo changes of structure ; no equilibrium can be arrived at between the whole and its parts. Under these conditions, then, an un-differentiated portion of the aggregate — a group of phy- siological units not bound up into a specialized tissue — will be able to arrange itself into the structure peculiar to the species ; and will so arrange itself, if freed from controlling forces, and placed in fit conditions of nutrition and temper- ature. Hence the continuance of agamogenesis in little- differ- entiated organisms, so long as assimilation continues to be greatly in excess of expenditure.

But let growth be checked and development approach its completion — let the units of the aggregate be severally exposed to an almost constant distribution of forces ; and they must begin to equilibrate themselves. Arranged as they will gradually be, into comparatively stable attitudes in relation to each other, their mobility will diminish ; and groups of them, partially or wholly detached, will no longer readily re- arrange themselves into the specific form. Agamogenesis will be no longer possible ; or, if possible, will be no longer easy.

When we remember that the force which keeps the Earth in its orbit, is the gravitation of each particle in the Earth towards every one of the group of particles existing 91,000,000 of miles off; we cannot reasonably doubt, that each unit in

GENESIS, HEREDITY, AND VARIATION. 289

ah organism, acts, by its polar forces, on all the other units, and is re-acted on by them. When, too, we learn that glass has its molecular constitution changed by light, and that substances so rigid and stable as metals, have their atoms re-arranged by forces radiated in the dark from adjacent objects ; we are obliged to conclude that the ex- cessively-unstable units of which organisms are built, must be sensitive in a transcendant degree, to all the forces pervading the organisms composed of them — must be tending ever to re- adjust, not only their relative positions, but their molecular structures, into equilibrium with these forces. Hence, if ag- gregates of the same species are differently conditioned, and re-act differently on their component units, their component units will be rendered somewhat different ; and they will become the more different the more widely the re-actiona of the aggregates upon them differ, and the greater the num- ber of generations through which these different re-actions of the aggregates upon them are continued.

If, then, unlikenesses of function among individuals of the same species, produce unlikenesses between the physiological units of one individual and those of another ; it becomes com prehensible that when groups of units derived from two indi viduals are united, the group formed will be more unstable than either of the groups was before their union : the mixed units will be less able to resist those re- distributing forces which cause evolution ; and may so have restored to them, the capacity for development which they had lost.

This view harmonizes with the conclusion which we saw reason to draw, that fertilization does not depend on any intrinsic peculiarities of sperm-cells and germ-cells ; but depends on their derivation from different individuals. It explains the fact that nearly-related individuals are less likely to have offspring than others ; and that their offspring, when they have them, are frequently feeble. And it gives us a key to the converse fact, that the crossing of varieties results in unusual fertility and vigour.

200 THE INDUCTIONS OF BIOLOGY.

Bearing in mind that the slightly-different orders of phy- siological units which an organism inherits from its parents, are subject to the same set of forces ; and that when the organism is fully developed, this set of forces, becoming con- stant, tends slowly to re-mould the two orders of units into the same form ; we see how it happens that self-fertilization becomes impossible in the higher organisms, while it remains possible in the lower organisms. In long-lived creatures that have tolerably-definite limits of growth, this assimilation of the some what- unlike physiological units, is liable to go on to an appreciable extent ; whereas in organisms which do not continuously subject their component units to constant forces, there will be much less of this assimilation. And where the assimilation is not considerable, the segregation of mixed

' O O

units, may cause the sperm-cells and germ-cells developed in the same individual, to be sufficiently different to produce, by their union, fertile germs ; and several generations of self- fertilizing descendants may succeed one another, before the two orders of units have had their unlikenesses so far diminish- ed, that they will no longer do this. The same principles explain for us the variable results of union between nearly- related organisms. According to the contrasts among the physiological units they inherit from parents and ancestors ; according to the unlike proportions of the contrasted units which they severally inherit ; and according to the degrees of segregation of such units in different sperm-cells and germ-cells ; it may happen that two kindred individuals will produce the ordinary number of offspring, or will produce none ; or will at one time be fertile and at another not ; or will at one time have offspring of tolerable strength, and at another time feeble offspring.

To the like causes are also ascribable the phenomena of Variation. These are unobtrusive while the tolerably-uni- form conditions of a species maintain tolerable uniformity among the physiological units of its members ; but they become obtrusive when differences of conditions, entailing

GENESIS, HEREDITY, AND VARIATION. 29.1

considerable functional differences, have entailed decided dif- ferences among the physiological units ; and when the differ- ent physiological units, differently mingled in every individual, come to be variously segregated and variously combined.

Did space permit, it might be shown that this hypothesis is a key to many further facts — to the fact that mixed races are comparatively plastic under new conditions ; to the fact that pure races show predominant influences when crossed with mixed races ; to the fact that while mixed breeds are often of larger growth, pure breeds are the more hardy — have functions less-easily thrown out of balance. But with- out further argument, it will, I think, be admitted, that the power of this hypothesis to explain so many phenomena, and to bring under a common bond phenomena that seem so little allied, is strong evidence of its truth. And such evidence gains greatly in strength on observing that this hypothesis brings the facts of Genesis, Heredity, and Yariation into har- mony with first principles. When we see that these plastic physiological units, which we find ourselves obliged to assume, are just such more integrated, more heterogeneous, more un- stable, and more multiform atoms, as would result from con- tinuance of the steps through which organic matter is reached — when we see that the differentiations of them assumed to oc- cur in differently- conditioned aggregates, and the equilibra- tions of them assumed to occur in aggregates which maintain constant conditions, are but corollaries from those universal principles implied by the persistence of force — when we see that the maintenance of life in the successive generations of a species, becomes a consequence of the continual incidence of new forces on the species, to replace the forces that are ever being rhythmically equilibrated in the propagation of the species — and when we thus see that these apparently-excep- tional phenomena displayed in the multiplication of organic beings, fall into their places as results of the general laws of Evolution ; we have weighty reasons for entertaining the hypothesis which affords us this interpretation.

CHAPTER XL

CLASSIFICATION.

§ 98. THAT orderly arrangement of objects called Classi- fication, has two purposes ; which, though not absolutely dis- tinct, are distinct in great part. It may be employed to facilitate identification ; or it may be employed to organize our knowledge. If a librarian places his books in the alpha- betical succession of the author's names, he places them in such way that any particular book may easily be found ; but not in such way that books of a given nature stand together. When, conversely, he makes a distribution of books accord- ing to their subjects, he neglects various superficial similari- ties and distinctions, and groups them according to certain primary and secondary and tertiary attributes, which sever- ally imply many other attributes — groups them so that any one volume being inspected, the general characters of all the neighbouring volumes may be inferred. He puts together in one great division, all works on History ; in another all Biographical works ; in another all works that treat of Science ; in another Voyages and Travels ; and so on. Each of his great groups he separates into sub-groups ; as when he puts different kinds of pure Literature, under the heads of Fiction, Poetry, and the Drama. In some cases he makes sub-sub-groups ; as when, having divided his Scientific treatises into abstract and concrete, putting in the one Logic and Mathematics, arid in the other Physios. Astronomy, Ge-

CLASSIFICATION. 293

ology, Chemistry, Physiology, &c. ; he goes on to sub-divide his books on Physics, into those which treat of Mechanical Motion, those which treat of Heat, those which treat of Light, of Electricity, of Magnetism.

Between these two modes of classification, note the essen- <ial distinctions. Arrangement according to any single con- jpicuous attribute is comparatively easy, and is the first tha; (suggests itself: a child may place books in the order of their sizes, or according to the styles of their bindings. But ar> rangement according to combinations of attributes, which, though fundamental, are not conspicuous, requires analysis i and does not suggest itself till analysis has made some pro- gress. Even when aided by the information which the author gives on his title page, it requires considerable kno\vdedge to classify rightly an essay on Polarization ; and in the absence of a title page, it requires much more knowledge. Again, classification by a single attribute, which the objects possess in different degrees, may be more or less serial, or linear. Books may be put in the order of their dates, in single file ; or if they are grouped as works in one volume, works in two volumes, works in three volumes, &c., the groups may be placed in an ascending succession. But groups severally formed of things distinguished by some common attribute which implies many other attributes, do not admit of serial arrangement. You cannot rationally say, either that His- torical Works should come before Scientific Works, or Scien- tific Works before Historical Works ; nor of the sub-divi- sions of creative Literature, into Fiction, Poetry, and the Drama, can you give a good reason why any one should take precedence of the others.

Hence this grouping of the like and separation of the un- like, which constitutes Classification, can reach its complete form only by slow steps. We saw (First Principles, § 36) that, other things equal, the relations among phenomena are recognized in the order of their couspicuousness ; and that-, things equal, they are recognized in the order of their

THE INDUCTIONS OF BIOLOGY.

simplicity. The first classifications are sure, therefore, to be. groupings of objects that resemble each other in external or easily-perceived attributes, and attributes that are not of com- plex characters. Those likenesses amoEg things which are due to their possession in common of simple obvious properties, may or may not coexist with further likenesses among them. When geometrical figures are classed as curvilinear and rectilinear, or when the rectilinear are divided into trilateral, quadrilateral, &c., the distinctions made, connote various other distinctions, with which they are necessarily bound up ; but if liquids be classed according to their visible cha- racters— if water, alcohol, sulphuret of carbon, &c., be grouped as colourless and transparent, we have things placed together which are unlike in their essential natures. Thus, where the objects classed have numerous attributes, the pro- babilities are, that the early classifications, based on simple and manifest attributes, unite under the same head many objects that have no resemblances in the majority of their attributes. As the knowledge of objects increases, it be- comes possible to make groups of which the members have more numerous properties in common ; and to ascertain what property, or combination of properties, is most characteristic of each group. And the classification eventually arrived at, is one in which the segregation has been carried so far, that the objects integrated in each group have more attributes in common with one another, than they have in common with any excluded objects ; one in which the groups of such groups are integrated on the same principle ; and one in which the degrees of differentiation and integration are proportioned to the degrees of intrinsic unlikeness and likeness. And the ultimate classification, while it serves most completely to identify the things, serves also to express the greatest amount of knowledge concerning the things — enables us to predicate the greatest number of facts concerning each thing ; and by so doing proves that it expresses the most precise corre- spondence between our conceptions and the realities.

CLASSIFICATION. 295

§ 99. Biological classifications illustrate well these phases, through which classifications in general necessarily pass. In early attempts to arrange organic beings in some sys- tematic manner, we see at first, a guidance by conspicuous and simple characters, and a tendency towards arrangement in linear order. In successively later attempts, we see more regard paid to combinations of characters which are essential but often inconspicuous ; and a gradual abandon- ment of a linear arrangement for an arrangement in di- vergent groups and re-diyergent sub-groups.

In the popular mind, plants are still classed under the heads of Trees, Shrubs, and Herbs ; and this serial classing according to the single attribute of magnitude, swayed the earliest observers. They would have thought it absurd to call a bamboo, thirty feet high, a kind of grass ; and would have been incredulous if told that the HartVtongue should be placed in the same great division with the Tree-ferns. The zoological classifications that were current before Ka- tural History became a science, had divisions similarly super- ficial and simple. Beasts, Birds, Fishes, and Creeping-things, are names of groups marked off from one another by con- spicuous differences of appearance and modes of life — crea- tures that walk and run, creatures that fly, creatures that live in the water, creatures that crawl. And these groups wTere thought of in the order of their importance.

The first arrangements made by naturalists were based either on single characters, or on very simple combinations of characters. Describing plant-classifications, Lindley says : — " Bivinus invented, in 1690, a system depend- ing upon the formation of the corolla ; Kamel, in 1693, upon the fruit alone ; Magnol, in 1720, on the calyx and corolla ; and finally, Linnaeus, in 1731, on variations in the stamens and pistil." In this last system, which has been for so long current as a means of identification, simple external attributes are still depended on ; and an arrangement, in great measure serial, is based on the degrees in which theae

296 THE INDUCTIONS OF BIOLOGY.

attributes are possessed. In 1703, some thirty years before the time of Linnaeus, our countryman Ray had sketched the outlines of a more advanced system. He said that —

•>

Plants are either

Flowerless, or Flowering ; and these are Dicotvledones. or

•/

Monocotyledones.

mi

Among the minor groups which he placed under these general heads, " were Fungi, Mosses, Ferns, Composites, Cichoracese Umbellifers, Papilionaceous plants, Conifers, La- biates, &c., under other names, but with limits not very dif- ferent from those now assigned to them." Being much in advance of his age, Ray's ideas remained dormant until the time of Jussieu ; by whom they were developed into what has become known as the Natural System. Passing through various modifications in the hands of successive botanists, the Natural System has now taken the following form ; which I copy (adding the alliances to the classes) from Prof. Lindley's Vegetable Kingdom*

* From this table I have omitted the class Rhizogcns, which other botanists do not agree with Lindley in regarding as a separate class. The plants respect- ing which there has arisen this difference of opinion, are certain flowering plants, which grow parasitically on the roots of trees. The reasons assigned bv Endlicher and Lindley, for erecting them into a separate group of Phsenogams, are, that in place of true leaves they have only cellular scales ; that the stem is an amorphous fungous mass, imperfectly supplied with spiral vessels ; ana tnit they are without chlorophyll. Mr Griffith and Dr Hooker, however, have gi^ui preponderating reasons why they should be restored to the class Exogens. It seems here worth remarking, that certain zoological facts suggest an explanation of these anomalous botanical facts ; and confirm the conclusion reached by Dr Hooker and Mr Griffith. It very commonly happens that animal-parasites are aberrant forms of the types to which they belong ; and, by analogy, we may not unreasonably expect to find among parasitic plants, the most aberrant forms of vegetal types. More than this is true. The kind of aberration which we see in the one case, we see in the other ; and in both cases, the meaning of the aberration is manifest. In such Epizoa as the Lernece, the Crustacean type is disguised by the almost entire loss of the limbs and organs of sense, by the simplification of the digestive aj paratus, and by the great development of the reproductive system :

CLASSIFICATION.

Asexual, or Floicerless Plants. Stems and leaves undistinguishable Stems and leaves distinguishable

Sexual, or Flowering Plants.

fAlgales

I. THALLOGENS < Fun.irales iLicheualca f Must-ales

II. ACEOGEXS \ Lycopodalea ^Filicales

Wood of stem youngest in centre ; cotyledon single.

Leaves parallel-veined, permanent ; wood confused

Leaves net-veined, deciduous ; wood, when perennial, arranged in a circle with a central pith

Wood of stem youngest at circum- ference, always concentric ; coty- ledons two or more.

Seeds quite naked

III. EXDOGENS

IV. DlCTYOGESS.

Glumales Arales Palmales Hydrales Narcissales Amomales Orchidales Xyridales Juncales Liliales L Alternates

V. GTM.NOGEXS.

fDiclinous

Seeds enclosed in seed-vessels

VI. EXOGENS

Hypogyiious

Perigynous

.Epigynous

/•Amentalea

\ Urticales | Euphorbiales I. &c. &c. rViolalea I Cistalea I Malvaies *• &c. &c.

{Picoidales Daplinales Resales &c. &c. rCampanales } Myrtales I Cactales »• &c. &c.

Here, linear arrangement has disappeared : there is a breaking up into groups and sub-groups and sub-sub-groups, which do not admit of being placed in serial order, but only in divergent and re-divergent order. Were there space to exhibit the way in which the Alliances are subdivided into Orders, and these into Genera, and these into Species ; the

the parts no longer needed, abort, and those parts develop which favour the preservation of the race. Similarly in the Rhizogcns, the abortive development of the leaves, the absence of chlorophyll, and the imperfect supply of spiral vessels, are changes towards a structure fit for a plant which lives on the juices absorbed from another plant; while the rapid and great development of the fructifying organs, are correlative changes advantageous to a plant, the seeds of which have but small chances of rooting themselves. And just the same reason tliat exists for the production of immensely numerous but extremely small eggs by Entozoa, exists for the production by Rhizogens^ of seeds that are great \ii number and almost spore-like in size.

298 THE INDUCTIONS OF BIOLOGY.

same principle of co-ordination would be still further mani- fested. On studying the definitions of these primary, se- condary, and tertiary classes, it will be found that the largest are marked off from each other by some attribute which connotes sundry other attributes ; that each of the smaller classes comprehended in one of these largest classes, is marked off in a similar way from the smaller classes bound up with it ; and that so, each successively smaller class, has an increased number of co-existing attributes.

§ 100. Zoological classification has had a parallel history. The first attempt which we need notice, to arrange animals in such a way as to display their affinities, is that of Lin- naeus. He grouped them thus :* —

CL. 1. MAMMALIA. Ord. Primates, Bruta, Perse, Glires, Pecora, Bellurc, Cete.

CL. 2. AVES. Ord. Accipitres, Picae, Anseres, Grallse, Galliuse, Passeres.

CL. 3. AMPHIBIA. Ord. Reptiles, Serpentes, Nantes.

CL. 4. PISCES. Ord. Apodes, Jugulares, Thoracici, Abdominales.

CL. 5. INSECTA. Ord. Coleoptera, Hemiptera, Lepidoptera, Neuroptera, Diptera, Aptera.

CL. 6. VERMES. Ord. Intestina, Mollusca, Testacea, Litliophyta, Zoo- phyta.

This arrangement of classes, is obviously based on ap- parent gradations of rank ; and the placing of the orders similarly betrays an endeavour to make successions, begin- ning with the most superior forms and ending with the most inferior forms. While the general and vague idea of perfection, determines the leading character of the classification, its detailed groupings are determined by the most conspicuous external attributes. Not only Lin- naeus, but his opponents, who proposed other systems, were "under the impression that animals were to be arranged together into classes, orders, genera, and species, according to their more or less close external resemblance." This con- ception survived till the time of Cuvier. "Naturalists,"

* This classification, and the three which follow it, 1 quote (^i undoing t,uiiie of them) from Prof. Agassiz's "Essay on Classin'cation."

CLASSIFICATION. 299

suys Agassiz, "were bent upon establishing one contiimal uniform series to embrace all animals, between the links of which it was supposed there were no unequal intervals. The watchword of their school was : Natura non facit saltum. They called their system la chaine dcs etres"

The classification of Cuvier, based on internal organization instead of external appearance, was a great advance. He asserted that there are four principal forms, or four general plans, on which animals are constructed ; and in pursuance of this assertion, he drew out the following scheme.

First Branch. ANIMALIA YERTEBRATA

CL. 1. MAMMALIA. CL. 2. BIRDS. CL. 3. REPTILIA. CL. 4. FISHES.

Second Branch. ANIMALIA MOLLUSCA.

CL. 1. CEPHALAPODA. CL. 2. PTEROPODA. CL. 3. GASTEROPODA. CL. 4. ACEPHALA. CL. 5. BRACHIOPODA. CL. 6. CIRRHOPODA.

Third Branch. ANIMALIA ARTICULAT&,

CL. 1. ANNELIDES. CL. 2. CRUSTACEA. CL. 3. ARACHNIDES. CL. 4. INSECTS.

Fourth Branch. ANIMALIA

CL. 1. ECHIXODERMS.

CL. 2. INTESTINAL WORMS.

CL. 3. ACALEPIL?E.

OL. 4. POLYPI.

CL. 5. INFUSORIA.

300

THE INDUCTIONS OF BIOLOGY.

But though Cuvier emancipated himself from tlio concep- tion of a serial progression throughout the Animal- King- dom ; sundry of his contemporaries and successors remained fettered by the old error. Less regardful of the differently- co-ordinated sets of attributes displayed by the different sub- kingdoms ; and swayed by the belief in a 'progressive develop- ment, which was erroneously supposed to imply the possibility of arranging animals in a linear series ; they persisted in thrusting organic forms into a quite unnatural order. The following classification of Lamarck illustrates this.

I.

INVEETEBEATA. APATHETIC ANIMALS.

CL. 1. INFUSORIA.

CL. 2. POLYPI.

CL. 3. RADIARIA.

CL. 4. TUNICATA.

CL. 5. YERMES.

II. SENSITIVE ANIMALS.

CL. CL. CL.

6. INSECTS.

7. ARACHNIDS.

8. CRUSTACEA. CL. 9. ANNELIDS. CL. 10. CIRRIPEDS. CL. 11. CONCHIFERA. CL. 12. MOLLUSKS.

Do not feel, and move only by their excited irritability. No brain, not elongated medullary mass ; no senses ; forms varied ; rarely articu- lations.

Peel, but obtain from their sensa- tions only perceptions of objects, a sort of simple ideas, which they are unable to combine to obtain complex • ones. No vertebral column ; a brain and mostly an elongated medullary mass; some distinct senses; mu,ccles attached under the skin ; form sym- metrical, the parts being in pairs.

VEETEBEATA.

Feel; acquire preservable ideas;

III. INTELLIGENT ANIMALS.

,*

CL. 13. FISHES. CL. 14. REPTILES. CL. 15. BIRDS. CL. 16. MAMMALIA

perform with them operations by which they obtain others ; are intel- ligent in different degrees. A vcr- tebral column ; a brain and a spinal marrow ; distinct senses ; the mus- cles attached to the internal skele- ton ; form symmetrical, the p?H.<» being in pairs.

CLASSIFICATION, 301

Passing over sundry classifications in which the serial arrangement dictated by the notion of ascending complexity, is variously modified by the recognition of conspicuous anatomical facts, we come to the classifications which recognize another order of facts — those of development. The embryo- logical inquiries of Von Baer, led him to arrange animals as follows : —

I. Peripherie Type. (RABIATA.) Evolutio radiata. The development proceeds from a centre, producing identical parts in a radiating order.

II. Massive Type. (MOLLUSCA.) Evolutio contorta. The development produces identical parts curved around a conical or other space.

III. Longitudinal Type. (ARTICULATA.) Evolutio yemina.

The development produces identical parts arising on both sides of an axis, and closing up along a line opposite the axis.

IV. Doubly Symmetrical type. (VERTEBRATA.) Ecolutio

ligemina. 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, Von Baer shows (among the Vertebrata 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-

(302 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 : —

VERTEBRATA

(Abranclriaia) Mammalia

Aves

Reptilia

(Branchiata)

Amphibia

Pisces

MOLLTJSCA ANNULOSA

Cephalopoda Heteropoda \ Articulata

Gasteropoda- Insecta Arachnida

dioecia ) Myriapoda Crustacea

( Pulmonata Gasteropoda- \ Pteropoda monoacia Annuloida

Lamellibranchiata Annellata Scoleidre

Echinodermata Trematoda Rotifera Treniadae

Turbellaria Nematoidea

CCELENTERATA Hydrozoa Actinozoa.

PROTOZOA

Infusoria Spongiadae Gre£arinid{e

NoctilucidcK Foraminifera Thallassicollida

What remnant there may seem 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. Huxley 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 Hunterian Lectures delivered 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.

303

inent perhaps not very much unlike that shown in the an-

•jexed diagram.

VERTEB'RATA

•

• • • •• ,

Am.AA.l6ia \ Pisces V

\

Ptcrcficda

• .Ga&leroficda. • • dicecict-

Gastercfoda.. ' tPu.tlKcna,6a

mcncecta, • •

MOLL

C A

/ ."/ 'Crustacea

ArticMfata.

A N N U L O

jScoledda • i • •

AKnuloida

.' /

^

/ / / /

/ /

' /

\Brachic/toda,

M ollu s c 6Tda

/PROTOZOA

• _

• ^ / iS/icjigida Jhfuscrio

* • * * *

Mydrossoa* •.

t

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 will 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, undiiferentiated 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 a.s

%>

another. Though this conviction is not a formulated one, yet the disputes continually arising among naturalists on the

CLASSIFICATION. 305

questions, whether such and such organisms are 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 ot' the decree of difference, which warrants each further eleva-

O '

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

heedful for us to remember, that while our successivelv

j

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 .ficc

14

in these classifications. On observing the natures of the

806 THE INDUCTIONS OF BIOLOGY.

attributes which 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 differ 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), 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." Now 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 Rhizopoda, 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 Cceknterata 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 itv; and the Hydrozoa and Actinozoa, which are sub-divisions of the Ccelenterata, 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, 307

the animals classed as Molluscoida, possess rude appliances for the transfer of force : the peri- visceral sac, or closed cavity between the 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 Vertebrate^ 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 hcemal 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 provided 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 distinguished from each other, either by modifications in the structures of fundamental parts, or by the presence or absence of subsidiary parts, or by both. Fishes and Am- phibia are unlike higher vertebrates in possessing branchiic ; 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 mammcc, 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 Placcntalia 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 anv one believes this, it is

CLASSIFICATION. 309

easy to prove to him. that the law of probabilities negatives the assumption. Even 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.

Xo 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 Vertebrate^ 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 vertebrae 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.

CHAPTER

DISTRIBUTION.

§ 104. THEKE is a distribution of organisms in Space, and [li ere 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 onlv life of a much lower order is found. While.

V

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.

312 THE ".NDUCTIONS OF BIOLOGY.

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 Globigerma 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 limita- tions, 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-flying

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 wrhich 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 Chelonobia uses the turtle, and as a certain Ac- tinia uses the shell inhabited bv a hermit-crab. We have

t,'

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. Wo

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. We 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 owrn area ; but also by 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

DISTRIBUTION. 315

with the influences, direct or more or less rtaoic, of nearly all co- existing 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 well 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 are 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 ttie 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 our 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 permanent settlement — perhaps in conquest over indigenous species. AVithin 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 illustra- tion 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 Blaita onentalis, shows us how these successful invasions last only until there come more powerful invaders. Organ

316 THE INDUCTIONS OF BIOLOGY.

%

isms encroach on one another's spheres of existence, in fur- ther 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

«/ o

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 organiza- tion. 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 ex- tent 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 phcenogam 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 dis- covered more than one species of fly living beneath the sur- face of the water, and corning up only occasionally for air. Birds, as a class, are especially fitted for an aerial existence ;

DISTRIBUTION. 317

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 mam- mals, too, which have limbs and lungs implying an organiza- tion for terrestrial life, may be named kinds that live moro or less in the water, and are 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 ex- cursions 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 in- habit 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 tend- ency 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 inor- ganic. And these expansive and repressive energies, vary- ing 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 incon-

318 THE INDUCTIONS OF BIOLOGY.

gruous 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 surround- ing 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, bet ween latitudes 25° and 35°, we shall find parts extremely similar in all their conditions, )Tet it would not be possible to point out three faunas and floras more utterly dissimilar. Or again we may compare the pro- ductions of South America south of lat. 35° with those north of 25°, which consequently inhabit a considerably different cli- mate, 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 closely- adjacent 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 — differ- ences not so marked as where the barriers are absolutely im- passable ; but much more marked than are necessitated bv unlikenesses of physical conditions.

Not less suggestive is the converse fact, that wide geogra- phical 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 tra-

DISTRIBUTION. 319

veiling, for instance, from north to south never fails to bo struck by the manner in which successive groups of beings, specifically distinct, yet clearly related, replace each other. He 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 inhabit- ed by one species of Hhea (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 Rodents, but they plainly display an American type of structure. T\re 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 musk- rat, but the coypu and capybara, rodents of the American type. Innumerable other instances could be given. If we look to the islands off the American shore, however much they may differ in geological structure, the inhabitants, though they may be all peculiar species, are essentially American."

What 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 dif- ferent Faunas. On the other hand, we have areas remote from eaoh 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 spe- cialty fit for these places, or more fit for them than all other

organisms.

The induction under which these facts come, and whieli

320 14IE INDUCTIONS OF BIOLOGY.

unites them with, various other facts, is a totally-different one. When we see that the similar areas peopled b}^ 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 re- minded 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 perpetually- recurring attempts to thrust itself into every accessible habitat, spreads until it reaches limits that are for the time insur- mountable.

-*

§ 107. A\re 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 com- paratively 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 form- ations once called " transition," and now re-named " meta- morphic," 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

DISTRIBUTION.

has everywhere resulted from tlie complete meltmg-up ot 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. ThuH we have but extremely-imperfect data for any conclusions respecting the distribution of organic forms in Time. Soni'i few generalizations, however, may be 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 carry- ing out the comparison, we find that past Faunas differ from each other ; and that the differences between them are pro- portionate to their degrees of remoteness from each other in Time, as measured by their relative positions in the sediment- ary series. So that if we take the assemblage of organic forms living now, and compare it with the successive assem- blages 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 common to the compared assemblages, becomes smaller and smaller ; and the assemblages differ more ana more in their general characters. Though a species of bracniopod 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 mol- luscs ; 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 i<« sudden and comparatively wide, wherever there occurs a groat break in the succession of strata. The contracts which

'622 THE INDUCTIONS OF BIOLOGY.

thus arise gradually or all at once, in formal 'ons 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 lay the unlikenesses between the types that are common to both. Such distinctions between Faunas as are due to the appear- ance 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 geologi- cally 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 be- comes 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.

DISTRIBUTION, 3^3

To form a conception of the total amount and general direction of the change that has arisen in organic forms during the geologic time measured by our sedimentary series, is at present impossible — the data are insufficient. The immense contrast between the few and low forms of the earliest-known Fauna, and the many and high forms of our existing Fauna, has been commonly supposed to prove, not only great change but great progress. Nevertheless, this appearance of progress may be, and probably is, mainly illusive. Vricler knowledge and increased power of interpretation, have made it manifest that remains of comparatively well-organized creatures, really existed in strata long supposed to be devoid of them ; and that where they are actually absent, the nature of the strata often supplies a sufficient explanation of their absence, without assuming that they did not exist when these strata were formed. It has now become a tenable hypothesis, that the successively-higher types fossilized in our successive- ly-later deposits, indicate nothing more than successive migra- tions from pre-existing continents, to continents that were step by step emerging from the ocean — migrations which necessarily began with the inferior orders of organisms, and included the successively-superior orders as the new lands became more accessible to them, and better fitted for them.*

TThile the evidence usually supposed to prove progres- sion, is thus untrustworthy, there is trustworthy evidence that there has been, in many cases, little or no progression. Though the types which have existed from palaeozoic and me- sozoic times down to the present day, are almost universally changed ; yet a comparison of ancient and modern members of these types, shows that the total amount of change is not relatively great, and that it is not manifestly towards a higher organization. Though nearly all the living forms which have prototypes in early formations, differ from these prototypes specifically, and in most cases generically ; yet ordinal pecu- liarities are, in very numerous cases, maintained from the earli-

* For explanations, sec " Illogical Geology." Essays : Second Series.

T11E IXDUCTIONI3 OF BIOLOGY.

est times geologically recorded, down to our own time ; and we have no visible evidence of superiority in the existing genera of these orders. In his lecture " On the Persistent Types of Animal Life," Prof. Huxley enumerates many cases. On the authority of Dr. Hooker, he stated " that there are Carbon- iferous plants which appear to be generically identical with some now living ; that the cone of the Oolitic Araucarla is hardily distinguishable from that of an existing species ; that a true Pinus appears in the Purbecks and a Juglans in the chalk." Among animals he named palseozoic and mesozoic corals which are very like certain extant corals ; genera of Silu- rian molluscs that answer to existing genera ; insects and arach- nids in the coal formations, that are not more than generically different from some of our own insects and arachnids. He instanced " the Devonian and Carboniferous Pleur acanthus ^ which differs no more from existing sharks than these do from one another;'1 early mesozoic reptiles "identical in the essential characters of their organization with those now liv- ing; r and Triassic mammals which did not differ " nearly so much from some of those which now live, as these differ from one another." Continuing the argument in his " Anniversary Address to the Geological Society " in 1862, Prof. Huxley gave many cases in which the changes that have taken place, are not changes towards a more specialized or higher organ- ization— asking " in what sense are the Liassic Chelonia infe- rior to those which now exist ? How are the Cretaceous Ichthyosauria, Plesiosauria, or Pterosauria less embryonic or more differentiated species than those of the Lias ? ' \Vhile, however, contending that in most instances " positive evidence fails to demonstrate any sort of progressive modifi- cation towards a less embryonic or less generalized type in a great many groups of animals, of long-continued geological existence ; ': Prof. Huxley added, that there are other groups " co-existing with them, under the same conditions, in which more or less distinct indications of such a process seem to be traceable." Ajid in illustration of this, he named that better

DISTRIBUTION. 325

development of the vertebrae which characterizes some of the more modern fishes and reptiles, when compared with an- cient fishes and reptiles of the same orders ; and the u regu- larity and evenness of the dentition of the Anoplotlierium as contrasting with that of existing Artiodaetyles."

The facts thus summed up, do not show that higher forms have not arisen on the Earth in the course of geologic time, any more than the facts commciily cited prove that higher forms have arisen ; nor are they regarded by Prof. Huxley as showing this. Were the types which have survived from palceozoic and mesozoic periods down to our own clay, the only types ; and did the modifications, rarely of more than generic value, which these types have undergone, give no better evidences of increased complexity than are actually given by them ; then it would be inferable that there has been no appreciable advance among organic forms. But there now exist, and have existed during the more recent geologic epochs, various types which are not known to have existed in earlier epochs — some of them widely unlike these persistent types, and some of them nearly allied to these persistent types. As yet, we know nothing respecting the origins of these new types. But it is quite possible that causes like those which have produced generic differences in the persistent types, may, in some or many cases, have pro- duced modifications great enough to constitute ordinal differ- ences— may have resulted in the formation of types that are now classed as separate. If structural contrasts not exceed- ing certain moderate limits, are held to mark only generic distinctions ; and if organisms displaying larger structural contrasts are considered ordinally or typically distinct ; it is r;lear that the persistence of a given type through a long geologic period without apparently undergoing deviations of more than generic value, by no means disproves the occurrence of far greater deviations ; since the forms resulting from such far greater deviations, being regarded as typically distinct forms, will not be taken as evidence of great change in the

326 THE INDUCTIONS OF BIOLOGY.

original type. That which Prof. Huxley's argument proves, and that only which he considers it to prove, is that organisms have no innate tendencies to assume higher forms, and that " any admissible hypothesis of progressive modification, must he compatible with persistence without progression through indefinite periods."

One very significant fact must be added, concerning the relation between distribution in Time and distribution in Space. I quote it from Mr Darwin : — " Mr Clift many years ago showed that the fossil mammals from the Australian caves were closely allied to the living marsupals of that con- tinent. In South America, a similar relationship is manifest ^ even to an uneducated eye, in the gigantic pieces of armour like those of the armadillo, found in several parts of La Plata ; and Professor Owen has shown in the most striking manner that most of the fossil mammals, buried there in such num- bers, are related to the South American types. This relation- ship is even more clearly seen in the wonderful collection of fossil bones made by MM. Lund and Clausen in the caves of Brazil. I was so much impressed with these facts that I strongly insisted, in 1839 and 1845, on this ' law of the suc- cession of types/- —on ' this wonderful relationship in the same continent between the dead and the living.' Professor Owen has subsequently extended the same generalization to the mammals of the Old World. We see the same law in this author's restorations of the extinct and gigantic birds of New Zealand. We see it also in the birds of the caves of Brazil. Mr Woodward has shown that the same law holds good, with sea- shells, but from the wide distribution of most genera of , molluscs, it is not well displayed by them. Other cases could be added, as the relation between the extinct and living land- shells of Madeira ; and between the extinct and living brack- ish-water shells of the Aralo-Caspian Sea.'!

The general results then, are these. Our knowledge of distribution in Time, being derived wholly from the evidence afforded by fossils, is limited to that geologic time of which

DISTRIBUTION.

some records remain : cannot extend to those pre-geologic times the records of which have been obliterated. From these remaining records, which probably form but a small fraction of the whole, the general facts deducible are :- -That such organic types as have lived through successive epochs, have almost universally undergone modifications of specific and generic values — modifications which have commonly been great in proportion as the period has been long. That besides the types that have persisted from ancient eras down to our own era, other types have from time to time made their ap- pearance in the ascending series of our strata — types of which some are lower and some higher than the types previously recorded ; but whence these new types came, and whether anj of them arose by divergence from the previously-recorded types, the evidence does not yet enable us to say. That in the course of long geologic epochs, nearly all species, most gerera, and a few orders, become extinct ; and that a species, geLus, or order, which has once disappeared from the Earth, never reappears. And, lastly, that the Fauna now occupying each separate area of the Earth's surface, is very nearly allied to the Fauna which existed on that area during recent geolo gic times.

§ 108. Omitting sundry minor generalizations, the exposi- tion of which would involve too much detail, what is to be said of these major generalizations ?

The distribution in Space cannot be said to imply that or- ganisms have been designed for their particular habitats, and placed in them ; since, besides the habitat in which an organ- ism is found there are commonly other habitats, as well or better for it, from which it is absent — habitats to which it is so much better fitted than organisms now occupying them, that it extrudes these organisms when allowed the oppor- tunity. Neither can we suppose that one end has been to establish varieties of Floras and Faunas ; since, if so, why are the Floras and Faunas but little divergent in widely-sundered

328

THE INDUCTIONS OF BIOLOGY.

areas between which migration is possible, while they are markedly divergent in adjacent areas between which migra- tion is impossible ?

Passing to distributions in Time, there arise the questions — ^hy during nearly the whole of that vast period geological- ly recorded, have there existed none of those highest organic forms which have now overrun the Earth ? — how is it that we find no traces of a creature endowed with large capacities for knowledge and happiness ? The answer that the Earth was not, in remote times, a fit habitation for such a creature, be- sides being unwarranted by the evidence, suggests the equally awkward question — why during untold millions of years did the Earth remain fit only for inferior creatures ? What, again, is the meaning of this extinction of types ? To conclude that the saurian type was replaced by other types at the beginning of the tertiary period, because this type was not adapted to the conditions which then arose, is to conclude that this type could not be modified into fitness for the conditions ; and this conclusion is quite at variance with the hypothesis that creative skill is shown in the multiform adaptations of one type to many ends.

What interpretations may rationally be put on these and other general facts of distribution in Space and Time, we shall see in the next division of this work ; to which let us now pass.

PART III,

THE EVOLUTION OF LIFE

15

CHATTEL I.

PRELIMINARY.

§ 109. IN the foregoing Part, we have contemplated the most important of the generalizations to which biologista have been led by observation of organisms. These Induc- tions of Biology have also been severally glanced at on their deductive sides ; for the purpose of noting the harmony that exists between them, and those primordial truths set forth in First Principles. Having thus studied the leading pheno- mena of life separately, we are prepared for studying them in their ensemble, with the view of arriving at the most general interpretation of them.

There is an ensemble of vital phenomena presented by each organism in the course of its growth, development, and decay ; and there is an ensemble of vital phenomena presented by the organic world as a whole. Neither of these can be properly dealt with apart from, the other. But the last of them may be separately treated more conveniently than the first. What interpretation we put on the facts of structure and function in each living body, depends entirely on our conception of the mode in which living bodies in general have* originated. To form some conclusion respecting this mode — a provisional if not a permanent conclusion — -must therefore be our first step.

"We have to choose between two hypotheses — the hypo- thesis of Special Creation and the hypothesis of Evolution.

332 THE EVOLUTION OF LIFE.

Either the multitudinous kinds of organisms that now exist, and the still more multitudinous kinds that have existed during past geologic eras, have been from time to time separ- ately made ; or they have arisen by insensible steps, through actions such as we see habitually going on. Both hypo- theses imply a Cause. The last, certainly as much as the first, recognizes this Cause as inscrutable. The point at issue is, how this inscrutable Cause has worked in the pro- duction of living forms. This point, if it is to be decided at all, is to be decided only by examination of evidence. L<st us inquire which of these antagonist hypotheses is most con- gruous with established facts.

CHAPTER H.

GENERAL ASPECTS OF THE SPECIAL-CREATIOX-EYPOTHESIS.*

§ 110. EARLY ideas are not usually true ideas. Unde- veloped intellect, be it that of an individual or that of the race, forms conclusions which require to be revised and re- revised, before they reach a tolerable correspondence with realities. Were it otherwise, there would be no discovery, no increase of intelligence. What we call the progress of knowledge, is the bringing of Thoughts into harmony with Things ; and it implies that the first Thoughts are either wholly out of harmony with Things, or in very incomplete harmony with them.

If illustrations be needed, the history of every science furnishes them. The primitive notions of mankind as to the structure of the heavens, were wrong ; and the notions which replaced them were successively less wrong. The original belief respecting the form of the Earth was wrong ; and this wrong belief survived through the first civilizations. The earliest ideas that have come down to us concerning the natures of the elements were wrong ; and only in quite recent times has the composition of matter in its various forms been better understood. The interpretations of me- chanical facts, of meteorological facts, of physiological facts,

\

* Several of the arguments used in this chapter and in that which follows it, formed parts of an essay on " the Development Hypothesis," originally published in 1852.

334 THE EVOLUTION OF LIFE.

were at first wrong. In all these cases men set out with beliefs which, if not absolutely false, contained but small amounts of truth disguised by immense amounts of error.

Hence the hypothesis that living beings resulted from special creations, being a primitive hypothesis, is probably an untrue hypothesis. If the interpretations of Nature given by aboriginal men, were erroneous in other directions, they were most likely erroneous in this direction. It would be strange if, while these aboriginal men failed to reach the truth in so many cases where it is comparatively conspicuous, they yet reached the truth in a case where it is compara- tively hidden.

§ 111. Besides the improbability given to the belief in special creations, by its association with mistaken early beliefs in general ; a further improbability is given to it by its association with a special class of mistaken beliefs. It belongs to a family of beliefs which have one after another been destroyed by advancing knowledge ; and is, indeed, almost the only member of the family that survives among educated people.

We all know that the savage thinks of each striking phe- nomenon, or group of phenomena, as caused by some separate personal agent ; that out of this fetishistic conception there grows up a polytheistic conception, in which these minor per- sonalities are variously generalized into deities presiding over different divisions of nature ; and that these are eventually further generalized. This progressive consolidation of causal agencies, may be traced in the creeds of all races ; and is far from complete in the creeds of the most advanced races. The unlettered rustics who till our fields, do not let the con- sciousness of a supreme power wholly absorb the aboriginal conceptions of good and evil spirits, and charms or secret potencies dwelling in particular objects. The earliest mode of thinking changes, only as fast as the constant relations among phenomena are established. Scarcely less

THE SPECIAL-CREATION-HYPOTHESIS. 335

familiar is tlie truth, that while accumulating knowledge makes these conceptions of personal causal agents gradually more vague, as it merges them into general causes, it also destroys the habit of thinking of them as working after the methods of personal agents. We do not now, like Kepler, assume guiding spirits to keep the planets in their orbits. It is no longer the universal belief that the sea was once for all mechanically parted from the dry, land; or that the mountains were placed where we see them by a sudden cre- ative act. All but a narrow class have ceased to suppose sunshine and storm to be sent in some arbitrary succession. The majority of educated people have given up thinking of epidemics as punishments inflicted by an angry deity. Nor do even the common people regard a madman as one pos- sessed by a demon. That is to say, we everywhere see fading away the anthropomorphic conception of the Un- known Cause. In one case after another, is abandoned that interpretation which ascribes phenomena to a will analogous to the human will, working by methods analogous to human methods.

If, then, of tins once-numerous family of beliefs, the im- mense majority have become extinct, we may not unrea- sonably expect that the few remaining members of the family will become extinct. One of these is the belief we are here considering — the belief that each species of organism was specially created. Many who in all else have abandoned the aboriginal theory of things, still hold this remnant of the aboriginal theory. Ask any tolerably-informed man whether he accepts the cosmogony of the Indians, or the Greeks, or the Hebrews, and he will regard the question as next to an insult. Yet one element common to these cosmogonies he very likely retains : not bearing in mind its origin. For whence did he get the doctrine of special creations ? Catechise him, and he is forced to confess that it was put into his mind in childhood, as one portion of a story which, as a whole, he has long since rejected. "Why this fragment is likely to be

THE EVOLUTION OF LIFE.

right while all the rest is wrong, he is unable to say. May we not then expect, that the relinquishment of all other parts of this story, will bye and bye be followed by the relinquishment of this remaining part of it ?

§ 112. The belief which we find thus questionable, both as being a primitive belief and as being a belief belonging to nn almost- extinct family, is a belief that is not countenanced by a single fact. No one ever saw a special creation ; no one ever found proof of an indirect kind, that a special creation had taken place. It is significant, as Dr Hooker remarks, that naturalists who suppose new species to be miraculously originated, habitually suppose the origination to occur in some region remote from human observation. Wherever the order of organic nature is exposed to the view of zoologists and botanists, it expels this conception ; and the conception survives only in connexion with imagined places, where the order of organic phenomena is unknown.

Besides being absolutely without evidence to give it exter- nal support, this hypothesis of special creations cannot sup- port itself internally — cannot be framed into a coherent thought. It is one of those illegitimate symbolic concep- tions, so continually mistaken for legitimate symbolic concep- tions (First Principles, § 9), because they remain untested. Immediately an attempt is made to elaborate the idea into anything like a definite shape, it proves to be a pseud-idea, admitting of no definite shape. Is it supposed that a new organism, when specially created, is created out of nothing ? If so, there is a supposed creation of matter ; and the crea- tion of matter is inconceivable — implies the establishment of a relation in thought between nothing and something — a relation of which one term is absent — an impossible rela- tion. Is it supposed that the matter of which the new or- ganism consists, is not created for the occasion, but is taken out of its pre-existing forms and arranged into a new form ? If so, we are met by the question — how is the re- arrangement

THE SPECIAL -CREATION-HYPOTHESIS. 337

effected ? Of the myriad atoms going to the composition of the new organism, all of them previously dispersed through the neighbouring air and earth, does each, suddenly dis- engaging itself from its combinations, rush to meet the rest, unite with them into the appropriate chemical compounds, and then fall with certain others into its appointed place in the aggregate of complex tissues and organs ? Surely thus to assume a myriad supernatural impulses, differing in their directions and amounts, given to as many different atoms, is a multiplication of mysteries rather than the solution of a mystery. For every one of these impulses, not being the result of a force locally existing in some other form, implies the creation of force ; and the creation of force is just as inconceivable as the creation of matter. And thus is it with all attempted ways of representing the process. The old Hebrew idea that God takes clay and moidds a new creature, as a potter might mould a vessel, is probably too grossly an- thropomorphic to be accepted by any modern defender of the Bpecial-creation doctrine. But having abandoned this crude belief, what belief is he prepared to substitute ? If a new organism is not thus produced, then in what way is a new organism produced ? or rather — in what way can a new organism be conceived to be produced ? We will not ask for the ascertained mode, but will be content with a mode that can be consistently imagined. No such mode, however, is assignable. Those who entertain the proposition that each kind of organism results from a divine interposition, do so because they refrain from translating words into thoughts. The case is one of those where men do not really believe, but rather believe they believe. For belief, properly so called, implies a mental representation of the thing believed ; and no such mental representation is here possible.

§ 113. If we imagine mankind to be contemplated by some creature as short-lived as an ephemeron, but possessing intelligence like our own — if we imagine such a being study

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&>8 THE EVOLUTION OF LIFE.

ing men and women, during his few hours of life, ana speculating as to the mode in which they came into existence ; it is manifest that, reasoning in the usual way, he would suppose each man and woman to have been separately created. No appreciable changes of structure occurring in any of them during the few hours over which his observa- tions extended, this being would probably infer that no changes of structure were taking place, or had taken place • and that from the outset, each man and woman had pos- sessed all the characters then visible — had been orginally formed with them. This would naturally be the first im- pression. The application is obvious. A human life is ephemeral compared with the life of a species ; and even the period over which the records of human experience extend, is ephemeral compared with the life of a species. There is thus a parallel contrast between the immensely-long series of changes that have occurred during the life of a species, and that small portion of the series open to our view. And there is no reason to suppose that the first conclusion drawn by mankind from this small part of the series visible to them, is any nearer the truth, than would be the conclu- sion of the supposed ephemeral being respecting men and women.

This analogy, suggesting as it does how the hypothesis of special creations is merely a formula for our ignorance, raises the question — what reason have we to assume special crea- ti ms of species but not of individuals ; iinless it be that in the case of individuals we directly know the process to be otherwise, but in the case of species do not directly know it to be otherwise ? Have we any ground for concluding that species were specially created, except the ground that we have no immediate knowledge of their origin ? And does our ignorance of the manner in which they arose, warrant us in asserting that they arose by special creation ?

Another question is suggested by this analogy. Those who, in the absence of immediate evidence of the WOT in

THE SPECIAL-CREATIOX-HYPOTHESIS. 339

which, species arose, assert that they arose not in any way analogous to that in which individuals arise, but in a totally distinct way, think that by this supposition they ' honour the Unknown Cause of things ; and they oppose any antagonist doctrine as amounting to an exclusion of divine power from the world. But if divine power is demonstrated by the separate creation of each species, would it not have been still better demonstrated by the separate creation of each indivi- dual ? Why should there exist this process of natural gene- sis ? Why should not omnipotence have been proved by the supernatural production of plants and animals everywhere throughout the world from hour to hour ? Is it replied that the Creator was able to make individuals arise from one mother in a natural succession, but not to make species thus arise ? This is to assign a limit to power instead of magni- fying it. Is it replied that the occasional miraculous origina- tion of a species was practicable, but that the perpetual miracu- lous origination of countless individuals was impracticable ? This also is a derogation. Either it was possible or not pos- sible to create species and individuals after the same general method. To say that it was not possible is suicidal in those who use this argument ; and if it was possible, it is required to say what end is served by the special creation of species that would not have been better served by the special creation of individuals. Again, what is to be thought of the

fact that the great majority of these supposed special creations took place before mankind existed ? Those who think that di- vine power is demonstrated by special creations, have to answer the question — to whom demonstrated ? Tacitly or avowedly, they regard the demonstrations as being for the benefit of mankind. But if so, to what purpose were the millions of these demonstrations which took place on the Earth when there were no intelligent beings to contemplate them ? Did the Unknowable thus demonstrate his power to himself? Few will have the hardihood to say that any such demon- stration was needful. There is no choice but to regard them,

340 THE EVOLUTION OF LIFE.

either as superfluous exercises of power, which is a derogatory supposition, or as exercises of power that were necessary because species could not be otherwise produced, which is< also a derogatory supposition.

§ 114. Those who espouse the hypothesis of special cre- ations, entangle themselves in other theological difficulties. This assumption that each, kind of organism was specially designed, carries with it the implication that the designer intended everything that results from the design. There is no escape from the admission, that if organisms were severally constructed with a view to their respective ends ; then the character of the constructor is indicated both by the ends themselves, and the perfection or imperfection with

which the organisms are fitted to them. Observe the con- sequences.

Without dwelling on the question put in a recent chap- ter, why during untold millions of years there existed on the Earth no beings endowed with capacities for wide thought and high feeling, we may content ourselves with asking why, at present, the Earth is largely peopled by creatures which inflict on each other, and on themselves, so much suffering ? Omitting the human race, whose defects and miseries the current theology professes to account for, and limiting ourselves to the lower creation, what must we think of the countless different pain-inflicting appliances and instincts with which animals are endowed ? Not only now, and not only ever since men have lived, has the Earth been a scene of warfare among all sentient creatures ; but palaeontology shows us that, from the earliest eras geologi- cally recorded, there has been going on this universal carn- age. Fossil structures, in common with the structures of existing animals, show us elaborate weapons for destroying other animals. We have unmistakable proof that through- out all past time, there has been a perpetual preying of the superior on the inferior — a ceaseless devouring of the weak

THE SPECIAL-CREATION-HYPOTHESIS. 341

by the strong. How is tliis to be explained ? How happens it that animals were so designed as to render this bloodshed necessary ? How happens it that in almost every species, the number of individuals annually born is such that the ma- jority die of starvation or by violence before arriving at ma- turity? Whoever contends that each kind of animal was special] y designed, must assert either that there was a deli- berate intention on the part of the Creator to produce those results, or that there was an inability to prevent them. Which alternative does he prefer ? To cast an imputation on the divine character, or assert a limitation of the divine power ? It is useless for him to plead that the destruction of the less powerful by the more powerful, is a means of pre- venting the miseries of decrepitude and incapacity, and therefore works beneficently. For even were the chief mor- tality among the aged instead of among the young, there would still arise the unanswerable question — why were not animals constructed in such ways as to avoid these evils ?

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why were not their rates of multiplication, their degrees of intelligence, and their propensities, so adjusted that these sufferings might be escaped ? And if decline of vigour was a necessary accompaniment of age, why was it not provided that the organic actions should end in sudden death, when- ever they fell below the level required for pleasurable exist- ence ? Will any one who contends that organisms were

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specially designed, assert that they could not have been designed so as to prevent suffering ? And if he admits that they could have been made so as to prevent suffering, will he assert that the Creator preferred so making them as to inflict suffering ?

Even as thus presented, the difficulty is sufficiently great ; but it appears immensely greater when we examine the facts more closely. So long as we contemplate only the preying of the superior on the inferior, some good appears to be extracted from the evil — a certain amount of life of a higher order, is supported by sacrificing a great deal of life of a

342 THE EVOLUTION OF LITE.

lower order. So long, too, as we leave out all mortality but that which, by carrying off the least perfect members of each species, leaves the most perfect members to continue the species ; we see some compensating benefit reached through the suffering inflicted. But what shall we say on finding innumerable cases in which the suffering inflicted brings no compensating benefit ? AVhat shall we say when we see the inferior destroying the superior ? What shall we say on discovering elaborate appliances for securing the prosperity of organisms incapable of feeling, at the expense of misery to organisms capable of happiness ?

Of the animal kingdom as a whole, more than half the species are parasites. " The number of these parasites," says Prof. Owen, " may be conceived when it is stated that almost every known animal has its peculiar species, and generally more than one, sometimes as many as, or even more kinds than, infest the human body." Passing over the evils thus inflicted on animals of inferior dignity, let us limit ourselves to the case of man. The Bothriocephalus latus and the Tcenia solttim, are two lands of tape-worm, which nourish in the human intestines ; producing great constitu- tional disturbances, sometimes ending in insanity ; and from the germs of the Tcenia, when carried into other parts of the body, arise certain partially-developed forms known as Cysti- ccrci, Echinococci, and Ccenuri, which cause disorganization more or less extensive in the brain, the lungs, the liver, the heart, the eye, &c., often ending fatally after long- continued suffering. Five other parasites, belonging to a different class, are found in the viscera of man — the Trichocephahis, the Oxyuris, the Strongyhts (two species), the Ancylostomum, and the Ascaris ; which, beyond that defect of nutrition which they necessarily cause, sometimes induce certain irritations that lead to complete demoraliza- tion. Of another class of entozoa, belonging to the sub- division Trcmatoda, there are five kinds found in different organs of the human body — the liver and gall ducts, the

THE SPECIAL-CREATION- HYPOTHESIS. 343

portal vein, the intestine, the bladder, the eye. Then we have the Trichina spiralis, which passes through one phase of its existence imbedded in the muscles and through another phase of its existence in the intestine ; and which, by the induced disease Trichinietsis, has lately committed such ra- vages in Germany, as to cause a panic. And to these we must add the Guinea-worm, which in some part of Africa and India, makes men miserable by burrowing in their legs. From this list of entozoa, which is by no means complete, let us pass to the epizoa. There are two kinds of Acari, one of them inhabiting the follicles of the skin, and the other producing the itch. There are other creatures that bury themselves beneath the skin, and lay their eggs there ; and there are three species of lice which infest the surface of the body. Nor is this all : besides animal parasites, there are sundry vegetal parasites, which grow and multiply at our cost. The Sardna ventriculi inhabits the stomach, and produces gastric disturbance. The Leptotlirix buccalis is extremely general in the mouth, and may have something to do with the decay of teeth. And besides these, there are microscopic fungi which produce ringworm, porrigo, pityri- asis, thrush, &c. Thus the human body is the

habitat of parasites, internal and external, animal and ve- getal, numbering, if all were set down, some two or three dozen species ; sundry of which are peculiar to man, and many of which produce in man great suffering and not un- frequently death. "WTiat interpretation is to be put on these facts by those who espouse the hypothesis of special crea- tions ? According to this hypothesis, all these parasites were designed with a view to their respective modes of life. They were endowed with constitutions fitting them to live by absorbing the jvices of the human body ; they were fur- nished with appliances, often of a formidable kind, enabling them to root themselves in and upon the human body ; and they were made prolific in an almost incredible degree, that their germs might have a sufficient number of chances of

34:4 THE EVOLUTION OF LIFE.

finding their way into the human body. In short, elaborate contrivances were combined to insure the continuance of their respective races ; and to make it impossible for the suc- cessive generations of men to avoid being preyed upon by them. What shall we say to this arrangement ? Shall we say that man, " the head and crown of things," was provided as a habitat for these parasites ? Or shall we say that these degraded creatures, incapable of thought or enjoyment, were created that they might cause unhappiness to man ? One or other of these alternatives must be chosen by those who contend that every land of organism was separately devised by the Creator. Which do they prefer ? With the concep- tion of two antagonistic powers, which severally work good and evil in the world, the facts are congruous enough. But with the conception of a supreme beneficence, this gratuitous infliction of misery on man, in common with all other terres trial creatures capable of feeling, is absolutely incompatible.

§ 115. See then the results of our examination. The belief in special creations of organisms, is a belief that arose among men during the era of profoundest darkness ; and it belongs to a family of beliefs which have nearly all died out as enlightenment has increased. It is without a solitary established fact on which to stand ; and when the attempt is made to put it into definite shape in the mind, it turns out to be only a pseud-idea. This mere verbal hypothesis, which men idly accept as a real or thinkable hypothesis, is of the same nature as would be one, based on a day's observation of human life, that each man and woman was specially created — an hypothesis not suggested by evidence, but by lack of evidence — an hypothesis which formulates absolute ignorance into a semblance of positive knowledge. Further, we see that this hypothesis, wholly without support, essentially inconceiv- able, and thus failing to satisfy men's intellectual need of an interpretation, fails also to satisfy their moral sentiment. It is quite inconsistent with those conceptions of the divine

THE SFEC1AL-CREATION-HYPOTHESIS. 3^5

nature \vliicli tliey profess to entertain. If infinite power was to be demonstrated, then, either by the special creation of every individual, or by the production of species after a method akin to that in which individuals are produced, it would be better demonstrated than by the use of the two methods which the hypothesis assumes to be necessary. And if infinite goodness was to be demonstrated, then, not only do the provisions of organic structure, if they are especially devised, fail to demonstrate it ; but there is an enormous mass of them which imply malevolence rather than bene- volence.

Thus, however regarded, the hypothesis of special creations turns out to be worthless — worthless by its derivation ; worthless in its intrinsic incoherence ; worthless as absolutely without evidence ; worthless as not supplying an intellectual need ; worthless as not satisfying a moral want. We must therefore consider it as counting for nothing, in opposition to any other hypothesis respecting the origin of organic beings.

CHAPTER III.

GENERAL ASPECTS OF THE EVOLUTION-HYPOTHESIS.

§ 116. JUST as the supposition that races of organisms have been specially created, is discredited by its origin ; so, conversely, the supposition that races of organisms have been evolved, is credited by its origin. Instead of being a conception suggested and accepted when mankind were profoundly ignorant, it is a conception born in times of com- parative enlightenment. Moreover, the belief that all organic forms have arisen in conformity with uniform laws, instead of through breaches of uniform laws, is a belief that has come into existence in the most-instructed class, living in these better-instructed times. Not among those who have paid no attention to the order of Nature, has this idea made its appearance ; but among those whose pursuits have famil- iarized them with the order of Nature. Thus the derivation of this modern hypothesis is as favourable as that of the ancient hypothesis is unfavourable.

§ 117. A kindred antithesis exists between the two fami- lies of beliefs, to which the beliefs we are comparing severally belong. "While the one family has been dying out, the other family has been multiplying. Just as fast as men have ceased to regard different classes of phenomena as caused by special personal agents, acting irregularly ; so fast have they come to regard these different classes of phe- nomena as caused by a general agency acting uniformly — the

THE EVOLUTION-HYPOTHESIS. 347

two changes being correlative. And as, on the one hand, the hypothesis that each species resulted from a supernatural act, having lost nearly all its kindred hypotheses, may be expected soon to become extinct ; so, on the other hand, the 1 .ypothesis that each species resulted from the action of na- mral causes, being one of an ever-increasing family of hypo- theses, may be expected to survive and become established.

Still greater will the probability of its survival and estab- lishment appear, when we observe that it is one of a particu- lar genus of hypotheses that has been rapidly extending. The interpretation of phenomena as resulting from Evolution, has been independently showing itself in various fields of inquiry, quite remote from one another. The supposition that the Solar System has been gradually evolved out of dif- fused matter, is a supposition wrholly astronomical in its origin and application. Geologists, without being led thereto by astronomical considerations, have been step by step ad- vancing towards the conviction, that the Earth has reached its present varied structure through a process of Evolution. The inquiries of biologists have proved the falsity of the once general belief, that the germ of each organism is a minute repetition of the mature organism, differing from it only in bulk ; and they have shown, contrariwise, that every organ- ism, arising out of apparently-uniform matter, advances to its ultimate multiformity through insensible changes. Among philosophical politicians, there has been spreading the per- ception that the . progress of society is an evolution : the truth that " constitutions are not made but grow," is a part of the more general truth that societies are not made but grow. It is now universally admitted by philologists, that languages, instead of being artificially or supernaturally formed, have been developed. And the histories of religion, of philosophy, of science, of the fine arts, and of the indus- trial arts, show that these have passed through stages as un- obtrusive as those through which the mind of a child passes on its way to maturity. If then, the recognition of evolu-

348 THE EVOLUTION OF LIFE.

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tion as the law of many diverse orders of phenomena, Las been spreading ; may we not say that there thence arises the probability that evolution will presently be recognized as the law of the phenomena we are considering ? Each further ad- vance of knowledge, confirms the belief in the unity of Nature ; and the discovery that evolution has gone on, or is going on, in so many departments of Nature, becomes a rea- son for believing that there is no department of Nature in which it does not go on.

§ 118. The hypotheses of Special Creation and Evolution, are no less contrasted in respect of their legitimacy as hy- potheses. "While, as we have seen, the one belongs to that order of symbolic conceptions which are proved to be illusive by the impossibility of realizing them in thought ; the other is one of those symbolic conceptions which are more or less completely realizable in thought. The production of all organic forms by the slow accumulation of modifications upon modifications, and by the slow divergences resulting from the continual addition of differences to differences, is mentallv

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representable in outline, if not in detail. Various orders of our experiences enable us to conceive the process. Let us look at one of the simplest.

There is no apparent similarity between a straight line and a circle. The one is a curve ; the other is defined as without curvature. The one encloses a space ; the other will not enclose a space though produced for ever. The one is finite ; the other may be infinite. Yet, opposite as the two are in all their properties, they may be connected together by a series of lines no one of which differs from the adjacent ones in any appreciable degree. Thus, if a cone be cut by a plane at right angles to its axis, we get a circle. If, instead of being perfectly at right angles, the plane subtends with the axis an angle of 89° 59', we have an ellipse which no human eye, even when aided by an accurate pair of compasses, can distinguish from a circle. Decreasing the angle minute

THE EVOLUTION-HYPOTHESIS. 349

by minute, the ellipse becomes first perceptibly eccentric, then manifestly so, and by and by acquires so immensely elongated a form, as to bear no recognizable resemblance to a circle. By continuing this process, the ellipse changes insensibly into a parabola. On still further diminishing the angle, the para- bola becomes an hyperbola. And finally, if the cone be made gradually more obtuse, the hyperbola passes into a straight line, as the angle of the cone approaches 180°. Now here we haye" five different species of line — circle, ellipse, parabola, hyperbola, and straight line — each having its pecu- liar properties and its separate equation, and the first and last of which are quite opposite in nature, connected together as members of one series, all producible by a single process of insensible modification.

But the experiences which most clearly illustrate to us the process of general evolution, are our experiences of special evolution, repeated in every plant and animal. Each organism exhibits, within a short space of time, a series of changes which, when supposed to occupy a period inde- finitely great, and to go on in various ways instead of one way, give us a tolerably clear conception of organic evo- lution in general. In an individual development, we have compressed into a comparatively infinitesimal space, a series of metamorphoses equally vast with those which the hypo- thesis of evolution assumes to have taken place during those immeasurable epochs that the Earth's crust tells us of. A tree differs from a seed immeasurably in every respect — in bulk, in structure, in colour, in form, in specific gravity, in chemical composition : differs so greatly that no visible resemblance of any kind can be pointed out between them. Yet is the one changed in the course of a few years into the other : changed so gradually, that at no moment can it be said — Now the seed ceases to be, and the tree exists. What can be more widely contrasted than a newly-born child and the small, semi-trap sparent, gelatinous spherule constituting the human ovum? The infant is so complex in structure

350 THE EVOLUTION OF LIFE.

that a cyclopaedia is needed to describe its constituent parts. The germinal vesicle is so simple that it may be defined in a line. Nevertheless, a few months suffice to develope the one out of the other ; and that, too, by a series of modifica- tions so small, that were the embryo examined at successive minutes, even a microscope would with difficulty disclose any sensible changes. Aided by such facts, the conception of general evolution may be rendered as definite a concep- tion as any of our complex conceptions can be rendered. If instead of the successive minutes of a child's foetal life, we take successive generations of creatures — if we regard the suc- cessive generations as differing from each other no more than the foetus did in successive minutes ; our imaginations must indeed be feeble if we fail to realize in thought, the evolu- tion of the most complex organism out of the simplest. If a single cell, under appropriate conditions, becomes a man in the space of a few years ; there can surely be no difficulty in understanding how, under appropriate conditions, a cell may, in the course of untold millions of years, give origin to the human race.

It is true that many minds are so unfurnished with those experiences of Nature out of which this conception is built, that they find difficulty in forming it. Habitually looking at things rather in their statical than in their dynamical aspects, they never realize the fact that, by small increments of modification, any amount of modification may in time be generated. That surprise which they feel on finding one whom they last saw as a boy, grown into a man, becomes incredulity when the degree of change is greater. To such, the hypothesis that by any series of changes a protozoon sbould ever give origin to a mammal, seems grotesque — as grotesque as did Galileo's assertion of the Earth's movement seem to the Aristotleans ; or as grotesque as the assertion of the Earth's sphericity seems now to the New Zealanders. II ut those who accept a literally-unthinkable proposition as

THE EVOLUTION-HYPOTHESIS. 351

quite satisfactory, may not unnaturally be expected to make a converse mistake.

§ 119. The hypothesis of evolution is contrasted with the hypothesis of special creations, in a further respect. It ia not simply legitimate instead of illegitimate, because repre- Sfcn table in thought instead of unrepresentable ; but it has the support of some evidence, instead of being absolutely unsupported by evidence. Though the facts at present as- signable in direct proof that by progressive modifications, races of organisms that are apparently distinct may result from antecedent races, are not sufficient ; yet there are nu- merous facts of the order required. It has been shown beyond all question that unlikenesses of structure gradually arise among descendants from the same stock. "We find that there is going on a modifying process of the kind alleged as the source of specific differences- : a process which, though slow in its action, does, in time, if the circumstances demand it, produce conspicuous changes — a process which, to all appearance, would produce in the millions of years, and under the great varieties of conditions which geological records imply, any amount of change.

In the chapters on " Heredity J; and "Variation," con- tained in the preceding Part, many such facts were given ; and plenty more might be added. Although comparatively little attention has been paid to the matter until recent times, the evidence already collected shows that there take place in successive generations, alterations of structure quite as marked as those which, in successive short intervals, arise ir a developing embryo — nay, often much more marked ; since, besides differences due to changes in the relative sizes of parts, there sometimes arise differences due to additions and suppressions of parts. The structural modification proved to have taken place since organisms have been observed, is not less than the hypothesis demands — bears as great a ratio

352 THE EVOLUTION OF LIFE.

to this brief period, as the total amount of structural changb seen in the evolution of a complex organism out of a simple germ, bears to that vast period during which living forms have existed on the Earth.

We have, indeed, much the same kind and quantity of direct evidence that all organic beings have gradually arisen through the actions of natural causes, which we have that all the structural complexities of the Earth's crust have arisen through the actions of natural causes. It may, I think, be fairly said, that between the known modifications undergone by organisms, and the totality of modifications displayed in their structures, there is no greater disproportion than between the geological changes which have been witnessed, and the to- tality of geological changes supposed to be similarly caused. Here and there are pointed out sedimentary deposits now slowly taking place. At this place, it is proved that a shore has been encroached on by the sea to a considerable extent within recorded times ; and at another place, an estuary is known to have become shallower within the space of some generations. In one region a general upheaval is going on at the rate of a few feet in a century ; while in another region occasional earthquakes are shown to cause slight variations of level. Appreciable amounts of denudation by water are visible in some localities ; and in other localities glaciers are detected in the act of grinding down the rocky sur- faces over which they glide. But the changes thus instanced, are infinitesimal compared with the aggregate of changes tc which the Earth's crust testifies, even in its still extant sys- tems of strata. If, then, from the small changes now being wrought on the Earth's crust by natural agencies, we may Legitimately conclude that by such natural agencies acting through vast epochs, all the structural complexities of the Earth's crust have been produced; may we not from the small known modifications produced in races of organisms b}r natural agencies, similarly infer that from natural agen-

IKE EVOLUTION-HYPOTHESIS. 353

tics have slowly arisen all those structural complexities which we see in them ?

The hypothesis of Evolution then, has direct support from facts which, though small in amount, are of the kind required; and the proportion wliieli these facts bear to the conclusion drawn, seems as great as is the proportion "between facts and conclusion which, in another case, produces acceptance of the conclusion.

§ 120. Let us put ourselves for a moment in the position of those who, from their experiences of human modes of action, draw inferences respecting the mode of action of that ultimate power manifested to us through phenomena. We shall find the supposition that each kind of organism was separately designed and put together, to be much less consistent with their professed conception of this ultimate power, than is the bupposition that all kinds of organisms have resulted from one unbroken process. Irregularity of method is a mark of weakness. Uniformity of method is a mark of strength. Con- tinual interposition to alter a pre-arranged set of actions, implies defective arrangement in those actions. The main- tenance of those actions, and the working out by them of the highest results, implies completeness of arrangement. If human workmen, whose machines as at first constructed require perpetual adjustment, show their increasing skill by making their machines self-adjusting ; then, those who figure to themselves the production of the world and its inhabitants by a " Great Artificer," must admit that the achievement of this end by a persistent process, adapted to all contingencies, implies greater skill than its achievement by the process of meeting the contingencies as they severally arise.

So, too, it is with the contrast under its moral aspect. We saw that to the hypothesis of special creations, a difficulty is presented by the absence of high forms of life during thoso immeasurable epochs of the Earth's existence which geology

16

C$54 THE EVOLUTION OF LIFE.

records. But to the hypothesis of evolution, this absence is no such obstacle. Suppose evolution, and this question is necessarily excluded. Suppose special creations, and this question, unavoidably raised, can have no satisfactory an- swer. Still more marked is this contrast between the two hypotheses, in presence of that vast amount of suf- fering entailed on all orders of sentient beings, by their imperfect adaptations to their conditions of life ; and the further vast amount of suffering entailed on them by enemies and by parasites. We saw that if organisms were severally designed for their respective places in Nature, the inevitable conclusion is, that these thousands of kinds of inferior organ- isms which prey upon superior organisms, were intended to inflict all the pain and mortality which results. But the hy- pothesis of evolution involves us in no such dilemma. Slowly, but surely, evolution brings about an increasing amount of happiness : all evils being but incidental. By its essen- tial nature, the process must everywhere produce greater fitness to the conditions of existence ; be they what they may. Applying alike to the lowest and the highest forms of organ- ization, there is in all cases a progressive adaptation ; and a survival of the most adapted. If, in the uniform working cut of the process, there are evolved organisms of low types, which prey on those of higher types, the evils inflicted form but a deduction from the average benefits. The universal and necessary tendency towards supremacy and multiplica- tion of the best, applying to the organic creation as a whole as well as to each species, is ever diminishing the damage done — tends ever to maintain those most superior organisms which, in one way or other, escape the invasions of the infe- rior, and so tends to produce a type less liable to the inva- sions of the inferior. Thus the evils accompanying evolu- tion are ever being self- eliminated. Though there may arise the question — "Why could they not have been avoided? there does not arise the question — Why were they deliber-

TIIE EVOLUTION- HYPOTHESIS. 355

ately inflicted? Whatever may be thought of them, it is clear that they do not imply gratuitous malevolence.

§ 121. In all respects, then, the hypothesis of evolution contrasts favourably with the hypothesis of special creation. It has arisen in comparatively-instructed times, and in the most cultivated class. It is one of those beliefs in the uni- form concurrence of phenomena, which are gradually sup- planting beliefs in their irregular and arbitrary concurrence ; and it belongs to a genus of these beliefs which has of late been rapidly spreading. It is a definitely- conceivable hypo- thesis : being simply an extension to the organic world at large, of a conception built from our experiences of individual organisms ; just as the hypothesis of universal gravitation, was an extension of the conception which our experiences of terrestrial gravitation had produced. This definitely- con- ceivable hypothesis, besides the support of numerous ana- logies, has the support of direct evidence : we have positive proof that there is going on a process of the kind alleged ; and though the results of this process, as actually witnessed, are minute in comparison with the totality of results ascribed to it, yet they bear to such totality, a ratio as great as that by which an analogous hypothesis is justified. Lastly, that senti- ment which the doctrine of special creations is thought neces- sary to satisfy, is much better satisfied by the doctrine of evolu- tion ; since this doctrine raises no contradictory implications respecting the Unknown Cause, such as are raised by the antagonist doctrine.

And now, having observed how, under its most general aspects, the hypothesis of evolution commends itself to us, by its derivation, by its coherence, by its analogies, by its direct evidence, by its implications ; let us go on to consider the several orders of facts which yield indirect support to it. We will begin by noting the harmonies that exist between it, and sundry of the inductions set forth in Part II.

CHAPTER IV.

THE ARGUMENTS FROM CLASSIFICATION.

§ 122. IN § 103, we saw that the relations which exist among the species, genera, orders, and classes of organisms, 9,re not interpretable as results of any such causes as have been usually assigned. We will here consider whether they are interpretable as the results of evolution. Let us first contemplate some familiar facts.

The Norwegians, Swedes, Danes, Germans, Dutch, and Anglo-Saxons, form together a group of Scandinavian races, that are but slightly divergent in their characters. Welsh, Irish, and Highlanders, though they have differences, have not differences such as to hide a decided community of na- ture : they are classed together as Celts. Between the Scandinavian race as a whole and the Celtic race as a whole, there is a recognized distinction greater than that between the sub-divisions which make up one or the other. And the several peoples inhabiting Southern Europe are more nearly allied to one another, than the aggregate they form is allied to the aggregates of Northern peoples. If, again, we compare these European varieties of man taken as a group, with that group of Eastern varieties which had a common origin with it, we see a stronger contrast than between the European varieties themselves. And once more, ethnolo- gists find differences of still higher importance, between the Aryan stock as a whole and the Mongolian stock as a whole,

THE AllGUMEXTS FROM CLASSIFICATION. 857

or tlie Negro stock as a whole. Though these contrasts are partially ohscured by intermixtures ; yet they are not so obscured as to hide the truths that the most-nearly-allied varieties of man, are those which diverged from one ano- ther at a comparatively-recent period ; that each group of nearly-allied varieties, is more strongly contrasted with other such groups that had a common origin with it at a remoter period ; and so on, until we coine to the largest groups, which are the most strongly contrasted, and of whose divergence no trace is extant.

The relations existing among the classes and sub- classes of languages, have been briefly referred to by Mr Darwin, in illustration of his argument. TTe know that languages have arisen by evolution. Let us then see what grouping of them evolution has produced. On comparing the dialects of adja- cent counties in England, we find that their differences are so small as scarcely to distinguish them. Between the dialects of the Northern counties taken together, and those of the Southern counties taken together, the contrast is stronger. These clusters of dialects, together with those of Scotland and Ireland, are nevertheless so similar, that we regard them as one language. The several languages of Scandinavian Eu- rope, including English, are much more unlike one ano- ther, than are the several dialects which each of them in- cludes ; in correspondence with the fact that they diverged from one another at earlier periods than did their respective dialects. The Scandinavian languages have nevertheless a certain community of character, which distinguishes them as a group from the languages of Southern Europe ; between which there are general and special affinities that similarly unite them into a group formed of sub-groups containing sub- sub-groups. And this wider divergence between the order of languages spoken in Northern Europe, and the order of languages spoken in Southern Europe, answers to the longer- rime that has elapsed since their differentiation commenced. Further, these two orders of modern European languages, as

358 THE EVOLUTION OF LIFE.

well as Latin and Greek and certain extinct and spoken languages of the East, are shown to have traits in common, which, notwithstanding the wide gaps between them, unite them together as one great class of Aryan lan- guages ; radically distinguished from the classes of lan- guages spoken by the other great divisions of the human race.

§ 123. Now this kind of subordination of groups, which we see arises in the course of continuous descent, multiplica- tion, and divergence, is just the kind of subordination of groups which plants and animals exhibit : it is just this kind of subordination which has thrust itself on the attention of naturalists, in spite of pre-conceptions.

The original idea was that of arrangement in linear order. We saw that even after a considerable acquaintance with the structures of organisms had been acquired, naturalists con- tinued their efforts to reconcile the facts with the notion of a uni- serial succession. The accumulation of evidence necessi- tated the breaking up of the imagined chain into groups and sub-groups. Gradually there arose the conviction that these groups do not admit of being placed in a line. And the conception finally arrived at, is, that of certain great sub- Idngdoms, very widely divergent, each made up of classes much less widely divergent, severally containing orders still less divergent; and so on with genera and species. The diagram on page 303, shows the general relations of these divisions in their degrees of subordination.

Hence this "grand fact in natural history of the subordina- tion of group under group, which from its familiarity does not always sufficiently strike us," is perfectly in harmony with the hypothesis of evolution. The extreme significance of this kind of relation among organic forms, is dwelt on by Mr Dai win ; who shows how an ordinary genealogical tree represents, on a small scale, a system of grouping analogous to that which exists among organisms in general, and which is

THE ARGUMENTS FROM CLASSIFICATION. 359

explained on the supposition of a genealogical tree by which all organisms are affiliated. If, wherever we can trace

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direct descent, multiplication, and divergence, this formation of groups within groups takes place ; there results a strong presumption that the groups within groups which constitute the animal and vegetal kingdoms, have arisen by direct descent, multiplication, and divergence — that is, by evolu- tion.

8 124. Strong confirmation of this inference is furnished

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by the fact, that the more marked differences which divide groups, are, in both cases, distinguished from the lesa marked differences which divide sub-groups, by this, that they are not simply greater in degree, but they are more radical in hind. Objects, as the stars, may present them- selves in small clusters, which are again more or less aggre- gated into clusters of clusters, in such manner that the in- dividuals of each simple cluster, are much closer together than are the simple clusters composing a compound cluster : in which case, the kinship that unites groups of groups differs from the kinship that unites groups, not in nature, but only in amount. But this is not the case either with the groups and sub-groups which we know have residted from evolution, or with those which we here infer have re- sulted from evolution. Among these, we find the highest or most general classes, are separated from one another by fundamental differences that have no common measure with the differences that separate small classes. Observe the pa- rallelism.

We saw that each sub-kingdom of animals is marked o.^ from the other sub-kingdoms, by a total unlikeness in its plan of organization : that is, the members of any sub-kingdom are bound together, not by some superficial attribute which they all have, but by some attribute determining the general nature of their organizations. TVhile, contrariwise, the members of the smallest groups are united together, and se- parated from the members of other small £roup«. h\- m '»<•!'•

360 THE EVOLUTION OF LIFE.

fications which, do not affect the essential relations of parti/. That this is just, the kind of arrangement which results from evolution, the case of languages will show.

If we compare the dialects spoken in different parts of England, we find scarcely any differences but those of pro- nunciation : the structures of the sentences are almost uuiform. Between English and the allied modern languages, there are decided divergences of structure : there are some unlikenesses of idiom ; some unlikenesscs in the ways of modifying the meanings of verbs ; and considerable unlike- nesses in the uses of genders. But these unlikenesses are not sufficient to hide a general community of organization. A greater contrast of structure exists between these modern lan- guages of Western Europe, and the classic languages. That differentiation into abstract and concrete elements, which is shown by the substitution of auxiliary words for inflections, has produced a higher specialization distinguishing these languages as a group from the older languages. Neverthe- less, both the ancient and modern languages of Europe, to- gether with some Eastern languages derived from the same original, have, under all their differences of organization, a fundamental community of organization ; inasmuch as all of them exhibit the formation of words by such a coalescence and integration of roots as destroys the independent meanings of the roots. These Aryan languages, and others which have the amalgamate character, are united by it into a class distinguished from the aptotic and agglutinate languages ; in which the roots are either not united at all, or so incompletely united that one of them still retains its independent meaning. And philologists find that these fundamental differences which severally determine the grammatical forms, or modes of com- bining ideas, are really characteristic of the primary divisions among languages.

That is to say, among languages, where we know that evolution has been going on, the greatest groups are marked off from one another by the strongest structural contrasts ; and as the like holds among grouTis of organisms, there re-

THE ARGUMENTS FROM CLASSIFICATION. 361

e lilts a further reason for inferring that these have been evolved.

§ 125. There is yet another parallelism of like meaning. We saw (§ 101) that the successively-subordinate classes, orders, genera, and species, into which zoologists and botan- ists segregate animals and plants, have not, in reality, those definite values conventionally given to them. There are \vell-marked species, and species so imperfectly defined that certain systcmatists regard them as varieties. Between genera, strong contrasts exist in many cases ; and in other cases, contrasts so much less decided as to leave it doubtiul whether they constitute generic distinctions. So, too, is it with orders and classes : in some of which there have been introduced intermediate sub-divisions, having no equivalents in others. Even of the sub-kingdoms the same truth holds. The contrast between the Molluscoida and the Mollusca, is far less than that between the M'ollusca and the Annulosa ; and there are naturalists who think that the Vertebrata are so much more widely separated from the other sub-kingdoms, than these are from one another, that the Vertebrata should have a classificatory value equal to that of all the other sub- kingdoms taken together.

Now just this same indefiniteness of value, or incomplete- ness of equivalence, is observable in those simple and com- pound and re-compound groups, which we see arising by evolution. In every case, the endeavour to arrange the divergent products of evolution, is met by a difficulty like that which would meet the endeavour to classify the branches of a tree, into branches of the first, second, third, fourth, &c., orders — the difficulty, namely, that branches of intermediate degrees of composition exist. The illustration furnished by languages will serve us once more. Some dia- lects of English are but little contrasted ; others are strongly contrasted. The alliances of the several Scandinavian tongues with one another are different in degree. Dutch is much

362 THE EVOLUTION OF LIFE

less distinct from. German than Swedish is ; while the Danish and Swedish there is so close a kinship, that they might almost be regarded as widely- diver gent dialects. Similarly on comparing the larger divisions, we see that the various languages of the Aryan stock, have deviated from the original to very unlike distances. The geneial conclusion is manifest. While the kinds of human speech fall into groups, and sub-groups, and sub-sub-groups ; yet the groups are not equal to one another in value, nor have the sub-groups equal values, nor the sub-sub-groups.

If, then, the classification of organisms results in several orders of assemblages, such that assemblages of the same order are but indefinitely equivalent ; and if, where evolution is known to have taken place, there have arisen assemblages between which the equivalence is similarly in- definite ; there is additional reason for inferring that organisms are products of evolution.

§ 126. A fact of much significance remains. If groups of organic forms have arisen by divergence and re-diver- gence ; and if, while the groups have been developing from simple groups into compound groups, each group and sub-group has been giving origin to more complex forms of its own type ; then it is inferable that there once ex- isted greater structural likenesses between the members of allied groups, than exist now. Hence, if we take the simplest members of any group to be those which have undergone the least change ; we may expect to find a greater likeness between them and the simplest members of an allied group, than we find between the more complex members of the two groups. This, speaking generally, proves to be so.

Between the sub-kingdoms, the gaps are extremely wide ; but such distant kinships as may be discerned, bear out an- ticipation. Speaking of that extremely- degraded vertebrate animal the Amphioxus, which has several molluscous traits

THE ARGUMENTS FROM CLASSIFICATION. 'J)(>''

in its organization, Dr Carpenter remarks, that it " furnishes an apt illustration of another important fact, that it is by the lowest rather than by the highest forms of two natural groups, that they are brought into closest relation." What are the faint traces of community between the Annulosa and the Mollusca ? They are the thread-cells which some of their inferior groups have in common with the Coelentcrata. More decided approximations exist between the lower members of classes. In tracing down the Crustacea and the Arachnida from their more complex to their simpler forms, zoologists meet with difficulties : respecting some of these simpler forms, it becomes a question which class they belong to. The Lepidosiren, about which there have been disputes whether it is a fish or an amphibian, is inferior in the organization of its skeleton, to the great majority of both fishes and amphibia. Widely as they differ from them, the lower mammals have some characters in common with birds, which the higher mammals do not possess.

Now since this kind of relationship of groups is not ac- counted for by any other hypothesis, while the hypothesis of evolution gives us a clue to it ; we must include it among the evidences of this hypothesis, which the facts of classification furnish.

§ 127. What shall we say of these several leading truths ttrhen taken together ? That naturalists have been gradually compelled to arrange organisms in groups within groups ; and that this is the arrangement which we see arises by descent, alike in individual families and among races of men, is a striking circumstance. That while the smallest groups are the most nearly related, there exist beween the great sub-kingdoms, structural contrasts of the profoundest kind ; cannot but impress us as remarkable, when we see that where it is known to take place, evolution actually produces these feebly-distinguished small groups, and these strongly-dis- tinguished great groups. The impression made by these two

364 THE EVOLUTION OF LIFE.

parallelisms, which, add meaning to each other, is deepened by the third parallelism, which enforces the meaning of both — the parallelism, namely, that as, between the species, genera, orders, classes, &c., which naturalists have formed, there are transitional gradations ; so between the groups, sub-groups, and sub- sub- groups, which we know to have been evolved, groups of intermediate values exist. And these three correspondences between the known results of evolution, and the results here ascribed to evolution, have further weight given to them by the circumstance, that the kinship of groups through their lowest members, is just the kinship which the hypothesis of evolution implies.

Even in the absence of these specific agreements, the broad fact of unity amid multiformity, which organisms so strik- ingly display, is strongly suggestive of evolution. Freeing ourselves from pre-conceptions, we shall see good reason to think with Mr Darwin, " that propinquity of descent — the only known cause of the similarity of organic beings — is the bond, hidden as it is by various degrees of modification, which is partly revealed to us by our classifications." When we consider that this only known cause of similarity, joined with the only known cause of divergence, which we have in the influence of conditions, gives us a key to these likenesses obscured by unliken esses, to which no consistent interpreta- tion can otherwise be given, even if purely hypothetical causes be admitted; we shall see that were there none of those very remarkable harmonies above pointed out, the truths of classification would still yield strong support to our conclusion.

CHAPTER V.

THE ARGUMENTS FROM EMBRYOLOGY.

$ 123. THERE was briefly set forth in § 52, a remarkable induction established by Yon Baer ; who " found that in its earliest stage, every organism has 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 displayed at corresponding phases by a less extensive multitude of organisms ; that at each subse- quent 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 class 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." Though this generalization is to be taken with qualifications, yet, as an average truth, it may be regarded as beyond question ; and as an average truth, it has a profound significance.

For if we follow out in thought the implications of this truth — if we conceive the germs of all kinds of organisms simultaneously developing ; if after taking their first step together, we imagine at the second step, one half of the vast multitude diverging from the other half; if, at the next step, we mentally watch each of these great assemblages beginning to take two or more routes of development ; if we represent to ourselves this bifurcation simultaneously going on, stage after stage, in all the

366 THE EVOLUTION OF LIFE

N.

branches ; we shall see that there must result an aggregate analogous, in its arrangement of parts, to a tree. If this vast genealogical tree be contemplated as a whole, made up of trunk, great branches, secondary branches, and so on, as far as the terminal twigs ; it will be perceived that all the various kinds of organisms represented by these terminal twigs, forming the periphery of the tree, will stand related to each other in small groups, which are united into groups of groups, and so on. The embryological tree, expressing the developmental relations of organisms, will be similar to the tree which symbolizes their classificatory relations. That subordination of classes, orders, genera, and species, to which naturalists have been gradually led, is just that subordination which results from the divergence and re-divergence of embryos, as they all unfold. On the hypothesis of evolution, this parallelism has a meaning — indicates that primordial kinship of all organisms, and that progressive differentiation of them, which the hypothesis alleges. But on any other hypothesis the parallelism is meaningless : or rather, it raises a difficulty ; since it implies either an effect without a cause, or a design without a purpose.

§ 129. It was said above, that this great embryological law is to be taken with certain qualifications. The resem- blances which hold together great groups of embryos in their early stages, and which hold together smaller and smaller groups in their later and later stages, are not special or exact, but general or approximate ; and in some cases, the conformity to this general law is very imperfect. These irregularities, however, instead of being at variance with the hypothesis of evolution, afford further support to it.

Observe, first, that the only two other possible suppositions respecting developmental changes, are negatived, the one by this general law and the other by the minor nonconformities to it. If it be said that the conditions of the case necessi- tated the derivation of all organisms from simple germs, and

TilE ARGUMENTS FROM EMBRYOLOGY. 367

therefore necessitated a morphological unity in their primitive states ; there arises the obvious answer, that the morphologi- cal unity thus implied, is not the only morphological unity to be accounted for. "Were this the only unity, the various kinds of organisms, setting out from a common primordial form, should all begin from the first to diverge individually, as so many radii from a centre ; which they do not. If, other- wise, it be said that organisms were framed upon certain types, and that those of the same type continue developing together in the same direction, until it is time for them to begin putting on their specialities of structure ; then, the answer is, that when they do finally diverge, they ought severally to develop in direct lines towards their final forms. No reason can be assigned why, having once parted company, some should progress towards their final forms by irregular or circuitous routes. On the hypothesis of design, such de- viations are inexplicable.

The hypothesis of evolution, however, while it pre- supposes those general relations among embryos which are found to exist, also affords explanations of these minor nonconformities. If, as any rational theory of evolution pre- supposes, the pro- gressive differentiations of organic forms from one another during past times, have resulted, as they are resulting still, from the direct and indirect effects of external conditions — if organisms have become different, either by immediate adaptations to unlike habits of life, or by the mediate adapta- tions resulting from preservation of the individuals most fitted for such habits of life, or by both ; and if the embryonic changes are related to the changes that were undergone by ancestral races ; then these irregularities must be expected. For the successive changes in modes of life pursued by successive ancestral races, can have had no regularity of sequence. In some cases they must have been more numerous than in others ; in some cases they must have been greater in degree than in others ; in some cases they must have been to lower modes, in some cases to higher modes, and in some

368 THE EVOLUTION OF LIFE.

cases to modes neither higher nor lower. Of two connate races which diverged in the remote past, the one may have had descendants that have remained tolerably constant in theii habits, while the other may have had descendants that have passed through widely- aberrant modes of life ; and yet some of these last may have eventually taken to modes of life like those of the divergent races derived from the same stock. And if the metamorphoses of embryos, indicate, in a general way, the changes of structure undergone by ancestors ; then, the later embryologic changes of such two allied races, will be somewhat different, though they may end in very similar forms. An illustration will make this clear. Mr Darwin says : — " Petrels are the most aerial and oceanic of birds, but in the quiet sounds of Tierra del Fuego, the Puffinuria berardi, in its general habits, in its astonishing power of diving, its manner of swimming, and of flying when un- willingly it takes flight, would be mistaken by any one for an auk or grebe ; nevertheless, it is essentially a petrel, but with many parts of its organization profoundly modified." Now if we suppose these grebe-like habits to be continued through a long epoch, the petrel-form, to be still more ob- scured, and the approximation to the grebe- form still closer ; it is manifest that while the chicks of the grebe and the Puffinuria will, during their early stages of development, display that likeness involved by their common derivation from some early type of bird, the chick of the Puffinuria will eventually begin to show deviations, representative of the ancestral petrel-structure, and will afterwards begin to lose these distinctions, and assume the grebe- structure.

Hence, remembering the perpetual intrusions of organisms on one another's modes of life, often widely different ; and remembering that these intrusions have been going on from the beginning ; we shall be prepared to find that the general law of embryologic parallelism, is qualified by irregularities that are mostly small, in many cases considerable, and

TILE ARGUMENTS FROM EMBRYOLOGY.

occasionally great. The hypothesis of evolution accounts for these : it does more — it implies the necessity of them.

§ 130. The substitutions of organs and the suppressions of organs, are among those secondary embryological phe- nomena which harmonize with the belief in evolution but cannot be reconciled with any other belief. There are cases where, during its earlier stages of development, an embryo possesses organs that afterwards dwindle away, as there arise other organs to discharge the same functions. And there are cases where organs make their appearance, grow to certain points, have no functions to discharge, and disappear by absorption.

We have a remarkable instance of this substitution in the successive temporary appliances for aerating the blood, which the mammalian embryo exhibits. During the first phase of its development, the mammalian embryo circulates its blood through a system of vessels distributed over what is called the area vasculosa — a system of vessels homologous with one which, among fishes, serves for aerating the blood until the permanent respiratory organs come into play. After a time, there buds out from the mammalian embryo, a

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vascular membrane called the allantois, homologous with one which, in birds and reptiles, replaces the first as a breathing apparatus. But while in the higher oviparous vertebrates, the allantois serves the purpose of a lung during the rest of embryonic life, it does not do so in the mamma- lian embryo. In iniplacental mammals, it aborts, having no function to discharge ; and in the higher mammals, it becomes " placentiferous, and serves as the means of inter- communication between the parent and the offspring " — be- comes an organ of nutrition more than of respiration. Now since the first system of external blood-vessels, not beinsr in

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contact with a directly-oxygenated medium, cannot be very serviceable to the mammalian embryo as a lung ; and sines

370 THE EVOLUTION OF LIFE.

the second system of external blood-vessels is, to the iin- placental embryo, of no greater avail than the first ; and since the communication between the embryo and tha placenta among placental mammals, might as well or better have been made directly, instead of by metamorphosis of the allantois ; these substitutions appear unaccountable aa results of design. But they are quite congruous with tho supposition, that the mammalian type arose out of lower vertebrate types. For in such case, the mammalian embryo, passing through states representing, more or less distinctly, those which its remote ancestors had in common with the lower Vertebrata, develops these subsidiary organs in like ways with the lower Vertebrata.

Even more striking than the substitutions of organs are the suppressions of organs. Mr Darwin names some cases as " extremely curious ; for instance, the presence of teeth in foetal whales, which when grown up have not a tooth in their heads ; * * * It has even been stated on good authority that rudiments of teeth can be detected in the beaks of certain embryonic birds." Not even temporary functions can be assigned for these organs that are first built up and then pulled down again. They are absolutely useless — their formation is absolutely superfluous. Irrecon- cilable with any teleological theory, they do not even har- monize with the theory of fixed types which are maintained by the development of all the typical parts, even where not wanted ; seeing that the disappearance of these incipient organs during foetal life, spoils the typical resemblance. But while to all other hypotheses these facts are stumbling- blocks, they yield strong support to the hypothesis of evolu- tion.

Allied to these cases, are the cases of what has been called retrograde development. Many parasitic creatures and creatures which, after leading active lives for a time, eventu- ally become fixed, lose, in their adult states, the limbs and senses which they had when young. It may be alleged.

rJIIE ARGUMENTS FROM EMBRYOLOGY. 371

however, that these creatures could not secure the habitats needful for them, without possessing during their larval stages, eyes and swimming appendages which eventually become useless , that though, by losing these, their organiza- tion retrogresses in one direction, it progresses in another direction ; and that, therefore, they do not exhibit the need- less development of a higher type on the way to a lower type. Nevertheless there are instances of a descent in organization, following an apparently-superfluous ascent. Mr Darwin says that in some genera of cirripedes, "the larvce become developed either into hermaphrodites having the ordinary structure, or into what I have called comple- mental males, and in the latter, the development has assuredly been retrograde ; for the male is a mere sack, which lives for a short time, and is destitute of mouth, stomach, or other organ of importance, excepting for reproduc- tion."

§ 131. Comparative embryology shows us that besides substitutions of organs, there are what may be called substi - tuted modes of development. The same kind of structure is not always produced in the same way; and some allied groups of organisms have modes of evolution which appear to be radically contrasted. The two modes are broadly dis- tinguishable as the direct and the indirect. They may severally characterize the general course of evolution as »i whole, and the course of evolution in particular organs.

Thus in the immense majority of articulate animals, metamorphoses, more or less marked and more or less numerous, are passed through on the way to maturity. The familiar transformations of insects show us how circuitous is the route by which the embryo-form arrives at the adult form, among some divisions of the ArticuJata. But there are other divisions, as the lower Arachnida, in which the unfold- ing of the egg into the adult takes place in the simplest manner : the substance grows towards its appointed shape

372 THE EVOLUTION OF 1.1 FK.

by the shortest route. Tlie Mollusca furnish contrasts whick> though less marked, are essentially of the same nature. Among some Gasteropods, according to Vogt, the germ-mass, after undergoing its earliest changes in the same way as germ- masses in general, begins to transform itself bodily into the finished structure : in one part, the component cells coalesce to form the heart, in another part to form the liver, and so on. But in other classes of molluscs, as the Cephalopods, the e7iibryo is moulded out of the blastoderm, or superficial layer of the germ-mass ; and the various organs, mostly aris- ing out of this blastoderm by a process of budding, reach their ultimate shapes through successive modifications, while they grow at the expense of the nutriment absorbed from the rest of the germ- mass. And this indirect development is universal among the Vertebrata.

JSTow on contemplating in their ensemble, the facts thus briefly indicated, we may trace among these irregularities something like a general rule. The indirect development characterizes the most-highly-organized forms. In the sub-kingdom Vertebrata, which, considered as a whole, stands far above the rest in complexity, the development is uniformly indirect. It is indirect in the great mass of the Articulate. It is indirect in the highest Mollusca. Conversely, it is direct in a large proportion of the lower types. The eggs of Protozoa, of Coelenterata, x>f inferior Annuloida, originate the respective structures proper to them, by transformations that are almost immediate ; each of the cycle of forms passed through, is assumed, when the proper time comes, in the simplest way ; and where they multiply by budding, the substance of the bud passes by as short a process as may be, into the finished form. Where among the simpler types of animals, the evolution is indirect, its indirectness generally appears to be related to some transitional mode of life, which the larva passes through on its way to maturity ; and where we find direct evolution among the more complex types, it is

THE ARGUMENTS FROM EMBRYOLOGY. o73

LU their most degraded members : instance the Acari among:

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the Articulate*

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We have before found that the facts of social organization

o

furnish us with hints towards interpreting the phenomena exhibited in individual organisms. Let us see whether analogies hence derived, do not help us here. A factory, or other producing establishment, or a town made up of such establishments, is an agency for elaborating some commodity consumed by society at large ; and may be regarded as analogous to a gland or viscus in an individual organism. If, now, we inquire what is the primitive mode in which one of these producing establishments grows up, we find it to be this. A single worker, who himself sells the produce of his labour, is the germ. His business increasing, he employs helpers — his sons or others ; and having done this, he be- comes a vendor not only of his own handiwork, but of that of others. A further increase of his business compels him to multiply his assistants, and his sale grows so rapid that he is obliged to confine himself to the process of selling ; that is, he ceases to be a producer, and becomes simply a channel through which the produce of others is conveyed to the public. Should his prosperity rise yet higher, he finds that he is unable to manage even the sale of his commodities, and has to employ others, probably of his own family, to aid him in selling ; that is, to him as a main channel are now added subordinate channels ; and so on continuously. Moreover,

* It may be urged that the mode of development is obviously related to the size of the mass which is to be transformed into the embryo. Doubtless it is true that direct transformation is characteristic of small ova, and indirect trans- formation of large ova ; and some such connexion may be necessary. Very pos- sibly that polarity of the physiological units, which determines the specific structure, will not act throughout a large mass in such way as to transform it bodily into the specific structure ; though it will thus act throughout a small mass. But that the bulk of the ovum is not the sole cause of this difference of method, is proved by the fact that in some cases where the development is comparatively direct, as in Acteon, the ovum is very much larger than in cases where it is comparatively indirect, as in minute insects.

374 THE EVOLUTION OF LIFE.

when there grow up in one place, as a Manchester or a Birmingham, many establishments of like kind, this process is carried still further. There arise factors and agents, who are the channels through which are transmitted the pro- duce of many mills ; and we believe that primarily, these factors were manufacturers who undertook to dispose of the produce of smaller houses as well as their own, and ultimately became salesmen only. Now this, which is the original mode in which social agencies of all kinds are evolved, does not continue to be the mode. There is a tendency everywhere manifested to substitute a direct process for this indirect process. Manufacturing establishments are no longer commonly developed through the series of modifica- tions above described ; but mostly arise by the immediate transformation of a number of persons into master, clerks, foremen, workers, &c. Instead of business-partnerships being formed, as they originally were, by some slow unob- trusive union between traders and their sons or assistants • we now have joint-stock-companies resulting by sudden metamorphoses of groups of citizens. The like is true with larger and more complex social agencies. A new town in the United States arises not at all after the old method of gradual accumulations round a nucleus, and successive small modifications of structure accompanying increase of size ; but it grows up over a large area, according to a pre-deter- mined plan ; and there are developed at the outset, those various civil, ecclesiastical, and industrial centres, which the incipient city will require. Even in the formation of colonies we may similarly see, that the whole type of social organization proper to the race from which the colony comes, begins at once to show itself. There is not a gradual passing through all those developmental phases passed through by the mother- society ; but there is a comparatively direct transformation of the assemblage of colonists, into a social organism allied in structure to the social organism of which was an offset.

THE ARGUMENTS FROM EMI3RYOLOG Y. 375

Let us now return to the development of individual organisms ; carrying back this idea with us. On the hypothesis of evolution, all organs must have been originally formed after the indirect method, by the accumulation of modifications upon modifications ; and if the development of the embryo repeats the development of ancestral races, organs must be thus formed in the embryo. To a consider-

*>

able extent they are thus formed. There is a striking parallelism between the mode in which, as above described, manufacturing agencies are originally evolved, and the mode in which secreting organs are evolved. Out of the group of bile-cells forming the germ of the liver, some oentrally-placed ones, lying next to the intestine, are trans- formed into ducts through which the secretion of the peri- pheral bile-cells is poured into the intestine ; and as the peri- pheral bile-cells multiply, there similarly arise secondary ducts emptying themselves into the main ones ; tertiary ones into these ; and so on. But while in this and in other organs, the development remains in a great degree indiiect ; there are organs, as the heart, in which it is comparatively direct. The heart of the vertebrate embryo does not arise from a bud ; but it is first traceable as an aggregated mass of cells, becoming distinct from the cells amid which it is imbedded : its transformation into a contractile chamber, is effected by the consolidation of its outer cells while its inner cells liquify. And the comparatively direct development thus displayed in some organs of the higher embryos, is, as we have seen, characteristic of the entire development in many lower embryos.

On the hypothesis of evolution, the direct mode of de- velopment in animals, must have been substituted for the indirect mode ; as we see that it is substituted in societies. How comes it to have been substituted ? By studying the cause of the substitution in the social organism, we may perhaps get some insight into its cause in the individual or- ganism. The direct mode of forming social agencies

376 THE EVOLUTION OF LIFE.

replaces the indirect mode, when these social agencies have either been so long established, or have become so prevalent, or both, as to modify the people's habits and ideas. Groups of citizens unite into corporate bodies which quickly organ- ize, because the habit of forming such combinations has bo far modified the thoughts and feelings of citizens, that it becomes natural to them thus to arrange themselves. So too, is it with the men who form a colony. The rapid as- sumption by them of a social structure, as similar as circum- stances permit to the structure of the mother- society, is manifestlv due to the fact, that the organization of the

fc- *~-2

mother-society has moulded the emotions and beliefs of its members into conformity with itself; so that when some of its members are transferred to a colony, £hey arrange themselves directly into a structure of like type with that of the mother-society : they do not repeat all the stages through which the mother-society passed, because their natures have been too far modified to allow of their doing this. That action and reaction between a social

organism and its units, which we here see accounts for changes in modes of social development, must be paralleled by the action and reaction between an individual organism and its units. Various classes of phenomena compelled us to conclude, that each kind of organism is composed cf physiological units, having certain peculiarites which force them to arrange themselves into the form of the species to which they are peculiar. And in the chapters on Genesis, Heredity, and Variation, we saw reason to believe, that while the polarities of the physiological units determine the structure of the organism as a whole ; the organism as a whole, if its structure is changed by incident forces, reacts on the physiological units, and modifies them towards con- formity with its new structure. Now this action and reac- tion between an organic aggregate and its units, tending ever to bring the two into absolute harmony, must be con- tinually making the developmental processes more direct ;

THE ARGUMENTS FROM EMBRYOLOGY. 377

and will show its effects in all kinds of ways and degrees, according to the ancestral history of each species. Suppos- ing it were possible for a race of organisms to have con- tinued propagating itself through an indefinitely-long period without any change of conditions, necessitating change of structure ; there would be reached so complete a congruity between the organic aggregate and its physiologi- cal units, that the units would arrange themselves directly into a structure like that of the adult organism : the germ would put on the proper characters of the species, with little or no transposition of substance. But in the absence of any such constancy of conditions and structure, w^hat may we expect ? We may expect that where the conditions and structure have been most constant, the mode of develop- ment will be the most direct ; and that it will be the most indirect, where there have been the greatest and most numerous changes in the habits and structures of ancestral races of organisms. And we may also expect that develop- mental changes corresponding to early ancestral forms, will undergo an obliteration that is great in proportion to the fixity of organization that has been since maintained. The facts appear in harmony with this conclusion. We see a comparatively- direct development in those inferior types of animals, which show us, by their inferiority, that they have not, since the commencement of organic life, passed through, many sets of changes. And where we find direct de- velopment among higher types of animals, it characterizes the simpler rather than the more complex members of the types.

Between different parts in the same embryo, there are un- likenesses in the method of formation, which seem to have kindred meanings. The heart, of which the development is in great measure direct, is an organ that appears compara- tively early among the ascending grades of organic forms ; and having appeared, retains throughout the character of a hollow muscle. Conversely, the organs which develop with

IT

378 THE EVOLUTION OF LIFE.

great indirectness, are the organs of external relation , which, in the progress of organic forms, undergo various metamorphoses. Some light, too, is thus thrown on

certain irregularities in the order of development of organs. If we contemplate those continuous actions and reactions \vliich tend ever to establish a balance between an organic aggregate and its units ; we shall see that the effect which the units composing any organ, produce on the organism as a whole, will depend, partly on the permanence of such organ, and partly on its proportional mass. The influence of any force, is a product of its amount multiplied into the time during which it has acted. Hence, a larger part of the aggregate acting for a shorter time, will impress itself on the phy- siological units, as much as a smaller part acting for a longer time ; and may thus begin to show its influence in the developmental changes, as soon as, or even earlier than, a part that has existed for a greater period. Thus it becomes comprehensible why, in certain Eiitozoa which have im- mensely-developed generative systems, the rudiments of the generative systems are the first to become visible. And thus are also explicable, anomalies such as those pointed out by Prof. Agassiz — the appearance, in some cases, of traits characterizing the species, at an earlier period of development than traits characterizing the genus.

§ 132. So that while the enibryologic law enunciated by Yon Baer, is in harmony with the hypothesis of evolution, and is, indeed, a law which this hypothesis implies ; the minor nonconformities to the law, are also interpretable by this hypothesis. Parallelism between the courses of develop- ment in species that had a common ancestry, is liable to be variously modified in correspondence with the later ancestral forms passed through after divergence of such species. The substitution of a direct for an indirect process of formation, which we have reason to believe will show itself, both in the unfolding of the entire organism and in the unfolding of par-

THE ARGUMENTS FROM EMBRYOLOGY. 379

ticular organs, must obscure the embryologic history. And the parts influencing the whole in degrees varying with their masses, there results a further influence which, from the out- set, must begin to modify the metamorphoses of each kind of embryo ; and cause it to show incipient divergences from embryos which had ancestral histories the same as its own. Thus we find three different causes conspiring in endless ways and degrees, to produce deviations from the general law — causes which are manifestly capable of producing, under special conditions, changes in apparent contradiction to tliia law.

CHAPTER VI.

THE ARGUMENTS FROM MORPHOLOGY.

§ 133. LEAVING out of consideration the parallelism of development which characterizes organisms belonging to each group, that community of plan which exists among them when they are mature, is extremely remarkable and extremely suggestive. As before shown (§ 103), neither the supposition that these combinations of attributes which unite classes are fortuitous, nor the supposition that no other combinations were practicable, nor the supposition of adherence to pre- determined typical plans, suffices to explain the facts. An instance will best prepare the reader for seeing the true meaning of these fundamental likenesses.

Under the immensely- varied forms of insects, greatly elon- gated like the dragon-fly, or contracted in shape like the lady-bird, winged like the butterfly, or wingless like the flea, we find this character in common — there are primarily twenty segments. These segments may be distinctly marked, or they may be so fused as to make it difficult to find the divisions between them. This is not all. It has been shown that the same number of segments is possessed by all the Crustacea. The highly-consolidated crab, and the squilla with its long, loosely-jointed divisions, are composed of the same number of somites. Though, in the higher crustaceans, some of these successive, indurated rings, forming the exo- skeleton, are never more than partially marked off from each

THE ARGUMENTS FROM MORPHOLOGY. 381

other ; yet they are indentinable as homologous with, segments, which, in other crustaceans, are definitely divided. What, now, can be the meaning of this

community of structure among these hundreds of thousands of

•> v_x

species filling the air, burrowing in the earth, swimming in the water, creeping about among the sea-weed, and having such enormous differences of size, outline, and substance, as that no community would be suspected between them ? TVTiy under the down-covered body of the moth and under the hard wing-cases of the beetle, should there be discovered the same number of divisions as in the calcareous framework of the lobster ? It cannot be by chance that there exist just twenty segments in all these hundreds of thousands of species. There is no reason to think it was necessary, in the sense that no other number would have made a possible organism. And to say that it is the result of design — to say that the Cre- ator followed this pattern throughout, merely for the purpose of maintaining vhe pattern — is to assign a motive which, if avowed by a human being, we should call whimsical. Ko rational interpretation of this and hosts of like morphological truths, can be given except by the hypothesis of evolution ; and from the hypothesis of evolution they are corollaries. If organic forms have arisen from common stocks by per- petual divergences and redivergences — if they have continued to inherit, more or less clearly, the characters of ancestral races; then there will naturally result these communities of fimda'inental structure among extensive assemblages of crea- tures, that have severally become modified in countless ways

• •>

and degrees, in adaptation to their respective modes of life. To this let it be added, that while the belief

in an intentional adhesion to a pre- determined pattern throughout a whole group, is totally negatived by the occur- rence of occasional deviations from the pattern ; such devi- ations are reconcilable with the belief in evolution. As pointed out in the last chapter, there is reason to think that remote ancestral trails, will be obscured more or less according

382 THE EVOLUTION OF I-TFE.

as the superposed modifications of structure, have or have not been great or long maintained. Hence, though the occur- rence of articulate animals, such as spiders and mites, having fewer than twe7ity segments, is fatal to the supposition that twenty segments was decided on for the three groups of superior Articulata; it is not incongruous with the supposition, that some primitive race of articulate animals, bequeathed to these three groups this common typical character — a character which has nevertheless, in many cases, become greatly ob- scured, and in some of the most aberrant orders of these classes, quite lost.

§ 134. Besides these wide-embracing and often deeply- hidden homologies, which hold together different animals, there are the scarcely-less significant homologies between different organs of the same animal. These homologies., like the others, are obstacles to the supernatural interpreta- tions, and supports of the natural interpretation.

One of the most familiar and instructive instances is furnished by the vertebral column. Snakes, which move sinuously through and over plants and stones, obviously need a segmentation of the bony axis from end to end ; and inasmuch as flexibility is required throughout the whole length of the body, there is advantage in the comparative uniformity of this segmentation : the creature's movements would be impeded if, instead of a chain of vertebrae varying but little in their lengths, there existed in the middle of the series some long bony mass that would not bend. But in most of the higher Vertebrata, the mechanical actions and reac- tions demand that while some parts of the vertebral axis shall be flexible, other parts shall be inflexible. Inflexibility is especially requisite in that part of the vertebral column called the sacrum ; which, in mammals and birds, forms a fulcrum exposed to the greatest strains which the skeleton has to bear. Now in both mammals and birds, this rigid portion of the vertebral column is not made of one long

THE ARGUMENTS FROM MORPHOLOGY. 383

segment or vertebra, but of several segments fused to- gether. In man there are five of these confluent sacral vertebrae ; and in the ostrich tribe they number from seven- teen to twenty. Why is this ? Why, if the skeleton of each species was separately contrived, was this bony mass made bv soldering together a number of vertebrae like those

»/ <— f

forming the rest of the column, instead of being made out of one simple piece ? And why, if typical uniformity was to be maintained, does the number of sacral vertebras vary within the same order of birds ? Why, too, should the develop- ment of the sacrum be by the round-about process of first forming its separate constituent vertebrae, and then de- stroying their separateness ? In the embryo of a mammal or bird, the substance of the vertebral column is, at the out- set, continuous. The segments that are to become vertebrae, arise gradually in the midst of this originally-homogeneous axis. Equally in those parts of the spine which are to remain flexible, and in those parts which are to grow rigid, these segments are formed ; and that part of the spine which is to compose the sacrum, having passed out of its ori- ginal unity into disunity, by separating itself into segments, passes again into unity by the coalescence of these segments. To what end is this construction and re-construction ? If, originally, the spine in vertebrate animals consisted from head to tail of separate moveable segments, as it does still in fishes and some reptiles — if, in the evolution of the higher Vertebrata, certain of these moveable segments were ren- dered less moveable with respect to each other, by the mechanical conditions to which they are exposed, and at length became relatively immoveable ; it is comprehensible why the sacrum formed out of them, should continue ever after to show more or less clearly its originally-segmented structure. But on any other hypothesis, this segmented structure is inexplicable. "We see the same law in

comparing the wonderfully complex jaws and legs in crusta- ceans," savs Mr Darwin • referring to the well-known fact

384 THE EVOLUTION OF LIFE.

that those numerous lateral appendages which, in the lom-r crustaceans most of them serve as legs, and have like shapes, are, in the higher crustaceans, some of them represented by enormously-developed claws, and others by variously-modi- fied foot-jaws. " It is familiar to almost every one," he continues, " that in a flower the relative position of the sepals, petals, stamens, and pistils, as well as their intimate structure, are intelligible on the view that they consist of metamorphosed leaves arranged in a spire. In. monstrous plants we often get direct evidence of the possibility of one organ being transformed into another ; and we can actually see in embryonic crustaceans and in many other animals, and in flowers, that organs, which when mature become extremely different, are at an early stage of growth exactly alike." * * * " Why should one crustacean, which has an ex- tremely complex mouth formed of many parts, consequently always have fewer legs ; or conversely, those with many legs have simpler mouths ? Why should the sepals, petals, stamens, and pistils in any individual flower, though fitted for such widely-different purposes, be all constructed on the same pattern ? '

To these and countless similar questions, the theory of evolution furnishes the only rational answer. In the course of that change from homogeneity to heterogeneity of struc- ture, displayed in evolution under every form, it will neces- sarily happen that from organisms made up of numerous like parts, there will arise organisms made up of parts more and more unlike : which unlike parts will nevertheless con- tinue to bear traces of their primitive likeness.

§ 185. One more striking morphological fact, near akin to some of the facts dwelt on in the last chapter, must be here set down --the frequent occurrence, in adult animals and plants, of rudimentary and useless organs, which are homologous with organs that are developed and useful in allied animals and plants. In the last chapter we saw that

THE ARGUMENTS FROM MORPHOLOGY.

din-ing the development of embryos, there often arise organs which disappear on being replaced by other organs dis- charging the same functions in different ways ; and that in some cases, organs develop to certain points, and are then re-absorbed without performing any functions. But very generally, the partially-developed organs are retained throughout life.

The osteology of the higher Vertebrata, supplies abundant examples. Vertebral processes which, in one tribe, are fully formed and ossified from independent centres, are, in other tribes, mere tubercles not having independent centres of ossification. • TVTiile in the tail of this animal, the vertebrae are severally composed of centrum and appendages, in the tail of that animal, they are simple osseous masses without any appendages ; and in another animal, they have lost their individualities by coalescence with neighbouring vertebrae into a rudimentary tail. From the structures of the limbs, analogous facts are cited by comparative anatomists. The undeveloped state of certain metacarpal bones, characterizes whole groups of mammals. In one case we find the normal number of digits ; and, in another case, a smaller number with an atrophied digit to make out the complement. Here is a digit with its full number of phalanges ; and there a digit of which one phalange has been arrested in its growth. Still more remarkable aye the instances of entire limbs being rudi- mentary ; as in certain snakes, which have hind legs hidden beneath the integument. So, too, is it with the dermal ap- pendages. Some of the smooth-skinned amphibia have scales buried in the skin. The seal, which is a mammal considerably modified in adaptation to an aquatic life, and which uses its feet mainly as paddles, has toes that still bear external nails ; but the manatee, which is a much more transformed mam- mal, has nailless paddles, which, when the skin is re- jnovecl, are said, by Ilumboldt, to display rudimentary nails at the ends of the imbedded digits. Nearly all birds are

O •/

covered with developed feathers, severally composed of a shaft

THK EVOLUTION OF LIFE.

bearing fibres, each of which again bears a fringe of down. But in some birds, as in the ostrich, various stages of arrested development of the feathers may be traced ; beginning with the unusually-elaborated feathers of the tail, and ending with those about the beak, which are reduced to simple hairs. Nor is this the extreme case. In the Apteryas we see the whole of the feathers Deduced t< a hair-like form. Again, the hair which commonly covers the body in mammals, is compara- tively rudimentary over the greater part of the human body, and is in some parts reduced to mere down — down which nevertheless proves itself to be homologous with the hair of mammals in general, by occasionally developing into the original form. Numerous cases of aborted organs are given by Mr Darwin, of which a few may be here added. " No- thing can be plainer," he remarks, "than that wings are formed for flight, yet in how many insects do we see wings so reduced in size as to be utterly incapable of flight, and not rarely lying under wing-cases, firmly soldered together?' * * * " In plants with separated sexes, the male flowers often have a rudiment of a pistil ; and Kolreuter found that by crossing such male plants with an hermaphrodite species, the rudi- ment of the pistil in the hybrid offspring was much increased in size ; and this shows that the rudiment and the perfect pistil are essentially alike in nature." And then, to complete the proof that these undeveloped parts are marks of descent from races in which they were developed, there are not a few direct experiences of this relation. " We have plenty of cases of rudimentary organs in our domestic productions — as the stump of a tail in tailless breeds — the vestige of an ear in earless breeds — the re-appearance of minute dangling horns in hornless breeds of cattle."

Here, as before, the teleological doctrine fails utterly ; for these rudimentary organs are useless, and occasionally even detrimental. The doctrine of typical plans is equally out of court ; for while, in some members of a group, rudi- mentary organs completing the general type are traceable,

THE ARGUMENTS FROM MORPHOLOGY. 387

in otlier members of the same group, such, organs are unre- presented. There remains only the doctrine of evolution ; and to this, these rudimentary organs offer no difficulties. On the contrary, they are among its most striking evi- dences.

§ 136. The general truths of morphology thus coincide in tneir implications. Unity of type, maintained under extreme dissimilarities of form and mode of life, is explicable as re- sulting from descent with modification ; but is otherwise inexplicable. The likenesses disguised by unlikenesses, which the comparative anatomist discovers between various organs in the same organism, are worse than meaningless if it be supposed that organisms were severally framed as we now see them ; but they fit in quite harmoniously with the belief, that each kind of organism is a product of accumulated modifications upon modifications. And the presence in all kinds of animals and plants, of functionally-useless parts corresponding to parts that are functionally-useful in allied animals and plants, while it is totally incongruous with the belief in a construction of each organism by miraculous in- terposition, is just what we are led to expect by the belief that organisms have arisen by progression.

CHAPTER VII.

THE ARGUMENTS FROM DISTRIBUTION.

§ 137. Ix §§ 105 and 106, we contemplated the phenomena of distribution in Space. The general conclusions reached, in great part based on the evidence brought together by Mr Darwin, were that, " on the one hand, we have similarly-con ditioned, 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." "Whence it was inferred that " as like organisms are not uni- versally, or even generally, found in like habitats ; nor very unlike organisms, in very unlike habitats ; there is no manifest pre-deterniined adaptation of the organisms to the habitats." In other words, the facts of distribution in Space, do not conform to the hypothesis of design. At the same

time we saw 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 ;" and these generalizations appeared to be in harmony with the abund- antly-illustrated 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."

By way of showing still more clearly the effects of thia competition among races of organisms, let me here add some *ecently-published instances of the usurpations of areas, and

THE ARGUMENTS FROM DISTRIBUTION. 389

changes of distribution hence resulting. In the Natural His- tory Review for January, 1864, Dr Hooker quotes as follows from some ISTew Zealand naturalists : — "You would be surprised at the rapid spread of European and other foreign plants in this country. All along the sides of the main lines of road through the plains, a PoTygonum (aviculare), called ' Cow Grass/ grows most luxuriantly, the roots sometimes two feet in depth, and the plants spreading over an area from four to five feet in diameter. The dock (Rumex obtusifolius or I?. ens-pus] is to be found in every river bed, extending into the valleys of the mountain rivers, until these become mere tor-

4/

rents. The sow-thistle is spread all over the country, growing luxuriantly nearly up to 6000 feet. The water- cress increases in our still rivers to such an extent, as to threaten to choke them altogether : * * * I have measured stems twelve feet long and three-quarters of an inch in diameter. In some of the mountain districts, where the soil is loose, the white clover is completely displacing the native grasses, forming a close sward. * * * In fact, the young native vegetation appears to shrink from competition with these more vigorous in- truders." " The native (Maori) saying is, ' as the white man's rat has driven away the native rat, so the European fly drives away our own, and the clover kills our fern, so will the Maoris disappear before the white man himself/

Given this universal tendency of the superior to over- run the habitats of the inferior ; let us consider what, on the hypothesis of evolution, will be the effects on the geo- graphical relationships of species.

§ 138. A race of organisms cannot expand its sphere of existence, without subjecting itself to new external conditions. Those of its members which spread over adjacent areas, inevitably come in contact with circumstances partially different from their previous circumstances ; and such of them as adopt the habits of other organisms, necessarily experience re-actions more or less contrasted with the re-

390 THE EVOLUTION OF LIFE.

actions before experienced. Now if changes of organic structure are caused, directly or indirectly, by changes in the incidence of forces ; there must result unlikenesses of structure between the divisions of a race which colonizes new habitats. Hence, in the absence of obstacles to migra- tion, we may anticipate manifest kinships between the animals and plants of one area, and those of areas adjoining it. This inference corresponds with an induction before set down (§106). In addition to the illustrations of it already quoted from Mr Darwin, his pages furnish others. One is that species which inhabit islands are habitually allied to species which inhabit neighbouring main lands ; and an- other is that the faunas of clustered islands show marked similarities. " Thus the several islands of the Galapagos Archipelago are tenanted," says Mr Darwin, "in a quite marvellous manner, by very closely related species ; so that the inhabitants of each separate island, though mostly dis- tinct, are related in an incomparably closer degree to each other than to the inhabitants of any other part of the world." Mr Wallace has traced " variation as specially influenced by locality " among the Papilionidce inhabiting the East Indian Archipelago : showing how " the species and varieties of Celebes possess a striking character in the form of the anterior wings, different from that of the allied species and varieties of all the surrounding islands ; " and how " tailed species in India and the western islands lose their tails as they spread eastward through the archipelago." During his travels on the Upper Amazons, Mr Bates found that " the greater part of the species of Ithomice changed from one locality to another, not further removed than 100 to 200 miles ; " that " many of these local species have the appear- ance of being geographical varieties ; ' and that in some species " most of the local varieties are connected with their parent form by individuals exhibiting all the shades of variation."

Further general relationships are to be inferred. If

THE ARGUMENTS FROM DISTRIBUTION. 391

races of organisms, ever being thrust by pressure of popula- tion into new habitats, undergo modifications of structure as they diverge more and more widely in space, it follows that, speaking generally, the widest divergences in Space will indicate the longest periods during which the descendants from a common stock have been subject to modifying con- ditions ; and hence that, among organisms of the same group, the smaller constrasts of structure will be limited to the smaller areas. This we find : " varieties being/ as Dr Hooker says in his Flora of Tasmania, "more re- stricted in locality than species, and these again than genera/' Again, if races of organisms spread, and

as they spread are altered by changing incident forces ; it follows that where the incident forces vary greatly within given areas, the alterations will be more numerous than in equal areas which are less-variously conditioned. This, too, proves to be the fact. Dr Hooker points out that the most uniform regions have the fewest species ; while in the most multiform regions the species are the most numerous.

§ 139. Let us consider next, how the hypothesis of evolution corresponds with the facts of distribution, not over different areas, but through different media. If all forms of organisms have descended from some primordial simplest form, it follows that, since this primordial simplest form must have inhabited some one medium out of the several media which organisms now inhabit, the peopling of other media by its descendants, implies migration from one medium to others — implies adaptations to media quite unlike the original medium. To speak specifically — water being the medium in which the lowest living forms exist, it is implied that the earth and the air have been colonized from the water. Great difficulties appear to stand in the "way of this assumption. Ridiculing those who contend for the uni- serial development of organic forms, who have, indeed, laid themselves open to ridicule by their many untenable pro-

392 THE EVOLUTION OF LIFE.

positions, Yon Bacr writes — "A fish, swimming towarda the shore, desires to take a walk, but finds his fins useless. They diminish in breadth for want of use, and at the same time elongate. This goes on with children and grandchil- dren for a few millions of years, and at last who can be as- tonished that the fins become feet ? It is still more natural that the fish in the meadow, finding no water, should gape after air, thereby, in a like period of time developing lungs ; the only difficulty being that in the meanwhile, a few generations must manage without breathing at all." Though, as thus presented, the belief in a

transition looks laughable ; and though such derivation of terrestrial vertebrates by direct modification of the piscine type, is untenable ; yet we must not therefore conclude that no migrations of the kind alleged can have taken place. The adage that " truth is stranger than fiction," applies quite as much to Nature in general as to human life. Besides the fact that there are certain fish which actually do " take a walk " without any very obvious reason ; and besides the fact that sundry fish ramble about on land when impelled to do so by the dry ing- up of the waters inhabited by them ; there is the still more astounding fact, that one kind of fish climbs trees. Few things seem more obviously impossible, than that a water-breathing creature without efficient limbs* should ascend eight or ten feet up the trunk of a palm ; and vet the Anabas scandens does as much. To previous testi-

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monies on this point, Capt. Mitchell has recently added others. Such remarkable cases of temporary changes of media, will prepare us for conceiving how, under special con- ditions, permanent changes of media may have taken place ; and for considering how the doctrine of evolution is eluci dated by them.

Both marine organisms and fresh-water organisms, are many of them left from time to time partially or completely without water ; and the creatures which show the power to change their media temporarily or permanently, are in very

THE ARGUMENTS fc-ROM DISTRIBUTION.

many cases, of the kinds most liable to be thus deserted by their medium. Let us consider what the sea- shore shows us. Twice a-day the rise and the fall of the tide,

covers and uncovers countless plants and animals, fixed and moving ; and through the alternation of spring and neap tides, it results that the exposure of the organisms living low down on the beach, varies both in frequency and duration : while some of them are left dry only once a fortnight for a very short time, others a little higher up, are left dry during two or three hours at several ebb tides every fortnight. Then by small gradations we come to such as, living at the top of the beach, are bathed by salt-water only at long in- tervals ; and still higher to some which are but occasionally splashed in stormy weather. What, now, do we find among the organisms thus subject to various regular and irregular alternations of media ? Besides many plants and many fixed animals, we find numerous moving animals ; some of which are confined to the lower zones of this littoral region, but others of which wander over the whole of it. Omitting the humbler animal forms, it will suffice to observe that each of the two great sub-kingdoms, Mollusca and Articulata, supplies ex- amples of creatures having a wide excursiveness within this region. Yv"e have gasteropods which, when the tide is down, habitually creep snail-like over sand and sea- weed, even up as far as high- water mark. We have several kinds of crustaceans, of which the crab is the most conspicuous, running about on the wet beach, and sometimes rambling beyond the reach of the water. And then note the striking fact, that each of these forms thus habituated to changes of media, is allied to forms that are mainly or wholly terrestrial. On the West Coast of Ireland, marine gasteropods are found on the rocks three hun- dred feet above the sea, where they are only at long intervals wetted by the spray ; and though between gasteropods of this class and land-gasteropods the differences are considerable, yet the land-gasteropods are more closely allied to them than to any other Mollusca. Similarly, the two highest orders of

394 THE EVOLUTION OF LIFE.

crustaceans have their species which live occasionally, or almost entirely, out of the water : there is a kind of lobster in the Mauritius which climbs trees ; and there is the land- crab of the West Indies, which deserts the sea when it reaches maturity, and re- visits it only to spawn. Seeing, thus, how there are many kinds of marine creatures whose habitat habitually exposes them to changes of media ; how some of the higher kinds so circumstanced, show a considerable adapt- ation to both media ; and how these amphibious kinds are allied to kinds that are mainly or wholly terrestrial ; we shall see that the migrations from one medium to another, which evolution pre-supposes, are by no means impracticable. With such evidence before us, the assumption that the dis- tribution of the Vertebrata through media so different as air and water, may have been gradually effected in some analogous manner, would not be altogether unwarranted, even had we no clue to the process. We shall find, however, a tolerably distinct clue. Though rivers, and lakes, and pools,

have no sensible tidal variations, they have their rises and falls, regular and irregular, moderate and extreme. Especially in tropical climates, we see them annually full for a certain number of months, and then dwindling away and drying up. This drying up may reach various degrees, and last for various periods : it may go to the extent only of producing a liquid mud, or it may reduce the mud to a hardened, fissured solid ; it may last for a day or two or for months. That is to say, aquatic forms which are in one place annually subject to a slight want of water for a short time, are elsewhere subject to greater wants for longer times : we have gradations of transition, analogous to those which the tides furnish. Now it is well known that creatures inhabiting such waters, have, in various degrees, powers of meeting these contingences. The contained fish either bury themselves in the mud when the dry season comes, or ramble in search of other waters. This is proved by evidence from India, Guiana, Siam, Ceylon ; and some of these fish, as the Anabas scandens, are

THE ARGUMENTS FROM DISTRIBUTION. 395

known to survive for clays out of the water. But the facts of greatest significance are furnished by an allied class of Fertebrata, almost peculiar to habitats of this kind. The Amphibia are not, like fish, habitually found in waters that are never partially or wholly dried up ; but they nearly all inhabit waters which, at certain seasons, evaporate, in great measure or completely — waters in which most kinds of fish cannot exist. And what are the leading structural traits of these Amphibia? They have two respiratory systems — pubnonic and branchial — variously developed in different orders ; and they have two or four limbs, also variously de- veloped. Further the class Amphibia consists of two groups, in one of which this duality of the respiratory system is permanent, and the development of the limbs always incom- plete ; and in the other of which the branchiae disappear as the lungs and limbs become fully developed. The lowest group, the Perennibranchiata, have organs homologous with the air-bladders of fishes, transformed in various degrees into lungs, until " in the Siren, the pulmonic respiration is more extensive and important than the branchial ; " and to these creatures, having a habitat partially aerial and partially aquatic, there are at the same time supplied, in the shallow water covering soft mud, the mechanical conditions which render swimming difficult and rudimentary limbs useful. In the higher group, the Caducibranchiata,vrefw.(L still more suggestive transformations. Having at first a structure re- sembling that which is permanent in the perennibranchiate amphibian, the larva of the caducibranchiate amphibian, pursues for a time a similar life ; but eventually, the changes are carried further in the same direction : the respir- ation of air, originally supplementary to the respiration of water, predominates over it more and more, till it replaces it entirely ; and an additional pair of legs is produced. This having been done, the creature either becomes, like the Triton, one which quits the water only occasionally ; or, like the Frog, one which pursues a life mainly terrestrial, and returns

396 THE EVOLTUICXN OF LIFE.

to the water now and then. Finally, if we ask under what conditions this metamorphosis of a water-breather into an air-breather completes itself, the answer is — it completes it- self at the time when the shallow pools inhabited by the larvre, are being dried up by the summer's sun..*

See, then? how significant are the facts when thus brought together. There are particular habitats in which animals are subject to changes of media. In such habitats exist animals having, in various degrees, the power to live in both media, consequent on various phases of transitional organization. Near akin to these animals, there are some that, after passing their early lives in the water, acquire more completely the structures fitting them to live on land, to which they then migrate. Lastly, we have closely-allied creatures like the Surinam toad and the terrestrial salamander, which, though they belong by their structures to the class Amphibia, are not amphibious in their habits — creatures the larvae of which do not pass their early lives in the water, and yet go through these same metamorphoses ! Must we then think that the distribution of kindred organisms through different media, presents an insurmountable difficulty ? On the contrary, with facts like these before us, the evolution-hypothesis supplies possible interpretations of many phenomena that are else unaccountable. Realizing the way in wrhich such changes of media are in some cases gradually imposed by physical conditions, and in other cases voluntarily commenced and slowly increased in the search after /ood ; we shall begin to understand how, in the course of evolution, there have arisen

* While these pages are passing through the press, Dr Hooker has obliged me by pointing out, that " plants afford many excellent examples " of analogous transitions. He says that among true " water plants," there are found, in the same species, varieties which have some leaves submerged and some floating ; other varieties in which they arc all floating ; and other varieties in which they are all submerged. Further, that many plants characterized by floating leaves, and which have all their leaves floating when they grow in deeper water, are found with partly aerial leaves when they grow in shallower water ; and th.it elsewhere they occur in almost dry soil with all their leaves aerial

THE ARGUMENTS FROM DISTRIBUTION. 39?

those strange obscurations of one type by the externals of another type. When we see land-birds occasionally feeding by the water-side, and then learn that one of them, the water- ouzel, an " anomalous member of the strictly terrestrial

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thrush family, wholly subsists by diving — grasping the stones with its feet and using its wings under water " — we are en- abled to comprehend how, under pressure of population, aquatic habits may be acquired by creatures organized for aerial life ; and how there may eventually arise an ornithic type, in which the traits of the bird are very much disguised. Finding among mammals, some that in search of prey or shelter, have taken to the water in various degrees, we shall cease to be perplexed on discovering the mammalian structure hidden under a fish-like form, as it is in the Cetacea. Grant that there has ever been going on that re-distribution of organisms, which we see still resulting from their intrusions on one another's areas, media, and modes of life ; and we have an explanation of those multitudinous cases in which hoinologies of structure are complicated with analogies. And while it accounts for the occurrence in one medium of or- ganic types fundamentally organized for another medium, the doctrine of evolution accounts also for the accompany- ing unfitnesses. Either the seal has descended from some mammal which little by little became aquatic in its habits, in which case the structure of its hinl limbs has a mean- ing ; or else it was specially framed for its present habi- tat, in which case the structure of its hind limbs is incom- prehensible.

§ 140. The facts respecting distribution in Time, which have more than any others been cited both in proof and in disproof of evolution, are too fragmentary to be conclusive either way. Were the geological record complete, or did it, as both Uniformitarians and Progressionists have habitually assumed, give us traces of the earliest organic forms ; the evidence hence derived, for or against, would have had more

398 THE EVOLUTION OF LIFE.

weight than any other evidence. As it is, all we can do is to see whether such fragmentary evidence as remains, is con- gruous with the hypothesis.

Palaeontology has shown that there is a "general relation between lapse of time and divergence of organic forms ' § 107) ; and that " this divergence is comparatively slow and continuous, where there is continuity in the geological forma- tions, but is sudden and comparatively wide, wherever there occurs a great break in the succession of strata." Now this is obviously what we should expect. The hypothesis implies structural changes that are not sudden but gradual. Hence, where conformable strata indicate a continuous record, we may expect to find successions of forms only slightly different . from one another ; while we may rationally look for consider- able contrasts between the groups of forms fossilized in adjacent strata, where there is evidence of a great blank in the record. The permanent disappearances of species, of genera, and of orders, which we saw to be a fact tolerably- well established, is also a fact for which the belief in evolution prepares us. If later organic forms have in all cases descended from earlier organic forms, and have diverged during their descent, both from their prototypes and from one another ; then it obviously follows, that such of them as become extinct at any epoch, will never re-appear at a subsequent epoch ; since there can never again arise a concurrence and succession of conditions, such as those under which each particular type was evolved.

Though comparisons of ancient and modern organic forms, prove that many types have persisted through enormous periods of time, without undergoing great changes ; it waa shown that such comparisons do not disprove the occur- rence in organic forms, of changes great enough to produce what are called different types. The result of inductive in- quiry we saw to be, that while a few modern higher types yield signs of having been developed from ancient lower types ; and while there are many modern types which may

THE ARGUMENTS FKOM DISTRIBUTION.

have been tlius developed, though we are without evidence that they have been so ; yet that " any admissible hypothesis of progressive modification must be compatible with persistence without progression through indefinite periods." Now these results are quite congruous with the hypothesis of evolution. As rationally interpreted, evolution must in all cases be understood to result, directly or indirectly, from the incidence of forces. If there are no changes of conditions, entailing organic changes, organic changes are not to be expected. Only in organisms which fall under conditions, in conformity to which there arise additional modifications answering to additional needs, will there be that increased heterogeneity which characterizes higher forms. Hence, though the facts of palaeontology cannot be held to prove evolution, yet they are in harmony with it ; and some few of them yield it support.

§ 141. One general truth respecting distribution in Time, is, however, profoundly significant. If, instead of contem- plating the relations among past forms of life taken by them- selves, we contemplate the relations between them and the forms now existing ; we find a connexion which is in perfect harmony with the belief in evolution, but quite irreconcil- able with any other belief.

Note, first, how full of meaning is the close kinship that exists between the aggregate of organisms now living, and the aggregate of organisms which lived in the most recent geologic times. In the last-formed strata, nearly all the imbedded remains are those of species which still flourish. Strata a little older, contain a few fossils of species now ex- tinct ; though, usually, species greatly resembling extant ones- Of the remains found in strata of still earlier date, the ex- tinct species form a larger per centage ; and the differences be- tween them and the allied species now living, are more marked. That is to say, the gradual change of organic types in Time, which we before saw is indicated by the geological record, is

iOO THE EVOLUTION OF LIFE.

equally indicated by tlie relation between existing organic types and organic types of the epoch preceding our own. The evidence completely accords with the belief in a descent of present life from past life. Doubtless such a

kinship is not incongruous with the doctrine of special crea_ tions. It may be argued that the introduction, from time to time, of new species better fitted to the somewhat changed conditions of the Earth's surface, would result in an apparent alliance between our living Flora and Fauna, and the Floras and Faunas that lately lived. £To one can deny it. But on passing from the most general aspect of the alliance, to its more special aspects, we shall find this interpretation com- pletely negatived.

For besides a close kinship between the aggregate of sur- viving forms and the aggregate of forms that have died out in recent geologic times ; there is a peculiar connexion of like nature between present and past forms in each great geographical region. The instructive fact before cited from Mr Darwin, is the " wonderful relationship in the same con- tinent between the dead and the living." This relationship is not explained by the supposition that new species have been at intervals supernaturally placed in each habitat, as the habitat became modified ; since, as we saw, species are by no means uniformly found in the habitats to which they are best adapted. It cannot be said that the marsupials imbedded in recent Australian strata, having become extinct because of unfitness to some new external condition, the existing mar- supials were then specially created to fit the modified en vironment ; since sundry animals found elsewhere, are so much more completely in harmony with these new Australian conditions, that, when taken to Australia, they rapidly extrude the marsupials. While, therefore, the simi- larity between the existing Australian Fauna and the Fauna which immediately preceded it over the same area, is just that which the belief in evolution leads us to expect ; it Ls a similarity which cannot be otherwise accounted for.

THE ARGUMENTS FROM DISTRIBUTION. 401

And so is it with parallel relations in ~New Zealand, in South America, and in Europe.

§ 142. Given, then, that pressure which species exercise on one another, in consequence of the universal overfilling of their respective habitats — given the resulting tendency to thrust themselves into one another's areas, and media, and modes of life, along such lines of least resistance as from time to time are found — given besides the changes in modes of life, hence arising, those other changes which physical alterations of habitats necessitate — given the structural modifications directly or indirectly produced in organisms by modified conditions ; and the facts of distribution in Space and Time are accounted for. That divergence and re- divergence of organic forms, which we saw to be shadowed forth by the truths of classification and the truths of embry- ology, we see to be also shadowed forth by the truths of distribution. If that aptitude to multiply, to spread, to separate, and to differentiate, which the human races have in all times shown, be a tendency common to races in general, as we have ample reason to assume ; then there will result that kind of relation among the species, and genera, and orders, peopling the Earth's surface, which we find exists. Those remarkable identities of type discovered between or- ganisms inhabiting one medium, and strangely-modified or- ganisms inhabiting another medium, are at the same time rendered comprehensible. And the appearances and disap- pearances of species which the geological record shows us, as well as the connexions between successive groups of species from early eras down to our own, cease to be inexplicable.

18

CHAPTER VIII.

HOW IS ORGANIC EVOLUTION CAUSED?

§ 143. ALREADY it lias been necessary to speak of the causes of organic evolution in general terms ; and now we are prepared for considering them specifically. The task before us is to deduce the leading facts of organic evolution, from those same first principles which evolution at large conforms to.

Before attempting this, however, it will be instructive to glance at the causes of organic evolution that have been from time to time alleged.

§ ]44. The theory that plants and animals of all kinds were gradually evolved, seems to have been at first accom- panied only by the vaguest conception of cause — or rather, by no conception of cause properly so called, but only by the blank form of a conception. One of the earliest who in modern times (1735) contended that organisms are indefi- nitely modifiable, and that through their modifications they have become adapted to various modes of existence, was De Maillet. But though De Maillet supposed all living beings to have arisen by a natural, continuous process, he does not appear to have had any definite idea of that which determines this process. In 1794, in his Zoonomia,

Dr Darwin gave reasons (sundry of them valid ones) for believing that organized beings of every kind, have de-

HOW IS ORGANIC EVOLUTION CAUSED? 403

Bcended from one, or a few, primordial germs ; and along with some observable causes of modification, which he points out as aiding the developmental process, he apparently ascribes it, in part, to a tendency given to such germ or germs when created. He suggests the possibility " that £.11 warm-blooded animals have arisen from one living filament, which THE GREAT FIRST CAUSE endued with animality, with the power of acquiring new parts, attended with new pro- pensities, directed by irritations, sensations, volitions, and associations ; and thus possessing the faculty of continuing to improve by its own inherent activity." In this passage we see the idea to be, that evolution is pre- determined by some intrinsic proclivity. "It is curious," says

Mr Charles Darwin, " how largely my grandfather, Dr Erasmus Darwin, anticipated the erroneous grounds of opinion, and the views of Lamarck." One of the anticipa- tions was this ascription of development to some inherent tendency. To the " plan general de la nature, et sa marche uniforme dans ses operations," Lamarck attributes " la progression evidente qui existe dans la composition de F organisation des animaux ; ' and " la gradation reguliere qu'ils devroient offrir dans la composition de leur organ- isation," he thinks is rendered irregular by secondary causes. Essentially the same in kind, though some-

what different in form, was the conception put forth in the Vestiges of Creation ; the author of which contends " that the several series of animated beings, from the simplest and oldest up to the highest and most recent, are, under the pro- vidence of God, the results, first, of an- impulse which has been imparted to the forms of life, advancing them, in defi- nite times, by generation, through grades of organization terminating in the highest dicotyledons and vertebrata ; '' and that the progression resulting from these impulses, is modified by certain other causes. The broad general con- trasts between lower and higher forms of life, are regarded by him as due to an innate aptitude to give birth to forms

404 THE EVOLUTION OF LIFE.

of more perfect structures. The last to re-enun-

ciate this doctrine has been Prof. Owen ; who asserts " the axiom of the continuous operation of creative power, or of the ordained becoming of living things." Though these highly-general expressions do not suggest any very definite idea, yet they imply the belief that organic progress is a result of some in-dwelling tendency to develop, supernatur- ally impressed on living matter at the outset — some ever- acting constructive force, which, independently of other forces, moulds organisms into higher and higher forms.

In whatever way it is formulated, or by whatever language it is obscured, this ascription of organic evolution to some aptitude naturally possessed by organisms, or miraculously imposed on them, is unphilosophical. It is one of those ex- planations which explains nothing — a shaping of ignorance into the semblance of knowledge. The cause assigned is not a true cause — not a cause assimilable to known causes — not a cause that can be anywhere shown to produce analogous effects. It is a cause unrepresentable in thought : one of those illegitimate symbolic conceptions which cannot by any mental process be elaborated into a real conception. In brief, this assumption of a persistent formative power, in- herent in organisms, and making them unfold into higher forms, is an assumption no more tenable than the assump- tion of special creations : of which, indeed, it is but a modi- fication ; differing only by the fusion of separate unknown processes into a continuous unknown process.

§ 145. Along with this intrinsic tendency to progress, supposed to be primordially impressed on them, Dr Darwin held that animals have a capacity for being modified by pro- cesses which their own desires initiate. He speaks of powers as " excited into action by the necessities of the creatures which possess them, and on which their existence depends ; ' and more specifically he says that " from their first rudiment or primordiuin, to the termination of their

HOW IS ORGANIC EVOLUTION CAUSED? 405

lives, all animals undergo perpetual transformations ; which are in part produced by their own exertions, in consequence of their desires and aversions, of their pleasures and their pains, or of irritations, or of associations ; and many of these acquired forms or properties are transmitted to their pos- teritv." While it embodies a belief for which a great deal

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is to be said, this passage involves the assumption that, desires and aversions, existing before experiences of the ac- tions to which they are related, were the originators of the actions, and therefore of the structural modifications caused by them. In his Philosophic Zoologize, Lamarck

much more specifically asserts " le sentiment interienr," to be in all creatures that have developed nervous sys- tems, an independent cause of those changes of form which are due to the exercise of organs : distinguishing it from that simple irritability possessed by inferior animals, which cannot produce what we call a desire or emotion ; and holding that these last, along with all "qui manquent de systeme nerveux, ne vivent qu'a 1'aide des excitations qu'ils recoivent de 1'exterieur." Afterwards he says — "je reconnus que la nature, obligee d'abord d'emprunter des milieux environnans la puissance excitatriced.es mouvemens vitaux et des actions des animaux imparfaits, sut, en com- posant de plus en plus 1'organisation animale, transporter cette puissance dans Pinterieiir meme de ces etres, et qu'a la fin, elle parvint a mettre cette meme puissance a la disposition de 1'individu." And still more definitely he contends that if one considers " la progression qui se montre dans la com- position de 1'organisation," * * * " alors on eut pu aperce- voir comment les lesoins, d'abord reduits a nullite, et dont le n ombre ensuite s'est accru graduellement, ont amene 10 penchant aux actions propres a y satisfaire ; comment les actions devenues habituelles et energiques, ont occasionne* le developpement des organes qui les executent."

Now though this conception of Lamarck is more precisely stated, and worked out with much greater elaboration and

THE EVOLUTION OF LIFE.

wider knowledge of the facts, it is essentially the same as that of Dr Darwin ; and along with the truth it contains, contains also the same error more distinctly pronounced. Merely noting that desires or wants, acting directly only on the nervo-inuscular system, can have no immediate in- fluence on very many organs, as the viscera, or such external appendages as hair and feathers ; and observing, further, that even some parts which belong to the apparatus of external action, such as the bones of the skull, cannot be made to grow by increase of function called forth by desire ; it will suffice to point out that the difficulty is not solved, but simply slurred-over, when needs or wants are introduced as independent causes of evolution. True though it is, as Dr Darwin and Lamarck contend, that desires, by leading to increased actions of motor organs, may induce further de- velopments of such organs ; and true as it probably is, that the modifications hence arising, are transmissible to offspring ; yet there remains the unanswered question — Whence do these desires originate ? The transference of the exciting power from the exterior to the interior, as described by Lamarck, begs the question. How comes there a wish to perform an action not before performed ? Until some beneficial result has been felt from going through certain movements, what can suggest the execution of such movements ? Every desire consists primarily of a mental representation of that which is desired, and secondarily excites a mental representation of the actions by which it is attained ; and any such mental representations of the end and the means, imply antecedent experience of the end and antecedent use of the means. To assume that in the course of evolution there from time to time arose new kinds of actions dictated by new desires, is simply to remove the difficulty a step back.

§ 146. Changes of external conditions are named by Dr Darwin, as causes of modifications in organisms. Assigning, as evidence of original kinship, that marked similarity of

HOW IS ORGANIC EVOLUTION CAUSED? 407

type which exists among animals, he regards their devia- tions from one another, as caused by differences in their modes of life : such deviations being directly adaptive. Enumerating various appliances for procuring food, he says they all " seem to have been gradually produced during many generations by the perpetual endeavour of the creatures to supply the want of food, and to have been delivered to their posterity with constant improvement of them for the pur- poses required." And the creatures possessing these va- rious appliances, are considered as having been rendered unlike, by seeking for food in unlike ways. As illustrating the alterations wrought by changed circumstances, he names the acquired characters of domestic animals. La-

marck has elaborated the same view in detail : using for the purpose, with great ingenuity, his extensive knowledge of the animal kingdom. From a passage in the Avertissement, it would at first sight seem, that he looks upon direct adapt- ation to new conditions, as the chief cause of evolution. He says — "Je regardai comme certain que le mouvement des fliddes dans I'interieur des aiiimaux, mouvement qui c'est progressivement accelere avec la composition plus grande de 1' organisation ; et que V influence des circonstances nouvelles, a mesure que les animaux s'y exposerent en se repandant dans tons les lieux habitables, furent les deux causes ge*ne*- rales qui ont amene les differens animaux a 1'etat ou nous les voyons actuellement." But elsewhere, the view he expresses appears decidedly different from this. He asserts that "dans sa marche, la nature a commence, et recommence encore tous les jours, par former les corps organises les plus simples ; " and that " les premieres ebauches de F animal et du vegetal etaiit formees dans les lieux et les circonstances conveiiables, les facultes d'une vie commencante et d'un mouvement or- ganique etabli, ont necessairement developpe peu a peu les organes, et qu'avec le temps elles les ont diversifies ainsi que les parties." And then, further on, he puts in italics this proposition: — " La progression dans la composition de Vor-

108 THE EVOLUTION OF LIFE.

ganisation subit, pa et la, dans la serie generate iles animaux, des anomalies operees par V influence dcs cir con stances d' habi- tation, et par cclle des habitudes contractees." These, and sundry other passages, joined with his general scheme of classification, make it clear that Lamarck conceived adaptive modification to be, not the cause of progression, but the cause of irregularities in progression. The inherent tend- ency which organisms have, to develop into more perfect forms, would, according to him, result in a uniform series of forms ; but varieties in their conditions work divergences of structure, which break up the series into groups : groups which he nevertheless places in uni-serial order, and regards as still substantially composing an ascending succession.

§ 147. These speculations, crude as they may be considered, show much sagacity in their respective authors, and have done good service. Without embod}Ting the truth in a de- finite shape, they contain adumbrations of it. Not directly, but by successive approximations, do mankind reach correct conclusions ; and those who first think in the right direction, loose as may be their reasonings, and wide of the mark as their inferences may be, yield indispensable aid by framing provisional conceptions, and giving a bent to inquiry.

Contrasted with the dogmas of his age, the idea of De Maillet was a great advance. Before it can be ascertained how organized beings have been gradually evolved, there must be reached the conviction that they have been gradu- ally evolved ; and this conviction he reached. His wild notions as to the way in which natural agencies acted in the production of plants and animals, must not make us forget the merit of his intuition that animals and plants were pro- duced by natural causes. In Dr Darwin's brief ex- position, the belief in a progressive genesis of organisms^ is joined with an interpretation having considerable definite- ness and coherence. In the space of ten pages he not only indicates several of the leading- classes of facts which support

HOW IS ORGANIC EVOLUTION CAUSED 409

the hypothesis of evolution, but he does something towards elucidating the process of evolution. His reasonings show us an unconscious mingling of the belief in a supernaturally- impressed tendency to develop, with the belief in a develop- ment arising from the changing incidence of conditions. Probably had he pursued the inquiry further, this last belief would have grown at the expense of the first. La-

marck, in elaborating this general conception, has given greater precision to both its truth and its error. Asserting the same imaginary factors and the same real factors, he has traced out their supposed actions in detail ; and has, in con- sequence, committed himself to a greater number of un- tenable positions. But while, in trying to reconcile the facts with a theory which is only an adumbration of the truth, he laid himself open to the criticisms of his con- temporaries ; he proved himself profounder than his con- temporaries, by seeing that evolution, however caused, has been going on. If they were wise in not indorsing a theory which fails to account for a great part of the facts ; they were unwise in ignoring that degree of congruity with the facts, which shows the theory to contain some fundamental verity.

Leaving out, however, the imaginary factors of evolution which these speculations allege, and looking only at the one actual factor which Dr Darwin and Lamarck assign as accounting for some of the phenomena ; it is manifest from our present stand-point, that this, so far as it is a cause of evolution, is a proximate cause and not an ultimate cause. To say that functional adaptation to conditions, produces either evolution in general, or the irregularities of evolution, is to raise the further question — why is there a functional adaptation to conditions ? — why do use and disuse generate appropriate changes of structure ? Neither this nor any other interpretation of biologic evolution which rests simply on (he basis of biologic induction, is an ultimate interpretation. The biologic induction must itself be interpreted. Only when

410 THE EVOLUTION OF LIFE.

the process of evolution of organisms, is affiliated on the process of evolution in general, can it be truly said to be explained. The thing required is to show that its various results are corollaries from first principles. We have to reconcile the facts with the universal laws of the re-distribu- tion of matter and motion.

CHAPTER IX.

EXTERNAL FACTORS.

§ 148. "WHEN illustrating the rhythm of motion (First Principles, § 94) it was pointed out that besides the daily and annual alternations in the quantities of light and heat which any portion of the Earth's surface receives from the Sun, there are alternations which require immensely- greater periods to complete. Reference was made to the fact, that " every planet, during a certain long period, presents more of its northern than of its southern hemisphere to the Sun at the time of its nearest approach to him ; and then again, during a like period, presents more of its southern hemisphere than of its northern — a recurring co-incidence which, though causing in some planets no sensible alterations of climate, in- volves in the case of the Earth an epoch of 21,000 years, during which each hemisphere goes through a cycle of tem- perate seasons, and seasons that are extreme in their heat and cold." Further, it was pointed out that there is a varia- tion of this variation. The slow rhythm of temperate and in- temperate climates, which takes 21,000 years to complete, itself undergoes exaggeration and mitigation, during epochs that are far longer. The Earth's orbit slowly alters in form : now approximating to a circle ; and now becoming more eccentric. During the period at which the Earth's orbit has least eccentricity, the temperate and intemperate climates which repeat their cycle in 21.000 years, are

412 THE EVOLUTION OF LIFE.

severally less temperate and less intemperate, than when, some one or two millions of years later, the Earth's orbit haa reached its extreme of eccentricity.

Thus, besides those daily variations in the quantities of light and heat received by organisms, and responded to by varia- tions in their functions ; and besides the annual variations in the quantities of light and heat which organisms receive, and similarly respond to by variations in their functions ; there are variations that severally complete themselves in 21,000 years and in some millions of years — variations to which there must also be a response in the changed functions of organisms. The whole vegetal and animal kingdoms, are subject to a quadruply-compounded rhythm in the in- cidence of the forces on which life primarily depends — a rhythm so involved in its slow working round, that at no time during one of these vast epochs, can the in- cidence of these forces be exactly the same as at any other time. To the direct effects so produced on organ-

isms, have to be added much more important indirect effects. Changes of distribution must result. Certain redistributions are occasioned even by the annual variations in the quantities of the solar rays received by each part of the Earth's surface. The migrations of birds thus caused, are familiar. So too are the migrations of certain fishes : in some cases from one part of the sea to another ; and in some cases from salt water to fresh water. Now just as the yearly changes in the amounts of light and heat falling on each locality, yearly extend and restrict the habitats of many organisms that are able to move about with some rapidity ; so must these alternations of temperate and intemperate climates produce extensions and restrictions of habitats. These extensions and restric- tions, though slow, will be universal — will affect the habitats of stationary organisms as well as those of locomotive ones. For if during an astronomic era, there is going on at any limit to a plant's habitat, a diminution of the winter's cold or summer's heat, which had before stopped its spread ai

EXTERNAL FACTORS. 413

that limit ; then, though the individual plants are fixed, yet the species will move : the seeds of plants living at the limit, will produce individuals that survive beyond the limit. The gradual spread so effected, having gone on for some ten thousand years, the opposite change of climate will begin to cause retreat : the tide of each species will during the one half of a long epoch, slowly flow into new regions, and then will slowly ebb away from them. Further, this rise and fall in the tide of each species, will, during far longer intervals, undergo increasing rises and falls and then de- creasing rises and falls. There will be an alternation of spring tides and neap tides, answering in its period to the changing eccentricity of the Earth's orbit.

These astronomical rhythms, therefore, entail on organisms unceasing changes in the incidence of forces in two ways. They directly subject them to variations of solar influences, in such a manner that each generation is somewhat differently affected in its functions ; and they indirectly bring about complicated alterations in the environing agencies, by carry- ing each species into the presence of new physical conditions.

§ 149. The power of geological actions to modify every- where the circumstances in which plants and animals are placed, is conspicuous. In each locality, denudation slowly uncovers different deposits ; and slowly changes the exposed areas of deposits already uncovered. Simultaneously, the alluvial beds that are being formed, are qualitatively affected by these progressive changes in the natures and proportions of the strata denuded. The inclinations of surfaces and their directions with respect to the Sun, are at the same time altered ; and the organisms existing on them are thus having their thermal conditions continually altered, as well as their drainage. Igneous action, too, complicates these gradual modifications. A flat region cannot be step by step thrust up into a protuberance, without unlike climatic changes being produced in its several parts, by their exposures to dif-

$14 THE EVOLUTION OF LIFE.

fereiit aspects. Extrusions of trap, wherever they take place, revolutionize the localities ; both over the areas covered, and over the areas on which their detritus is left. And where volcanoes are formed, the ashes they occasionally send out, modify the character of the soil throughout large sur- rounding tracts.

In like manner alterations in the Earth's crust, cause the ocean to be ever subjecting the organisms it contains to new combinations of conditions. Here the water is being deep- ened by subsidence, and there shallowed by upheaval. While the falling upon it of sediment brought down by neighbour- ing large rivers, is raising the sea-bottom in one place ; in another, the habitual rush of the tide is carrying away the sediment previously deposited. The mineral character of the submerged surface on which sea- weeds grow and molluscs crawl, is everywhere occasionally changed : now by the bringing away from an adjacent shore some previously un- touched strata ; and now by the accumulation of organic remains, such as the shells of pteropods or of foraminifera. A further series of alterations in the circumstances of marine organisms, is entailed by changes in the movements of the water. Each modification in the outlines of neighbouring shores, makes the tidal streams vary their directions or velocities, or both. And the local temperature is from time to time raised or lowered, because some far-distant re-ar- rangement of the Earth's crust, has wrought a divergence in those circulating currents of warm and cold water which pervade the ocean.

These geologically-caused changes in the physical charac- ters of each environment, occur in ever-new combinations, and ^dth ever-increasing complexity. As already shown (First Principles, § 118), it follows from the law of the multiplication of effects, that during long periods, each tract of the Earth's Burface increases in heterogeneity of both form and substance. Hence plants and animals of all kinds, are, in the course of generations, subject by these alterations in the crust of the

EXTERNAL FACTORS. 415

Earth, to sets of incident forces which differ from previous sets, both by changes in the proportions of the factors, and, occasionally, by the addition of new factors.

§ 150. Variations in the astronomical conditions joined with variations in the geological conditions, bring about variations in the meteorological conditions. Those extremely slow alternations of elevation and subsidence, which there is reason to believe take place over immense areas, here pro- ducing a continent where once there was a fathomless ocean, and there causing wide seas to spread where in a long past epoch there stood snow-capped mountains, gradually work great atmospheric changes. While yet the highest parts of an emerging surface of the Earth's crust, exist as a cluster of islands, the plants and animals which in course of time migrate to them, have climates that are peculiar to small tracts of land surrounded by large tracts of water. As, by successive upheavals, greater areas are exposed, there begin to arise sensible contrasts between the states of their peripheral parts and their central parts : the sea and land breezes, which daily moderate the extremes of temperature near the shores, cease to affect the interiors ; and the interiors, less qualified too in their heat and cold by such ocean-currents as bathe the shores, acquire more decidedly the characters due to their latitudes. Along with the further elevations which unite the members of the archipelago into a continent, there come new meteorologic changes, as well as exacerbations of the old. The winds, which were comparatively uniform in their directions and periods when only islands existed, grow involved in their distribution, and widely-different in dif- ferent parts of the continent. The quantities of rain which they discharge and of moisture which they absorb, vary everywhere according to the proximity to the sea "and to surfaces of land having special characters.

Other complications result from variations of height above the sea : elevation producing a decrease of heat and conse-

416 THE EVOLUTION OF LIFE.

quently an increase in the precipitation of water — a precipit- ation that takes the shape of snow where the elevation is very great, and of rain where it is not so great. The gather- ing of clouds and descent of showers around mountain tops, are familiar to every tourist. Inquiries in the neighbouring valleys, prove that within distances of a mile or two the recurring storms differ in their frequency and violence. Nay, even a few yards off, the meteorologic conditions vary in such regions : as witness the way in which the condensing vapour keeps eddying round on one side of some high crag, while the other side is clear ; or the way in which the snow- line runs irregularly to many different heights, in all the minor valleys and ravines and hollows of each mountain side.

Climatic variations that are thus geologically produced, being compounded with those which result from the slow astronomical changes ; and no correspondence existing be- tween the geologic and the astronomic rhythms ; it results that the same plexus of actions never recurs. Hence the incident forces to which the organisms of every locality are exposed by atmospheric agencies, are ever passing into un- paralleled combinations ; and these are on the average ever becoming more complex.

§ 151. Besides changes in the incidence of inorganic forces, there are equally continuous, and still more involved, changes in the incidence of forces which organisms exercise on one another. As before pointed out (§ 105), the plants and animals inhabiting each locality, are held together in so entangled a web of relations, that any considerable modifica- tion which one species undergoes, acts indirectly on many other species ; and eventually changes, in some degree, the circumstances of nearly all the rest. If an increase of heat, or modification of soil, or decrease of humidity, causes a par- ticular kind of plant either to thrive or to dwindle ; an unfavourable or favourable effect is wrought on all such competing kinds of plants, as are not immediately influenced

EXTERNAL FACTORS. 417

in the same way. The animals which eat the seeds or browse on the leaves either of the plant primarily affected or those of its competitors, are severally altered in their states of nutri- tion and in their numbers ; and this change presently tells on various predatory animals and parasites. And since each of these secondary and tertiary changes, becomes itself 'a centre of others ; the increase or decrease of each species, produces waves of influence which spread and reverberate and re-reverberate, throughout the whole Flora and Fauna of the locality.

More marked and multiplied still, are the ultimate effects of those causes which make possible the colonization of neigh- bouring areas. Each intruding plant or animal, besides the new inorganic conditions to which it is subject, is subject to organic conditions considerably different from those to which it has been habituated. It has to compete with some organ- isms unlike those of its preceding habitat. It must preserve itself from enemies not before encountered. Or it may meet with a species over which it has some advantage greater than any that it had over the species it was previously in contact with. Even where migration does not bring it face to face with new competitors or new enemies or now prey, it inevitably experiences new proportions among these. Further, an expanding species is almost certain to invade more than one adjacent region. Spreading north or south, it will come among the plants and animals, here of a level district and there of a hilly one — here of an inland tract, and there of a tract bordered by the sea. And while differ- ent groups of its members will thus expose themselves to the actions and re-actions of different Floras and Faunas, these different Floras and Faunas will simultaneously have their organic conditions changed by the intruders.

This process becomes gradually more active and more complicated. Though in particular cases, a plant or animal may fall into simpler relations with the living things around, than those it was before placed in ; yet it is manifest that,

418 THE EVOLUTION OF LIFE.

on the average, the organic environments of organisms have been increasing in heterogeneity. As the number of species with which each species is directly or indirectly implicated, multiplies, each species is oftener subject to changes in the organic actions which influence it. These more frequent changes severally grow more involved. And the corre- sponding reactions affect larger Floras and Faunas, in ways increasingly complex and varied.

*

§ 152. When the astronomic, geologic, meteorologic, and organic agencies that are at work on each species of organ- ism, are contemplated as becoming severally more compli- cated in themselves, and at the same time as co-operating in ways that are always more or less new ; it will be seen that throughout all time, there has been an exposure of organisms to endless successions of modifying causes which gradually acquire an intricacy that is scarcely conceivable. Every kind of plant and animal may be regarded as for ever pass- ing into a new environment — as perpetually having its relations to external circumstances altered, either by their changes with respect to it when it remains stationary, or by its changes with respect to them when it migrates, or by both.

Yet a further cause of progressive alteration and compli- cation in the incident forces, exists. All other things con- tinuing the same, every additional faculty by which an organism is brought into relation with external objects, as well as every improvement in such faculty, becomes a means of subjecting the organism to a greater number and variety of external stimuli, and to new combinations of external stimuli. So that each advance in complexity of organization, itself becomes an added source of complexity in the incidence of external forces.

Once more, every increase in the locomotive powers of animals, increases both the multiplicity and the multiformity of the actions of things upon them, and of their reactions

EXTERNAL FACTORS. 419

upon tilings. Doubling a creature's activity, quadruples the area that comes within the range of its excursions : thus augmenting in number and heterogeneity, the external agencies which act on it during any given interval.

By compounding the actions of these several orders of factors, there is produced a geometric progression of changes, increasing with immense rapidity. And there goes on an equally rapid increase in the frequency with which the com- binations of the actions are altered, and the intricacies of their co-operations enhanced.

CHAPTER X.

INTERNAL FACTORS.

§ 153. WE saw at the outset (§§ 10 — 16), that organic matter is built up of molecules so extremely unstable, that the slightest variation in their conditions destroys their equilibrium ; and causes them either to assume altered structures or to decompose. But a substance which is beyond all others changeable by the actions and reactions of the forces liberated from instant to instant within its own mass, must be a substance that is beyond all others change- able by the forces acting on it from without. If their composition fits organic aggregates for undergoing with special facility and rapidity those re- distributions of matter and motion whence result individual organization and life : then their composition must make them similarly apt to undergo those permanent re- distributions of matter and mo- tion which are expressed by changes of structure, in corre- spondence with permanent re-distributions of matter and motion in their environments.

Already in First Principles, when considering the phe- nomena of Evolution in general, the leading characters and causes of those changes which constitute organic evolution, were briefly traced. Under each of the derivative laws of force to which the passage from an incoherent, indefinite homogeneity to a coherent, definite heterogeneity, conforms, were given illustrations drawn from the metamorphoses of

INTERNAL FACTORS. 421

living bodies. Here it •will be needful to contemplate the several resulting processes as going on at once, in both individuals and species.

§ 154. Our postulate being that organic evolution in ge- neral commenced with homogeneous organic matter, just as the evolution of individual organisms commences, we have first to remember that the state of homogeneity is an un- stable state (First Principles, § 109). In any aggregate "the relations of outside and inside, and of comparative nearness to neighbouring sources of influence, imply the re- ception of influences that are unlike in quantity or quality, or both ; and it follows that unlike changes will be produced in the parts thus dissimilarly acted upon." Further, "if any given whole, instead of being absolutely uniform through- out, consists of parts distinguishable from each other — if each of these parts, while somewhat unlike other parts, is uniform within itself; then, each of them being in unstable equilibrium, it follows that while the changes set up within it must render it multiform, they must at the same time render the whole more multiform than before ; ' and hence, " whether that state with which we commence be or be not one of perfect homogeneity, the process must equally be towards a relative heterogeneity." This loss of

homogeneity which the special instability of organic aggre- gates fits them to display more promptly and variously than any other aggregates, must be shown in more numerous ways in proportion as the incident forces are more numerous. Every differentiation of structure being a result of some difference in the relations of the parts to the agencies acting on them, it follows that the more multiplied and more unlike the agencies, the more varied must be the differentiations wrought. Hence the gravitation from a state of homogeneity to a state of heterogeneity, will be conspicuously shown in proportion as the environment is complex. This

transition from a uniform to a multiform state, must con-

422 THE EVOLUTION OF LIFE.

thine through successive individuals. Given a series of or- ganisms, each of which is developed from a portion of a preceding organism, and the question is, whether, after exposure of the series for a million years to changed incident forces, one of its members will be the same as though the incident forces had only just changed. To say that it will, is implicitly to deny the persistence of force. In relation to any cause of divergence, the whole series of such organisms may be considered as fused together into a continuously- existing organism ; and when so considered, it becomes manifest that a continuously- acting cause will go on working a continuously-increasing effect, until some counteracting cause prevents any further effect.

But now if any primordial organic aggregate, must, in itself and through its descendants, gravitate from uniformity to multiformity, in obedience to the more or less multiform forces acting on it ; what must happen if these "multiform forces are themselves ever undergoing slow variations and complications ? Clearly the process, ever-advancing towards a temporary limit but ever having its limit removed, must go on unceasingly. On those structural changes wrought in the once homogeneous aggregate by an original set of in- cident forces, will be superposed further changes wrought by a modified set of incident forces ; and so on throughout all time. Omitting for the present those circumstances which check and qualify its consequences, the instability of the homogeneous must be recognized an ever-acting cause of

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organic evolution, as of all other evolution.

While it follows that every organism, considered as an in- dividual and as one of a series, tends thus to pass into a more heterogeneous state ; it also follows that every species, con- sidered as an aggregate of individuals, tends to do the like. Throughout the area it inhabits, the conditions can never be absolutely uniform : its members must, in different parts of its area, be exposed to different sets of incident forces. Still more decided must this difference of exposure be when

INTERNAL FACTORS. 428

its members spread into other habitats. Those expansive and repressive energies which set to each species a limit that perpetually oscillates from side to side of a certain mean, are, as we lately saw, frequently changed by new combinations of the external factors — astronomic, geologic, meteorologic, and organic. Hence there from time to time arise lines of di- minished resistance, along which the species flows into new localities. Such portions of the species as thus migrate, are subject to circumstances markedly contrasted with its average circumstances. And from multiformity of the circumstances, must come multiformity of the species.

Thus the law of the instability of the homogeneous, has here a three-fold corollary. As interpreted in connexion with the ever-progressing, ever- complicating changes in external fac- tors, it brings us to the conclusion that there must be a pre- vailing tendency towards greater heterogeneity in all kinds of organisms, considered both individually and in successive generations ; as well as in each assemblage of organisms con- stituting a species ; and, by consequence, in each genus, order, and class.

§ 155. When considering the causes of evolution in general, we further saw (First Principles, § 116), that the multiplication of effects aids continually to increase that heterogeneity into which homogeneity inevitably lapses. It was pointed out that since " the several parts of an aggre- gate are differently modified by any incident force ; ' and that since " by the reactions of the differently modified parts the incident force itself must be divided into differently modified parts ; ' it follows that " each differentiated di- vision of the aggregate thus becomes a centre from which a differentiated division of the original force is again diffused. And since unlike forces must produce unlike results, each of these differentiated forces must produce, throughout the aggregate, a further series of differentia- tions/' And to this it was added, that in proportion aa

424 THE EVOLUTION OF LIFE.

the heterogeneity increases, the complications arising from this multiplication of effects grow more marked ; since the more strongly contrasted the parts of an aggregate become, the more different must be their reactions upon incident forces, and the more unlike must be the secondary sets of effects which these modified incident forces initiate ; and since every increase in the number of unlike parts increases the number of such differentiated incident forces, and such secondary sets of effects.

How this multiplication of effects conspires with the in- stability of the homogeneous, to work an increasing multi- formity of structure in an organism, was shown at the time ; and the foregoing pages contain further incidental illustra- tions. Under the head of Adaptation (§ 69), it was shown that a change in one function must act and re-act through ever-complicating perturbations on the rest; and that, eventu- ally, all parts of the organism must be modified in their states. Suppose that the head of a mammal becomes very much more weighty — what must be the indirect results ? The muscles of the neck are put to greater exertions ; and its vertebrae have to bear additional tensions and pressures, caused both by the increased weight of the head, and the stronger contractions of the muscles that support and move the head. These muscles also affect their own attachments : several of the dorsal spines have augmented strains put on them ; and the vertebrae to which they are fixed, are more severely taxed. Further, this heavier head and the more massive neck it necessitates, require a stronger fulcrum : the whole thoracic arch, and the fore limbs which support it, are sub- ject to greater continuous stress and more violent occasional shocks. And the required strengthening of the fore- quarters cannot take place, without the centre of gravity being changed, and the hind limbs being differently reacted upon during locomotion. Any one who compares the outline of the bison with that of its congener, the ox, will clearly see how profoundly a heavier head affects the entire osseous

INTERNAL FACTORS. 425

and muscular systems. Besides this multiplica-

tion of mechanical effects, there is a multiplication of physiological effects. The vascular apparatus is modified throughout its whole structure, by each considerable modifi- cation in the proportions of the body. Increase in the size of any organ, implies a quantitative, and often a qualitative, reaction on the blood ; and so alters the nutrition of all other organs. Such physiological correlations are exemplified in the many differences that accompany difference of sex. That the minor sexual peculiarities are brought about by the physio- logical actions and reactions, is shown both by the fact that they are commonly but faintly marked until the fundamentally distinctive organs are developed ; and that when the de- velopment of these is prevented, the minor sexual peculiarities do not arise. ISTo further proof is, I think, needed,

that in any individual organism or its descendants, a new external action must, besides the primary internal change which it works, work sundry secondary changes, as well as tertiary changes still more multiplied. That tendency to- wards greater heterogeneity which is given to an organ- ism by disturbing its environment, is helped by the tendency which every modification lias to produce other modifications — modifications which must become more numerous in pro- portion as the organism becomes more complex. And then, lastly, among the indirect and involved manifestations of this tendency, we must not omit the innumerable small irregularities of structure that result from the crossing of dissimilarly-modified individuals. It was shown (§§ 89, 90) ^,hat what are called "spontaneous variations," are inter- pretable as results of miscellaneously compounding the changes wrought in different lines of ancestors by different conditions of life. These still more complex and multi- tudinous effects so produced, are thus further illustrations of the multiplication of effects.

Equally in the aggregate of individuals constituting a species, does multiplication of effects become the continual 19

426 THE EVOLUTION OF LIFE.

cause of increasing multiformity. The lapse of a species into divergent varieties, initiates freyh combinations of forces tending to work further divergences. The new varieties compete with the parent species in new ways ; and so add new elements to its circumstances. They modify somewhat the conditions of other species existing in their habitat, or into whose habitat they have spread; and the modificationn wrought in such other species, become additional sources of influence. The Flora and Fauna of every region are united by their entangled relations into a whole, of which no part can be affected without affecting the rest. Hence, each dif- ferentiation in a local assemblage of species, becomes the cause of further differentiations in such assemblage.

§ 15G. One of the universal principles to which we saw that the re-distribution of matter and motion conforms, is that in any aggregate made up of mixed units, incident forces produce segregation — separate unlike units and unite like units ; and it was shown that the increasing integration and definiteness which characterizes each part of an evolving organic aggregate, as of every other aggregate, results from this (First Principles, § 126). It remains here to be pointed out, that while the actions and reactions going on between organisms and their ever-changing environments, add to the heterogeneity of organic structures, they also give to the heterogeneity this growing distinctness. At first sight the reverse might be inferred. It might be argued that any new set of effects wrought in an organism by some new set of external forces, must tend more or less to obliter- ate the effects previously wrought — must produce confusion or indefiniteness. A little consideration, however, will dissi- pate this impression.

Doubtless the condition under which alone increasing de- finiteness of structure can be acquired by any part of an or- ganism, either in an individual or in successive generations, is that such paH shall be exposed to some set of tolerably-con -

INTERNAL FACTORS. 427

slant forces; and doubtless, continual change of circumstances interferes with this. 13ut the interference can never be con- siderable. For the pre-existing structure of an organism pre- vents it from living under any new conditions except such as are congruous with the fundamental characters of its organiza- tion— such as subject its essential organs to actions substan- tially the same as before. Great changes must kill it. Hence, it can continuously expose itself and its descendants, only to those moderate changes which do not destroy the general har- mony between the aggregate of incident forces and the ag- gregate of its functions. That is, it must remain under influences calculated to make greater the definiteness of the chief differentiations already produced. If, for ex- ample, we set out with an animal in which a rudimentary vertebral column with its attached muscular svstem has

i/

been established; it is clear that the mechanical arrange- ments have become thereby so far determined, that sub- sequent modifications are extremely likely, if not certain, to be consistent with the production of movement by the action of muscles on a flexible central axis. Hence, there will con- tinue a general similarity in the play of forces to which the flexible central axis is subject ; and so, notwithstanding the metamorphoses which the vertebrate type undergoes, there will be a maintenance of conditions favourable to increasing definiteness and integration of the vertebral column. More- over, this maintenance of such conditions becomes secure in proportion as organization advances. Each further com- plexity of structure, implying some further complexity in the relations between an organism and its environment, must tend to specialize the actions and reactions between it and its environment — must tend to increase the stringency with which it is restrained within such environments as admit of those special actions and reactions for which its structure fits it ; that is, must further guarantee the continuance of those actions and reactions to which its essential organs respond, and therefore the continuance of the segregating process.

428 THE EVOLUTION OF LIFE.

\

How in eacli species, considered as an aggregate of indi- viduals, there must arise stronger and stronger contrasts between those divergent varieties which result from, the instability of the homogeneous and the multiplication of effects, needs only be briefly indicated. It has already been shown (First Principles, § 126), that in conformity to the universal law that mixed units are segregated by like incident forces, there are produced increasingly-definite distinctions among varieties, wherever there occur definitely- distinguished sets of conditions to which the varieties are re- spectively subject.

§ 157. Probably in the minds of some, the reading of this chapter has been accompanied by a running commentary, to the effect that the argument proves too much. The apparent implication is, that the passage from an indefinite, incohe- rent homogeneity to a definite, coherent heterogeneity in organic aggregates, must have been going on universally ; whereas we find that in many cases there has been persist- ence without progression. This apparent implication, how- ever, is not a real one.

For though every environment on the Earth's surface undergoes changes ; and though usually the organisms which each environment contains, cannot escape certain resulting new influences ; yet occasionally such new in- fluences are escaped, by the survival of species in the un- changed parts of their habitats, or by their spread into neighbouring- habitats which the change has rendered like

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their original habitats, or by both. Any alteration in the temperature of a climate or its degree of humidity, is tin- likely to affect simultaneously the whole area occupied by a species ; and further, it can scarcely fail to happen that the addition or subtraction of heat or moisture, will give to a part of some adjacent area, a climate like to that to wrhich the species has been habituated. If, again, the circumstances of a species are modified by the intrusion of some foreign

INTERNAL FACTORS. 429

kind of plant or animal, it follows that since the intruders will probably not spread throughout its whole habitat, the species will, in one or more localities, remain unaffected by them. Especially among marine creatures, must there fre- quently occur cases in which modifying causes are con- tinually eluded. Much more uniform as are the physical conditions to which the sea exposes its inhabitants, it becomes possible for such of them as live on widely-diffused food, to be widely distributed ; and wide distribution generally pre- vents the members of a species from being all subject to the same cause. Our commonest cirrhiped, for instance, subsisting on minute creatures that are everywhere dispersed through the sea ; needing only to have some firm surface on which to build up its shell ; and in scarcely any danger from sur- rounding animals ; is able to exist on shores so widely remote from one another, that nearly every change in the actions of incident forces, must fall within narrower areas than that which the species occupies. In nearly every case, therefore, a portion of the species will survive unmodified. Its easily- transported germs will take possession of such new habitats as have been rendered fitter by the change that has unfitted some parts of its original habitat. Hence, on successive occasions, while some parts of the species are slightly trans- formed, another part may continually escape transformation by migrating hither and thither, where the simple condi- tions needed for its existence recur in nearly the same com- binations as before. And it will so become possible for it to survive, with comparatively trifling structural changes, throughout long geologic periods.

§ 158. The results to which we find ourselves led, are these.

In subordination to the different amounts and kinds of forces to which its different, parts are exposed, every in- dividual organic aggregate, like all other aggregates, tends- to pass from its original indistinct simplicity towards a more

130 THE EVOLUTION OF LIFE.

distinct complexity. Unless we deny the persistence of force, we must admit that the gravitation of an organism's structure from an indefinitely homogeneous to a definitely heterogeneous state, must be cumulative in successive genera- tions, if the forces causing it continue to act. And for the like reasons, the increasing assemblage of individuals arising from, a common stock, is also liable to lose its original uniformity; and, in successive generations, to grow more pronounced in its multiformity.

These changes, which would go on to but a comparatively small extent were organisms exposed to constant external conditions, are kept up by the continual changes in external conditions, produced by astronomic, geologic, meteorologic, and organic agencies : the average result being, that on previous complications of structure wrought by previous incident forces, new complications are continually superposed by new incident forces. And hence simultaneously arises increasing heterogeneity in the structures of individuals, in the structures of species, and in the structures of the Earth's Flora and Fauna.

But while, in very many or in most cases, the ever- changing incidence of forces is ever adding to the complexity of organisms, and to the complexity of the organic world as a whole ; it does this only where its action cannot be eluded. And since, by migration, it is possible for species to keep themselves under conditions that are tolerably constant ; there must be a proportion of cases in which greater hetero- geneity of structure is not produced.

Uniting these three propositions, we are brought to a con elusion which, so far as it goes, appears to be in harmony with the facts. We find progression to result, not from a special, inherent tendency of living bodies, but from a general average effect of- their relations to surrounding agencies. While we are not called on to suppose that there exists in organisms any primordial impulse which makes them con- tinually unfold into more heterogeneous forms ; we see

INTERNAL FACTORS. 431

that a liability to be unfolded arises from the actions and reactions between organisms and their fluctuating environ- ments. And we see that the existence of such a cause of development, presupposes the non-occurrence of development where this fluctuation of actions and reactions does not come into play.

To show, however, that there must arise a certain general tendency to the production of more heterogeneous aggregates, is not sufficient. It is quite conceivable that aggregates should be rendered more heterogeneous by changing incident forces, without having given to them that peculiar form of heterogeneity required for carrying on the functions of life. Hence it remains now to inquire, how the production and maintenance of this peculiar form of heterogeneity is insured.

CHAPTER XL

DIRECT EQUILIBRATION.

§ 159. EVERY change is of necessity towards a balance of s; and of necessity can never cease until a balance offerees is reached. When treating of equilibration under its general aspects (First Principles, Part II., Chap, xvi.), we saw that in every aggregate having compound movements, there tends continually to be established a moving equilibrium ; since any unequilibrated force to which such an aggregate is subject, if not of a kind to overthrow the aggregate al- together, must continue modifying its state until an equi- librium is brought about. And we saw that the structure simultaneously reached must be " one presenting an arrange- ment of forces that counterbalance all the forces to which the aggregate is subject ; ' since, " so long as there remains a residual force in any direction — be it excess of a force exercised by the aggregate on its environment, or of a force exercised by its environment on the aggregate, equilibrium does not exist ; and therefore the re-distribution of matter must continue."

It is essential that this truth should here be fully under- stood ; and to the end of insuring a clear comprehension of it, some re-illustration is desirable. The case of the Solar System will best serve our purpose. An assemblage of bodies, each of which has its simple and compound motions, that severally alternate between two extremes, and the whole of

DIRECT EQUILIBRATION. 433

which has its involved perturbations, that now increase and now decrease, is heie presented to us. Suppose a new force were brought to bear on this moving equilibrium — say by the arrival of some wandering mass, or by an additional momentum given to one of the existing masses — what would be the result ? If the strange body or the extra force were very large, it might so derange the entire system as to cause its collapse : by overthrow of its rhythmical movements, the moving equilibrium might rapidly be changed into a com- plete equilibrium. But what if the incident force, falling on the system from without, proved insufficient to overthrow it ? There would then arise a set of perturbations which would, in the course of an enormous period, slowly work round into a modified moving equilibrium. The effects primarily im- pvessed on the adjacent masses, and in a smaller degree on the remoter masses, would soon become complicated with the secondary effects impressed by the disturbed masses on one another ; and these again with tertiary effects. Waves of perturbation would continue to be propagated throughout the entire system ; until, around a new centre of gravity, there had been established a set of planetary motions more or less different from the preceding ones. All this would necessarily follow from the truth;, that any new force brought to bear on a moving equilibrium, must gradually be used up in overcoming the forces that resist the divergence it gener- ates : which antagonizing forces, being then no longer op- posed, set up a counter-action, ending in a compensating divergence in the opposite direction, that is followed by a re-compensating divergence ; and so on, until there is either established some additional rhythmical movement, or some equivalent modification of the pre-existing rhythmical move- ments. Now though instead of being, like the Solar System, in a state of independent moving equilibrium, an organism is in a state of dependent moving equilibrium (First Principles, § 130) ; yet this does not prevent the manifestation of the same law. Every animal d-.iily obtains

TIIF EVOLUTION OF LIFE

from without, a supply of force to replace the foren which it expends ; but this continual giving to its parts a new momentum, to make up for the momentum continually lost, does not interfere with the carrying on of actions and reactions like those just described. Here, as before, we have a definitely- arranged aggregate of parts, which we call organs, having their definitely-established actions and re- actions, which we call functions. These rhythmical actions or functions, and the various compound rhythms resulting from their combinations, are in such adjustment as to balance the actions to which the organism is subject : there is a con- stant or periodic genesis of forces, which, in their kinds, amounts, and directions, suffice to antagonize the forces which the organism has constantly or periodically to bear. If then there exists this state of moving equilibrium among a definite set of internal actions, exposed to a definite set of ex- ternal actions ; what must result if any of the external actions are changed ? Of course there is no longer an equilibrium. Some force which the organism habitually generates, is too great or too small to balance some incident force ; and there arises a residuary force exerted by the environment on the organism, or by the organism on the environment. This residuary force — this unbalanced force, of necessity expends itself in producing some change of state in the organism. Acting directly on some organ and modifying its function, it indirectly modifies dependent functions, and remotely influences all the functions. As we have already seen (§§ 68, 69), if this new force is permanent, its effects must be gradually diffused throughout the entire system ; until it has come to be equilibrated in working those structural re- arrangements which produce an exactly counterbalancing force.

The bearing of this general truth on the question we are now dealing with, is obvious. Those modifications upon modifications, which the unceasing mutations of their en- vironments have been all along generating in organisms,

DIRECT EQUILIBRATION. 435

have been in each case modifications involved by the establishment of a new balance with the new combination of conditions. In every species throughout all geologic time, there has been perpetually going on a rectification of the equilibrium, that has been perpetually disturbed by the alteration of surrounding circumstances ; and every further heterogeneity has been the addition of a structural change entailed by a new equilibration, to the structural changes entailed by previous equilibrations. There can be no other ultimate interpretation of the matter, since change can have no other goal. Any fresh force brought to bear on an aggregate in a state of moving equilibrium, must do one of two things : it must either overthrow the moving equi- librium altogether, or it must alter without overthrowing it ; and the alteration must end in the establishment of a new moving equilibrium. Hence in organisms, death or restora- tion of the physiological balance, are the only alternatives.

This equilibration between the functions of an organism and the actions in its environment, may be either direct or indirect. The new incident force may either immediately call forth some counteracting force, and its concomitant structural change ; or it may be eventually balanced by some otherwise-produced change of function and structure. These two processes of equilibration are quite distinct, and must be separately dealt with. We will devote this chapter to the first of them.

§ 160. Direct equilibration is that process currently known as adaptation. We have already seen (Part II., Chap, v.), that individual organisms become modified when placed in new conditions of life — so modified as to re-adjust the powers to the requirements ; and though there is great difficulty in disentangling the evidence, we found reason for thinking (§ 82) that structural changes thus caused by functional changes are inherited. In the last chapter, it was argued that if, instead of the succession of individuals

436 THE EVOLUTION OF LIFE.

constituting a species, there were a continuously-existing individual, any such functional and structural divergence as we see produced by a new incident force, would necessarily go on increasing until the new incident force was counter- poised ; and that the replacing of a continuously-existing individual by a succession of individuals, each formed out of the modified substance of its predecessor, will not prevent the like effect from, being produced — the persistence of force negativing any other inference. Here we further find, that this limit towards which any such organic change advances, in the species as in the individual, is a new moving equi- librium adjusted to the new arrangement of external forces.

But now, what are the conditions under which alone, direct equilibration can occur ? Are all the modifications that serve to re-fit organisms to their environments, directly adaptive modifications ? And if otherwise, which are the directly adaptive and which are not ? How are we to distinguish between them ?

Manifestly, for any moving equilibrium to be gradually altered, it is needful, first, that some force shall operate upon it ; and, second, that the force shall not be such as to over- throw it. If in the environment there exists some agency that would act advantageously on an organism were the or- ganism a little modified, but which does not act on it in the absence of the required modification ; it is clear that this agency cannot itself tend to produce the modification. On the other hand, if the external agency be of such kind, that individuals of the species whenever affected by it, are either killed or so injured that the production of vigorous offspring is much interfered with, there cannot be directly wrought in the species, any such alteration as will fit it to cope with this external agency. The only new incident forces which can work the changes of function and structure required to bring any animal or plant into equilibrium with them, are such incident forces as operate on this animal or plant, either continuously or frequently. They must be capable

DIRECT EQUILIBRATION. 437

of appreciably changing that set of complex rhythmical actions and reactions constituting the life of the organism ; and yet must not usually produce perturbations that are fatal. Let us see what are the limits to direct equilibra- "ion hence arising.

§ 161. In plants, organs engaged in nutrition, and exposed to variations in the amounts and proportions of matters and forces utilized in nutrition, may be expected to undergo cor- responding variations. We find evidence that they do this. The " changes of habit ' ' which are common in plants, when taken to places unlike in climate or soil to those before in- habited by them, are changes of parts in which the modified external actions directly produce modified internal actions. The characters of the stem and shoots as woody or succulent, erect or procumbent ; of the leaves in respect of their sizes, thicknesses, and textures ; of the roots in their degrees of development and modes of growth ; are obviously in imme- diate relation to the characters of the environment. A per- manent difference in the quantity of light or heat, affects, day after day, the processes going on in the leaves. Habitual rain or drought, alters all the assimilative actions, and appreciably influences the organs that carry them on. Some particular substance, by its presence in the soil, gives new qualities to some of the tissues ; causing greater rigidity or flexibility, and so affecting the general aspect. Here, then, we have, in plants, changes tending to bring about in them, modified arrangements of functions and structures, in equi- librium with modified sets of external forces.

But now let us turn to other classes of organs possessed by plants — organs which are not at once affected in their actions by the variations of incident forces. Take first the organs of defence. Many plants are shielded against animals that would else devour them, by formidable thorns ; and others, like the nettle, by stinging hairs. These must be counted among the appliances by which equilibrium is maintained

438 THE EVOLUTION OF LIFE.

between tlie actions in the organism and tlie actions in its environment ; seeing that all other things remaining the same, if these defences were absent, the destruction by herb- ivorous animals would be so increased, that the number of young plants annually produced would not suffice, as it now does, to balance the mortality, and the species would there- fore disappear. But these defensive appliances, though they aid in maintaining the balance between inner and outer actions, cannot have been directly called forth by the outer actions which they serve to neutralize; for these outer actions do not continuously affect the functions of the plant even in a general way, still less in the special way required. Suppose a species of nettle bare of poison-hairs, to be habit- ually eaten by some mammal intruding on its habitat ; the agency of this mammal would have 110 direct tendency to develop poison-hairs in the plant; since the individuals devoured could not bequeath changes of structure, even were the actions of a kind to produce them ; and since the in- dividuals that perpetuated themselves, would be those on which the new incident force had not fallen. An-

other class of organs similarly circumstanced, are those of reproduction. Like the organs of defence, these are not, during the life of the individual plant, variably exercised by variable external actions ; and therefore do not fulfil those conditions under which structural changes may be directly caused by changes in the environment. The generative apparatus contained in every flower, acts only once during its existence ; and even then, the parts subserve their ends in a passive rather than an active way. Functionally-pro- duced modifications are therefore out of the question. If a plant's anthers are so placed, that the insect which most commonly frequents its flowers, is sure to come in contact with the pollen, and to fertilize with it other flowers of the same species ; and if this insect, dwindling away or dis- appearing from the locality, leaves behind no insects that have such shapes and habits as cause them to do the same

DIRECT EQUILIBRATION. 439

thing efficiently, but only some which do it inefficiently ; it is clear that the change of its conditions, has no immediate tendency to work in the plant any such structural change as shall bring about a new balance with its conditions. For the anthers, which, even when they discharge their functions, do it simply by standing in the way of the insect, are, under the supposed circumstances, left untouched by the insect ; and this remaining untouched, cannot have the effect of so modifying the stamens as to bring the anthers into a position to be touched by some other insect. Only those individuals whose parts of fructification so far differed from the average form of the species, that some other insect could serve them as pollen-carrier, would be sufficiently prolific to have good chances of perpetuating themselves. And on their progeny, inheriting the deviation, there would act no external force directly calculated to make the deviation greater, and the adaptation more complete ; since the new circumstances to which re- adaptation is required, are such as do not in the least alter the equilibrium of functions constituting the life of the individual plant.

§ 162. Among animals, adaptation by direct equilibration is similarly traceable, wherever, during the life of the indi- vidual, an external change generates some constant or re- peated change of function. This is conspicuously the case with such parts of an animal as are immediately exposed to diffused influences, like those of climate, and with such parts of an animal as are occupied in its mechanical actions on the environment. Of the one class of cases, the darkening or

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lightening of the skin, that follows exposure to greater or less heat, may be taken as an instance ; and with the other class of cases, we are made familiar by the increase and de- crease which use and disuse cause in the organs of motion

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and manipulation. It is needless here to exemplify these : they were treated of in the Second Part of this work.

But in animals, as in plants, there are many indispensable

140 THE EVOLUTION OF LIFE.

offices fulfilled by parts, between which and the external con ditions they respond to, there is no such action and reaction as can directly produce an equilibrium. This is especially manifest with dermal appendages. Some ground, perhaps, exists for the conclusion that the greater or less development of hairs, is in part immediately due to increase or decrease of demand on their passive function, as non-conductors of heat ; but be this as it may, it is impossible that there can exist any such cause for those immense developments of hairs which we see in the quills of the porcupine, or those complex de- velopments of them known as feathers. Such an enamelled armour as is worn by the Lepidosteus, is inexplicable as a direct result of any functionally- worked change. For purposes of defence, such an armour is as needful, or more needful, for hosts of other fishes ; and did it result from any direct re- action of the organism against any offensive actions it was subject to, there seems no reason why other fishes should not have developed similar protective coverings. Of

sundry reproductive appliances, the like may be said. The secretion of an egg-shell round the substance of an egg, in the oviduct of a bird, is quite inexplicable as a consequence of some functionally- wrought modification of structure, im- mediately caused by some modification of external con- ditions. The end fulfilled by the egg-shell, is that of protecting the contained mass against certain slight pressures and collisions, to which it is liable during incubation. How, by any process of direct equilibration, could it come to have the required thickness ? or, indeed, how could it come to exist at all? Suppose this protective envelope to be too weak, so that some of the eggs a bird lays are broken or cracked. In the first place, the breakages or crackings are actions of a kind which cannot react on the maternal organ- ism, in such way as to cause the secretion of thicker shells for the future : to suppose that they can, is to suppose that the bird understands the cause of the evil, and that the secretion of thicker or thinner shells can be controlled by its

DIRECT EQUILIBRATION. 441

will. In the second place, such developing chicks as are contained in the shells which crack or break, are almost certain to die ; and cannot, therefore, acquire any appro- priately-modified constitutions : even supposing any con- ceivable relation could be shown, between the impression received and the change required. Meanwhile, such eggs as escape breakage, are not influenced at all by the require- ment ; and hence, on the birds developed from them, there cannot have acted any force tending to work the needful adjustment of functions. In no way, therefore, can a direct equilibration between constitution and conditions be here produced. Even in organs that can be modified

by certain incident forces into correspondence with such incident forces, there are some re-adjustments which cannot be effected by the direct balancing of inner and outer actions. It is thus with the bones. The majority of the bones have to resist muscular strains ; and it is a familiar fact that variations in the muscular strains, call forth, by reaction, variations in the strengths of the bones. Here there is direct equilibration. But though the greater massiveness acquired by bones subject to greater strains, may be ascribed to a counter-acting force evoked by a force brought into action ; it is impossible that the acquirement of greater lengths by bones can be thus accounted for. It has been supposed that the elongation of the metatarsals in wading birds, has resulted from direct adaptation to conditions of life. To justify this supposition, however, it must be shown that the mechanical actions and reactions in the legs of a wading bird, differ from those in the legs of other birds ; and that the differential actions are equilibrated by the extra lengths. There is not the slightest evidence of this. The metatarsals of a bird, have to bear 110 appreciable strains but those due to the superincumbent weight. Standing in the water does not appreciably alter these strains ; and even if it did, an increase in the lengths of these bones would not fit them any better to meet the altered strains.

442 THE EVOLUTION OF LIFE.

§ 163. The conclusion at which we arrive i*. then, that there go on in all organisms, certain changes of function and

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structure that are directly consequent on changes in the incident forces — inner changes by which the outer changes are balanced, and the equilibrium restored. Such re-equi- librations, which are often conspicuously exhibited in in- dividuals, we have reason to believe continue in successive generations ; until they are completed by the arrival at structures fitted to the modified conditions. But, at the same time, we see that the modified conditions to which or- ganisms may be adapted by direct equilibration, are con- ditions of certain classes only. That a new external action may be met by a new internal action, it is needful that i^ shall either continuously or frequently be borne by the in- dividuals of the species, without killing or seriously injuring them ; and shall act in such wav as to affect their functions.

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And we find on examination, that many of the environing changes to which organisms have to be adjusted, are not of these kinds : being changes which either do not immediately affect the functions at all, or else affect them in ways that prove fatal.

Hence there must be at work some other process, which equilibrates the actions of organisms with the actions they are exposed to. Plants and animals that continue to exist, are necessarily plants and animals whose powers balance the powers that act on them ; and as their environments change, the changes which plants and animals undergo, must necessarily be changes towards a re-establishment of the balance. Besides direct equilibration, there must therefore be an indirect equilibration. How this goes on we have to inquire.

CHAPTER XII.

INDIRECT EQUILIBRATION.

§ 164. BESIDES those perturbations produced in the moving equilibrium of any organism by special disturbing forces, there are ever going on many other perturbations — some which are the still-reverberating effects of disturbing forces previously experienced by the individual, and others which are the still-reverberating effects of disturbing forces expe- rienced by ancestral individuals ; and the multiplied devia- tions of function so caused, imply multiplied deviations of structure. In § 155 there was re- illustrated the truth, set forth at length when treating of Adaptation (§ 69), that an organism in a state of moving equilibrium, cannot have extra function thrown on any organ, and extra growth pro- duced in such organ, without there being entailed correlative changes throughout all other functions, and eventually throughout all other organs. And when treating of Varia- tion (§ 90), we saw that individuals which have been made, by their different circumstances, to deviate functionally and structurally from the average type in different directions, will bequeath to their joint offspring, compound perturbations of function and compound deviations of structure, endlessly varied in their kinds and amounts. That is to say, besides the primary perturbations and deviations directly caused in organisms by altered actions in their environments, there are ever being indirectly caused, secondary and tertiary per-

444 THE EVOLUTION OF LIFE.

turbations and deviations, which, when compounded with one another from generation to generation, work innumerable slight modifications in the moving equilibria and correlative structures throughout the species.

Now if the individuals of a species are thus necessarily made unlike, in countless ways and degrees — if the compli- cated sets of rhythms which we call their functions, though similar in their general characters, are dissimilar in their details — if in one individual the anioimt of action in a par- ticular direction is greater than in any other individual, or if here a peculiar combination gives a resulting force which is not found elsewhere ; then, among all the individuals, some will be less liable than others to have their equilibria over- thrown by a particular incident force, previously unexperi- enced. Unless the change in the environment is of so vio- lent a kind as to be universally fatal to the species, it must affect more or less differently the slightly different moving equilibria which the members of the species present. It cannot but happen that some will be more stable than others, when exposed to this new or altered factor. That is to say, it cannot but happen that those individuals whose functions are most out of equilibrium with the modified aggregate of external forces, will be those to die ; and that those will sur- vive whose functions happen to be most nearly in eqeulibrium with the modified aggregate of external forces.

But this survival of the fittest, implies multiplication of the fittest. Out of the fittest thus multiplied, there will, as before, be an overthrowing of the moving equilibrium wher- ever it presents the least opposing force to the new incident force. And by the continual destruction of the individuals Lhat are the least capable of maintaining their equilibria in presence of this new incident force, there must eventually be arrived at an altered type completely in equilibrium with the altered conditions.

§ 165. Tliis survival of the fittest, which I have here

INDIRECT EQUILIBRATION. 445

sought to express in mechanical terms, is that which Mr Dar- win has called " natural selection, or the preservation of favoured races in the struggle for life." That there is going on a process of this kind throughout the organic world, Mr Darwin's great work on the Origin of Species has shown to the satisfaction of nearly all naturalists. Indeed, when once enunciated, the truth of his hypothesis is so obvious as scarcely to need proof. Though evidence may be required to show that natural selection accounts for everything ascribed to it, yet no evidence is required to show that natural selec- tion has always been going on, is going on now, and must ever continue to go on. Recognizing this as an a priori cer- tainty, let us contemplate it under its two distinct aspects.

That organisms which live, thereby prove themselves fit to live, in so far as they have been tried ; while organisms which die, thereby prove themselves in some respects unfitted for living ; are facts no less manifest, than is the fact that this self-acting purification of a species, must tend ever to insure adaptation between it and its environment. This adaptation may be either so maintained or so produced. Doubt-

less many who have looked at Nature with philosophic eyes, have observed that death of the worst and multiplication of the best, must result in the maintenance of a constitution in harmony with surrounding circumstances. That the aver- age vigour of any race would be diminished, did the diseased and feeble habitually survive and propagate ; and that the destruction of such, through failure to fulfil some of the con- ditions to life, leaves behind those which are able to fulfil the conditions to life, and thus keeps up the average fitness to the conditions of life ; are almost self-evident truths. But to recognize " natural selection " as a means of preserving an already-established balance between the powers of a spe- cies and the forces to which it is subject, is to recognize it only in its simplest and most general mode of action. It is the more special mode of action with which we are here con- rorned. This more special mode of action, Mr Dar-

446 THE EVOLUTION OF LIFE.

win lias been the first to perceive. To him we owe the dis- covery that natural selection is capable of producing fitness between organisms and their circumstances ; and he, too, has the merit of appreciating the immensely-important conse- quences that follow from this. He has worked up an enormous mass of evidence into an elaborate demonstration, that this " preservation of favoured races in the struggle for life," is an ever-acting cause of divergence among organic forms. He has traced out the involved results of the process with marvellous subtlety. He has shown how hosts of otherwise inexplicable facts, are fully accounted for by it. In brief, he has proved that the cause he alleges is a true cause ; that it is a cause which we see habitually in action ; and that the results to be inferred from it, are in harmony with the phe- nomena which the Organic Creation presents, both as a whole and in its details. Let us glance at a few of the more im- portant interpretations which the hypothesis furnishes.

A soil possessing some ingredient in unusual quantity, may supply to a plant an excess of the matter required for a certain class of its tissues ; and may cause all the parts formed of such tissues to be abnormally developed. Suppose that among these are the hairs clothing its surfaces, including those which grow on its seeds. Thus furnished with some- what longer fibres, its seeds, when shed, are carried a little further by the wind before they fall to the ground. The young plants growing up from them, being rather more widely dispersed than those produced by other individuals of the same species, will be less liable to smother one another ; and a greater number may therefore reach maturity and fructify. Supposing the next generation subject to the same- peculiarity of nutrition, some of the seeds borne by its mem- bers will not simply inherit this increased development of hairs, but will carry it further ; and these, still more advan- taged in the same way as before, will, on the average, have still more numerous chances of continuing the race. Thus, by the survival, generation after generation, of those possess-

INDIRECT EQUILIBRATION. 447

ing these longer hairs, and the inheritance of successive incre- ments of growth in the hairs, there may result a seed deviat- ing greatly from, the original. Other individuals of the same species, subject to the different physical conditions of other localities, may develop somewhat thicker or harder coatings to their seeds : so rendering their seeds less digest- ible by the birds that devour them. Such thicker- coated seeds, by escaping undigested more frequently than thinner- coated ones, will have additional chances of growing up and leaving offspring ; and this process, acting in a cumulative manner through successive years, will produce a seed diverg- ing in another direction from the ancestral type. Again, elsewhere, some modification in the physiologic actions of the plant, may lead to an unusual secretion of an essential oil in the seeds ; which rendering them unpalatable to crea- tures that would otherwise feed on them, may diminish the destruction of the seeds, so giving an advantage to the variety in its rate of multiplication ; and this incidental peculiarity proving a preservative, will, as before, be gradually increased by natural selection, until it constitutes another divergence. Now in these and countless analogous cases, we see that plants may become better adapted, or re-adapted, to the aggregate of surrounding agencies, not through any direct action of such agencies upon them, but through their indirect action — through the destruction by them of the individuals which are least congruous with them, and the survival of those which are most congruous with them. All these slight variations of function and structure, arising among the members of a species, serve as so many experiments ; the great majority of which fail, but a few of which succeed. Just as we see that each plant bears a multitude of seeds, out of which some two or three happen to fulfil all the conditions required for reaching maturity, and continuing the race; so we see that each species is perpetually producing numerous slightly-modified forms, deviating in all directions from the average, out of which most fit the surrounding conditions no better than their pa-

448 THE EVOLUTION OF LIFE.

rents, or not so well, but some few of which fit the conditions better; and doing so, are enabled the better to preserve them- selves, and to produce offspring similarly capable of preserv- ing themselves. Among animals the like process re- sults in the like development of various structures which cannot have been affected by the performance of functions — their functions being purely passive. The thick shell of u mollusk, is inexplicable as a result of direct reactions of the organism against the external actions to which it is exposed ; but it is quite explicable as a result of the survival, genera- tion after generation, of individuals whose thicker coverings protected them against enemies. Similarly with such a dermal structure as that of the tortoise. Though we have evi- dence that the skin where it is continually exposed to pres- sure and friction may thicken, and so re-establish the equi- librium, by opposing a greater inner force to a greater outer force ; yet we have no evidence that a coat of armour like that of the tortoise can be so produced. Nor, indeed, are the conditions under which only its production in such a man- ner could be accounted for, fulfilled ; since the surface of the tortoise is not exposed to greater pressure and friction than the surfaces of other creatures. This massive carapace, and the strangely-adapted osseous frame-work which supports it, are unaccountable as results of evolution, unless through the process of natural selection. Thus, too, is it with the pro- duction of colours in birds and in insects ; the formation of odoriferous glands in mammals ; the growth of such excres- cences as those of the camel. Thus, in short, is it with all those organs of animals, which do not play active parts in the compound rhythms of their functions.

Besides giving us explanations of structural characters that are otherwise unaccountable, Mr Darwin shows how natural selection explains peculiar relations between indi- viduals in certain species. Such facts as the dimorphism of the primrose and other flowers, he proves to be quite in har- mony with his hypothesis, though stumbling-blocks to all

INDIRECT EQUILIBRATION. 449

other hypotheses. "While the production of neuters among bees and ants, is inexplicable as a result of direct adaptation, natural selection affords a feasible solution of it. The various differences that accompany difference of sex, sometimes slight, sometimes very great, are similarly accounted for. As before suggested (§ 79), natural selection appears capa- ble of producing and maintaining the right proportion of the sexes in each species ; and it requires but to contemplate the bearings of the argument, to see that the formation of different sexes may itself have been determined in the same way.

To convey here an adequate idea of Mr Darwin's doctrine, in the immense range of its applications, is of course impos- sible. The few illustrations just given, serving but dimly to indicate the many classes of phenomena interpreted by it, are set down simply to remind the reader what Mr Darwin's hypothesis is, and what are the else insoluble problems which it solves for us.

§ 166. But now, though it seems to me that we are thus supplied with a key to phenomena which are multitudinous and varied beyond all conception ; it also seems to me that there is a moiety of the phenomena which this key will not unlock. Mr Darwin himself recognizes use and disuse of parts, as causes of modifications in organisms ; and does this, indeed, to a greater extent than do some who accept his general conclusion. But I conceive that he does not re- cognize them to a sufficient extent. While he conclusively shows that the inheritance of changes of structure, caused by changes of function, is utterly insufficient to explain a great mass — probably the greater mass — of morphological pheno- mena ; I think he leaves unconsidered a mass of morphological phenomena that are explicable as results of functionally- acquired modifications, transmitted and increased, and which are not explicable as results of natural selection.

By induction, as well as by inference from the hypothesis 20

450 THE EVOLUTION OF LIFE.

of natural selection, we know that there exists a balance among the powers of organs which habitually act together — such proportions among them, that no one has any consider- able excess of efficiency. We see, for example, that through- out the vascular system, there is maintained an. equilibrium between the powers, that is, the developments, of the com- ponent parts : in some cases, under excessive exertion, the heart gives way, and we have enlargement ; in other cases the large arteries give way, and we have aneurisms ; in other cases the minute blood-vessels give way — now bursting, now becoming chronically congested. That is to say, in the average constitution, no superfluous strength is possessed by any of the appliances for circulating the blood. Take, again, a set of motor organs. Great strain here causes the fibres of a muscle to tear. There the muscle does not yield but the tendon snaps. Elsewhere neither muscle nor tendon is damaged, but the bone breaks. Joining with these instances the general fact, that under the same adverse conditions, different individuals show their slight differences of consti- tution by going wrong some in one way and some in an- other ; and that even in the same individual, similar adverse conditions will now affect one viscus and now another ; it becomes manifest that though there cannot be maintained an accurate or absolute balance among the powers of the organs composing an organism, yet the excesses and de- ficiencies of power are extremely slight. That they must be extremely slight, is, as before said, a deduction from the hypothesis of natural selection. Mr Darwin himself argues " that natural selection is continually trying to economize in every part of the organization. If under changed conditions of life a structure before useful becomes less useful, any diminution, however slight, in its development, will be seized on by natural selection, for it will profit the individual not to have its nutriment wasted in building up an useless struc- ture." In other words, if any muscle has more fibres than can be utilized, or if a bone be stronger than needful, no ad-

INDIRECT EQUILIBRATION. 45 1

vantage results, but rather a disadvantage — a disadvantage which will decrease the chances of survival. Hence

it becomes a corollary, that among any organs which habit- ually act in concert, an increase of one can be of no service unless there is a concomitant increase of the rest. The co operative parts must vary together ; otherwise variation will be detrimental. A stronger muscle must have a stronger bone to resist its contractions ; must have stronger correlated mus- cles and ligaments to secure the neighbouring articulations ; must have larger blood-vessels to bring it supplies; must have a more massive nerve to bring it stimulus, and some extra development of a nervous centre to supply this extra stimulus. The question arises, then, — does spontaneous variation occur simultaneously in all these co-operative parts ? Have we any reason to think that they spontaneously increase or decrease together ? The assumption that they do, seems to me untenable ; and its untenability will, I think, become conspicuous if we take a case, and observe how ex- tremely numerous and involved are the variations which must be supposed to occur together. In illustration

of another point, we have already considered the modifica- tion required to accompany increased weight of the head. Instead of the bison, however, the moose deer, or the extinct Irish elk, will here best serve our purpose. In this species the male has enormously- developed horns, which are used for purposes of offence and defence. These horns, weighing up- wards of a hundred- weight, are carried at great mechanical disadvantage — supported as they are along with the massive skull which bears them, at the extremity of the outstretched neck. Further, that these heavy horns may be of use in fighting, the supporting bones and imiscles must be strong enough, not simply to carry them, but to put them in motion with the rapidity required for giving blows. Let us, then, ask how, by natural selection, this complex apparatus of bones and muscles can have been developed, pan passu with the horns ? If we suppose the horns to have originally

452 THE EVOLUTION OF LIFE.

been of like size with those borne by other kinds of deer ; and if we suppose that in certain individuals, they became larger by spontaneous variation ; what would be the con- comitant changes required to render their greater size useful ? Other things equal, the blow given by a larger horn would be a blow given by a heavier mass moving at a smaller velocity : the momentum would be the same as before ; and the area of contact with the body struck being somewhat increased, while the velocity was decreased, the injury done would be less. That the horns may become better weapons, the whole apparatus which moves them must be so strength- ened as to impress more force on them, and to bear the more violent reactions of the blows given. The bones of the skull on which the horns are seated must be thickened ; otherwise they will break. To render the thickening of these bones advantageous, the vertebrae of the neck must be further de- veloped ; and without the ligaments that hold together these vertebrae, and the muscles which move them, are also enlarged^ nothing will be gained. Such modifications of the neck will be useless, or rather will be detrimental if its fulcrum be not made capable of resisting intenser strains : the upper dorsal vertebrae and their spines must be strengthened, that they may withstand the more violent contractions of the neck- muscles ; and like changes must be made on the scapular arch. Still more must there be required a simultaneous de- velopment of the bones and muscles of the fore-legs ; since each of these extra growths in the horns, in the skull, in the neck, in the shoulders, adds to the burden which the fore- legs have to bear ; unless this deer with its heavier horns, head, neck, and shoulders, had stronger fore-legs, it would not only suffer from loss of speed but would even fail in fight. Hence, to make larger horns of use, additional sizes must be acquired by numerous bones, muscles, and ligaments, as well as by the blood-vessels and nerves, on which their actions depend. On calling to mind how the spraining of a single small muscle in the foot, incapacitates for walking, or how a

INDIRECT EQUILIBRATION. 453

permanent weakness in one of its ligaments will greatly diminish the power of a limb, it will be seen that unless all these many changes are simultaneously made, they may as well be none of them made — or rather, they had better be none of them made ; since, the enlargements of some parts, by putting greater strains on connected parts, would render them relatively weaker if they remained unenlarged. Thus, then, to account by the hypothesis of natural selection, for such a structure as that of the moose deer, or the extinct Irish elk, we must suppose a spontaneous increase in the size of the horns, to be accompanied by a spontaneous increase in each of these numerous bones and muscles and ligaments directly and indirectly implicated in the use of the horns. Can we with any propriety do this ? I think not. It would be a strong supposition that the vertebrae and muscles of the neck, spontaneously enlarged at the same time as the horns. It would be a still stronger supposition that the upper dorsal vertebrae not only at the same time spontaneously became more massive, but also spontaneously altered their pro- portions in appropriate ways, by the development of their immense neural spines. And it would be an assumption still more straining our powers of belief, that along with heavier horns there should spontaneously take place the re- quired strengthenings of the scapular arch and the fore-legs. Besides the multiplication of directly-cooperative organs, the multiplication of organs that do not cooperate, save in the degree implied by their combination in the same organ- ism, seems to me a further hindrance to the development of special structures by natural selection alone. "Where the life is comparatively simple, or where surrounding circumstances render some one function supremely important, the survival of the fittest may readily bring about the appropriate struc- tural change, without any aid from the transmission of func- tionally-acquired modifications. But in proportion as the life grows complex — in proportion a, a healthy existence cannot be secured by a large endowment of come one power,

454 THE EVOLUTION OF LIFE.

but demands many powers ; in the same -proportion do there arise obstacles to the increase of any particular power, by "the preservation of favoured races in the struggle for life." As fast as the faculties are multiplied, so fast does it become possible for the several members of a species to have various kinds of superiorities over one another. While one saves its life by higher speed, another does the like by clearer vision, another by keener scent, another by quicker hearing, another by greater strength, another by unusual power of enduring cold or hunger, another by special sagacity, another by special timidity, another by special courage; and others by other bodily and mental attributes. Now it is unquestionably true that, other things equal, each of these attributes, giving its possessor an extra chance of life, is likely to be transmitted to posterity. But there seems no reason to suppose that it will be increased in subsequent generations by natural selection. That it may be thus increased, the individuals not possess- ing more than average endowments of it, must be more- fre- quently killed off than individuals highly endowed with it ; and this can happen only when the attribute is one of greater importance, for the time being, than most of the other attri- butes. If those members of the species which have but ordinary shares of it, nevertheless survive by virtue of other superiorities which they severally possess ; then it is not easy to see how this particular attribute can be developed by natural selection in subsequent generations. The probability seems rather to be, that by gamogenesis, this extra endow- ment will, on the average, be diminished in posterity — -just serving in the long run to compensate the deficient endow- ments of other individuals, whose special powers lie in other directions ; and so to keep tip the normal structure of the species. The working out of the process is here somewhat difficult to follow ; but it appears to me that as fast as the number of bodily and mental faculties increases, and as fast as the maintenance of life comes to depend less on the amount of any one, and more on the combined action of all ; so

INDIRECT EQUILIBRATION. 455

fast does the production of specialities of character by natural selection alone, become difficult. Particularly does this seem to be so with a species so multitudinous in its powers as mankind ; and above all does it seem to be so with such of the human powers as have but minor shares in aiding the struggle for life — the aesthetic faculties, for example.

It by no means follows, however, that in cases of this kind, and cases of the preceding kind, natural selection plays no part. Wherever it is not the chief agent in working organic changes, it is still, very generally, a secondary agent. The survival of the fittest must nearly always further the produc- tion of modifications which produce fitness ; whether they be modifications that have arisen incidentally, or modifications that have been caused by direct adaptation. Evidently, those individuals whose constitutions or circumstances have facili- tated the production in them of any structural change con- sequent on any functional change demanded by some new external condition, will be the individuals most likely to live and to leave descendants. There must be a natural selection of functionally-acquired peculiarities, as well as of incidental peculiarities ; and hence such structural changes in a species as result from changes of habit necessitated by changed cir- cumstances, natural selection will render more rapid than they would otherwise be.

There are, however, some modifications in the sizes and forms of parts, which cannot have been aided by natural selection ; but which must have resulted wholly from the inheritance of functionally-produced alterations. The dwind- ling away of organs of which the undue sizes entail no appreciable evils, furnishes the best evidence of this. Take, for an example, that diminution of the jaws and teeth which characterizes the civilized races, as contrasted with the savage races.* How can the civilized races have been bene-

* I am indebted to Mr Flower for the opportunity of examining the collection of skulls in the Museum of the College of Surgeons for verification of this. Un

456 THE EVOLUTION OF LIFE.

filed in the struggle for life, by the slight decrease in these comparatively-small bones ? No functional superiorly possessed by a small jaw over a large jaw, in civilized life, can be named as having caused the more frequent survival of small-jawed individuals. The only advantage which smallness of jaw might be supposed to give, is the advantage of economized nutrition ; and this could not be great enough to further the preservation of men possessing it. The de- crease of weight in the jaw and co-operative parts, that has arisen in the course of many thousands of years, does not amount to more than a few ounces. This decrease has to be divided among the many generations that have lived and died in the interval. Let us admit that the weight of these parts diminished to the extent of an ounce in a single gener- ation (which is a large admission) ; it still cannot be con- tended that the having to carry an ounce less in weight, or the having to keep in repair an ounce less of tissue, could sensibly affect any man's fate. And if it never did this — nay, if it did not cause a, frequent survival of small-jawed in- dividuals where large-jawed individuals died ; natural selec- tion could neither cause nor aid diminution of the jaw and

fortunately the absence, in most cases, of some or many teeth, prevented me from arriving at that specific result which would have been given by weighing a number of the under jaws in each race. Simple inspection, however, disclosed a sufficiently-conspicuous difference. The under jaws of Australians and Negroes, when placed side by side with those of Englishmen, were visibly larger, not only relatively but absolutely. One Australian jaw only, did I observe, that was about of the same actual size as an average English jaw ; and this (probably the jaw of a woman) belonging as it did to a much smaller skull, bore a much greater ratio to the whole body of which it formed part, than did an English jaw of the same actual size. In all the other cases, the under jaws of these inferior races (con- taining larger teeth than our own) were absolutely more massive than our own- often exceeding them in all dimensions ; and relatively to the smaller skeletons of these inferior races, they were very much more massive. Let me add that the Australian and Negro jaws are thus strongly contrasted, not with all Brit.sh jaws, but only with the jaws of the civilized British. An ancient British skull in the collection, possesses a jaw almost or quite as massive as those of the Australian skulls. And this is in harmony with the alleged relation between greater size ^* jaws and greater action of jaws, involved by the habits of savages.

INDIRECT EQUILIBRATION. 457

its appendages. Here, therefore, the decreased action of these parts which has accompanied the growth of civilized habits (the use of tools and the disuse of coarse food), must have heen the sole cause at work. During civilization this decrease of function has affected, more or less, all individuals. Through direct equilibration, diminished external stress on these parts, has resulted in diminution of the internal forces by which this stress is met. From generation to generation, this lessening of the parts consequent on functional decline has been inherited. And since the survival of individuals must always have been determined by more important struc- tural traits, this trait can have neither been facilitated nor retarded by natural selection.

§ 167. Returning from these extensive classes of facts for which Mr Darwin's hypothesis does not account, to the still more extensive classes of facts for which it does account, and which are unaccountable on any other hypothesis ; let us consider in what way this hypothesis is expressible in terms of the general doctrine of evolution. Already it has been pointed out that the evolving of modified types by " natural selection or the preservation of favoured races in the struggle for life," must be a process of equilibration, since it results in the production of organisms that are in equilibrium with their environments ; and at the outset of this chapter, some- thing was done towards showing how this continual survival of the fittest, may be understood as the progressive estab- lishment of a balance between inner and outer forces. Here, however, we must consider the matter more closely. It re- mains to be shown that this process conforms to the same general mechanical principles as do all other equilibrations.

On previous occasions we have contemplated the assem- blage of individuals composing a species, as an aggregate in a state of moving equilibrium. We have seen that its powers of multiplication give it an expansive force which is bntagonized by other forces ; and that through the rhyth'

$58 THE EVOLUTION OF LIFE.

mical variations in these two sets of forces, there is main- tained an oscillating limit to its habitat, and an oscillating limit to its numbers. On another occasion (§ 96) it was shown that the aggregate of individuals constituting a species, has a kind of general life, which, " like the life of an individual, -is maintained by the unequal and ever- varying actions of incident forces on its different parts." We saw that "'just as, in each organism, incident forces constantly produce divergences from the mean state in various direc- tions, which are constantly balanced by opposite divergences indirectly produced by other incident forces ; and just as the combination of rhythmical functions thus maintained, con- stitutes the life of the organism ; so, in a species, there is through gamogenesis a perpetual neutralization of those con- trarv deviations from the mean state, which are caused in its

if

different parts by different sets of incident forces ; and it is similarly by the rhythmical production and compensation of these contrary deviations, that the species continues to live," Hence, to understand the way in which a species is affected by causes which destroy some of its units and favour the multi- plication of others, we must consider it as a whole whose units are held together by complex forces that are ever balancing themselves and ever being disturbed — a whole whose moving equilibrium is continually being modified, and through which waves of perturbation are continually being pro- pagated. Thus much premised, let us next call to mind in what way moving equilibria in general are changed. In the first place, the necessary effect wrought by a new in- cident force falling on any part of an aggregate with balanced motions, is to produce a new motion in the direction of least resistance. In the second place, the new incident force is gradually used up in overcoming the opposing forces, and when It is all expended the opposing forces produce a recoil — a reverse deviation that counter-balances the original de- viation. Consequently, to consider whether the moving equi- librium of a species is modified in the same way as moving

INDIRECT EQUILIBRATION. 459

equilibria in general, is to consider whether, when exposed to a new force, a species yields in the direction of least resist- ance ; and whether, by its thus yielding, there is generated in the species a compensating change in the opposite direc- tion. "We shall find that it does both these things.

For what, expressed in mechanical terms, is the effect wrought on a species by some previously-unknown enemy, that kills such of its members as fail in defending them- selves ? The disappearance of those individuals which meet the destroying forces by the smallest defensive forces, is tan- tamount to the yielding of the species as a whole at the places where the resistances are the least. Or if by some general influence, such as alteration of climate, the members of a species are subject to any increase of certain external actions that are ever tending to overthrow their equilibria, and which they are ever counter-balancing by the absorp- tion of nutriment, which are the first to die ? Those that are least able to generate the internal actions which antagon- ize these external actions. If the change be an increase of the winter's cold, then such members of the species as have unusual powers of getting food or of digesting food, or such as are by their constitutional aptitude for making fat, fur- nished with reserve stores of force, available in times of scarcity, or such as have the thickest coats and so lose least

«.- •*

heat by radiation, survive ; and their survival implies that in each of them the moving equilibrium of functions presents such an adjustment of internal forces, as prevents its over- throw by the modified aggregate of external forces. Con- versely, the members that die, are, other things equal, those deficient in the power of meeting the new action by an equi- ^alent counter-action. Thus, in all cases, a species con- sidered as an aggregate in a state of moving equilibrium, has its state changed by the yielding of its fluctuating mass wherever this mass is weakest in the relation to the special forces acting on it. The conclusion is, indeed, a truism. But now, what must follow from the de-

460 THE EVOLUTION OF LIFE.

struction of the least-resisting individuals and survival of the most-resisting individuals ? On the moving equilibrium of the species as a whole, existing from generation to gener- ation, the effect of this deviation from the mean state is to produce a compensating deviation. For if all such as are de- ficient of power in a certain direction are destroyed, what must be the influence on posterity ? Had those which are destroyed lived and left offspring, the next generation would have had the same average balance of powers as preceding generations : there would have been a like proportion of in- dividuals less endowed with this power, and individuals more endowed with this power. But the more-endowed individuals being alone left to continue the race, there must result a new generation characterized by a larger average endowment of this power. That is to say, on the moving equilibrium con- stituted \)j a species, an action producing change in a given direction, is followed, in the next generation, by a reaction producing an opposite change. Observe, too, that these effects correspond in their degrees of violence. If the altera- tion of some external factor is so great that it leaves alive only a few individuals, characterized by extreme endowments of the power required to antagonize it ; then, in succeeding generations, there is a rapid multiplication of individuals similarly characterized by extreme endowments of this power — the force impressed calls out an equivalent conflicting force. Moreover, the change is temporary where the cause is temporary, and permanent where the cause is permanent. All that are deficient in the needful attribute having been killed off; and the survivors having the needful attribute in a comparatively high degree ; there will descend from them, not only some possessing equal amounts of this attribute with themselves, but also some possessing less amounts of it. If the agency which proves fatal to them has not continued in action, such less-endowed individuals will multiply ; and the species, after sundry oscillations, will return to its previous mean state. But if this agency be a persistent one, such less

INDIRECT EQUILIBRATION. 461

endowed individuals will be continually killed off; and eventually none but the highly-endowed individuals will be produced — a new moving equilibrium, adapted to the new environing conditions, will result.

It may be objected that this mode of expressing the facts, does not include the numerous cases in which a species be- comes modified in relation to surrounding agencies that do not actively influence it — cases like that of the plant which acquires hooked seed-vessels, by which it lays hold of the skins of passing animals, and makes them the distributors of its seeds — cases in which the outer agency has no direct tendency at first to affect the species, but in which the species so alters itself as to take advantage of the outer agency To cases of this kind, however, the same mode of inter pretation applies on simply changing the terms. TThile, in the aggregate of influences amid which a species exists, there are some which tend to overthrow the moving equilibria ol its members, there are others which facilitate the maintenance of their moving equilibria, and some which are capable of giving their moving equilibria increased stability : instance the spread into their habitat of some new kind of prey, which is abundant at seasons when other prey is scarce. Now what is the process by which the moving equilibrium in any species, becomes adapted to some additional external factor which furthers its maintenance ? Instead of an increased resistance to be met and counter-balanced, there is here a diminished resistance ; and the diminished resistance is equilibrated in the same way as the increased resistance. As, in the one case, there is a more frequent survival of those in- dividuals whose peculiarities of constitution enable them best to resist the new adverse factor ; so, in the other case, there is a more frequent survival of individuals whose peculiarities of constitution enable them to take advantage of the new favourable factor. In each member of the species, the balance of functions and correlated arrangement of structures, differ slightly from those existing in other members. To say th/ii

£62 THE EVOLUTION OF LIFE.

among all its members, one is better adapted than the rest to take advantage of some before-unused agency in the environ- ment, is to say that its moving equilibrium is, in so far, more stably adjusted with respect to the aggregate of surrounding influences. And if, as a consequence, this individual main- tains its moving equilibrium when others fail to do so, and produces offspring which do the like — that is, if individuals thus characterized multiply and supplant the rest ; there is evidently, as before, a process by which an equilibration be- tween the organism and its environment is effected, not im- mediately but mediately, through the continuous intercourse between the species as a whole and the environment.

§ 168. Thus we see that indirect equilibration does what- ever direct equilibration cannot do. It is scarcely possible too much to emphasize the conclusion, that all these processes by which organisms are re-fitted to their ever-changing environments, must be equilibrations of one kind or other. As authority for this conclusion, we have not simply the universal truth that change of every order is towards equi- librium ; but we have also the truth which holds throughout the organic world, that life itself is the maintenance of amoving equilibrium between inner and outer actions — the continuous adjustment of internal relations to external relations ; or the maintenance of a correspondence between the forces to which an organism is subject and the forces which it evolves. For if the preservation of life is the preservation of such a moving equilibrium, it becomes a corollary that those changes which enable a species to live under altered conditions, are changes towards equilibrium with the altered conditions.

Hence, all such changes being equilibrations, their differ- ences can be nothing but differences in the ways through which they result. If they are not effected immediately, they must be effected mediately. A priori, therefore, we may be certain that all processes of modification which do

INDIRECT EQUILIBRATION. 463

not come within the class of direct equilibrations, must come within the class of indirect equilibrations.

Examination of the facts confirms this conclusion. The external factors to which a species is exposed, are of two kinds. They are such as act continuously or frequently on the individuals ; or they are such as do not act continuously or frequently on the individuals. To a factor which continuously or frequently acts on the individuals, the functions of the in- dividuals re-adjust themselves — there is direct equilibration. While a factor which does not act continuously or fre- quently on the individuals, acts continuously on the species as a whole — either destroying such of the members as are least capable of resisting it, or fostering such of the members as are most capable of taking advantage of it. And by the abstraction, generation after generation, of those least in equilibrium with the new factor ; or by the extra multipli- cation, generation after generation, of those most in equi- librium with the new factor ; the species as a whole is event- ually brought into complete equilibrium with the new factor —there is indirect equilibration.

CHAPTER XIII.

-

THE CO-OPERATION OF THE FACTORS.

8 169. THUS the phenomena of organic evolution, ma^v be interpreted in the same way as the phenomena of all other evolution. Those universal laws of the re-distribution of matter and motion, to which things in general conform, are conformed to by all living things ; whether considered in their individual histories, in their histories as species, or in their aggregate history. However otherwise they may ordin- arily be expressed, the truths of development as exhibited in the animal and vegetal kingdoms, prove to be expressible as manifestations of those abstract truths set forth in First Principles. Fully to see this, it will be needful for us to con- template in their ensemble, the several processes separately described in the four preceding chapters.

If the forces acting on any aggregate remain the same, the changes produced by them in the aggregate will presently reach a limit, at which the constant outer forces are balanced by the constant inner forces ; and thereafter no further me- tamorphosis will take place. Hence, that there may be continuous changes of structure in organisms, there must be continuous changes in the incident forces. This condition to the evolution of animal and vegetal forms, we find to be fully satisfied. The astronomic, geologic, and meteorologic changes that have been slowly but incessantly going on, and have been increasing in the complexity of their combinations.

THE CO-OPERATION OF THE FACTORS. 465

have been perpetually altering the circumstances of organ- isms ; and organisms, as they have become more numerous in their kinds and higher in their kinds, have been perpetually altering one another's circumstances. Thus, for those pro- gressive modifications upon modifications which organic evo- lution implies, we find a sufficient cause in the modifications after modifications, which every environment over the Earth's surface has been undergoing, throughout all geologic and pre- geologic times. The progressive inner changes for

which we thus find a cause in the continuous outer changes, conform, so far as we can trace them, to that universal law of the instability of the homogeneous, which is manifested throughout evolution in general. We see that in organisms, as in all other things, the exposure of different parts to different kinds and amounts of incident forces, has necessi- tated their differentiation ; and that for the like reason, aggregates of individuals have been lapsing into varieties, and species, and genera, and classes. We also see that in each type of organism, as in the aggregate of types, the mul- tiplication of effects has continually aided this transition from a more homogeneous to a more heterogeneous state. And yet again, we see that that increasing segregation, and con- comitant increasing definiteness, which characterizes the growing heterogeneity of organisms, has been insured by the necessary maintenance of them under combinations of forces not greatly unlike preceding combinations — by the continual destruction of those which expose themselves to aggregates of external actions markedly incongruous with the aggregates of their internal actions, and the survival of those subject only to comparatively small incongruities. Finally,

we have found that each change of structure, superposed on preceding changes, has been a re-equilibration necessitated by the disturbance of a preceding equilibrium. The maintenance of life being the maintenance of a balanced combination of functions, it follows that individuals and species that have continued to live, are individuals and species in which the

166 THE EVOLT TI<-,N OF LIFE.

balance of functions lias not been overthrown. Inevitably, therefore, survival through successive changes of conditions, implies successive adjustments of the balance to the new con- ditions. This deduction we find to be inductively verified. What is ordinarily called adaptation, is, when translated into mechanical terms, direct equilibration. And that process which, under the name of natural selection, Mr Darwin has shown to be an ever-acting means of fitting the structures of organisms to their circumstances, we find, on analysis, to be expressible in mechanical terms as indirect equilibration.

The actions that are here specified in succession, are in reality simultaneous ; and they must be so conceived before organic evolution can be rightly understood. Some aid towards so conceiving them, will be given by the annexed table, representing the co-operation of the factors.

§ 170. Respecting this co-operation of these factors, it re- mains only to point out their respective shares in producing the total result ; and the way in which the proportions of their respective shares vary as evolution progresses.

At first, changes in the amounts and combinations of ex- ternal inorganic forces, astronomic, geologic, and meteoro- logic, were the only causes of the successive modifications undergone by organisms ; and these changes have continued, and must still continue, to be causes of such modifications. As, however, through the diffusion of organisms, and the consequent differential actions of inorganic forces on them, there arose unlikenesses among organisms, producing varieties, species, genera, orders, classes, &c. ; the actions of organisms on one another became new sources of organic modifications. And as fast as types have multiplied, and become more com- plex ; so fast have the mutual actions of organisms come to be more influential factors in their respective evolutions. Until, eventually, fts we see exemplified in the human race, they have come to be the chief factors.

Passing from the external causes of change to the internal

THE CO-OPERATION OF THE FACTORS.

467

which, partially in the first gen oration, and completely in the course of gener- ations, are directly equiliuratcd with the changed agencies.	which have their direct equilibration with the changed agencies, aided by indirect equilibration, through the more frequent survival of those in which the direct equilibration is most rapid.	positively — byaidingthemultiplication of those whose moving equilibria hap- pen to be most congruous with the changed agencies : thus, in the course of generations, indirectly equilibrating certain individuals with them.	negatively — by killing those whose moving equilibria are most incon- gruous with the changed agencies : thus, in the course of generations, in- directly equilibrating each of its sur- viving individuals with them.	some parts of the habitat more than in s differentiating the species into local	' and thus causing differentiations of the species into varieties, irre- spective of locality.	and thus causing modification of the species as aw hole.by abstract- . ing a certain class of its units.
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468 THE EVOLUTION OF LIFE.

processes of change entailed by them, we see that these, too, have varied in their proportions — that which was originally the most important and almost the sole process, becoming gradually less important, if not at last the least important. Always there must have been, and always there must con-' tinue to be, a survival of the fittest : natural selection must have been in operation at the outset, and can never cease to operate. While yet organisms had comparatively feeble powers of co-ordinating their actions, and adjusting them to environing actions, natural selection worked almost alone in moulding and re-moulding organisms into fitness for their changing environments ; and natural selection has re- mained almost the sole agency by which plants and in- ferior orders of animals have been modified and developed. The equilibration of organisms that are comparatively passive, is necessarily effected indirectly, by the action of incident forces on the species as a whole. But along with the gradual evolution of organisms having some activity, there grows up a kind of equilibration that is relatively direct. In propor- tion as the activity increases, direct equilibration plays a more important part. Until, when the nervo-muscular apparatus becomes greatly developed, and the power of vary- ing the actions to fit the varying requirements becomes con- siderable, the share taken by direct equilibration rises into co-ordinate importance. We have seen reason to think that as fast as essential faculties multiply, and as fast as the num- ber of organs that co-operate in any given function increases, indirect equilibration through natural selection, becomes less and less capable of producing specific adaptations ; and re- mains fully capable only of maintaining the general fitness of constitution to conditions. Simultaneously, the production of adaptations by direct equilibration, takes the first place — indirect equilibration serving to facilitate it. Until at length, among the civilized human races, the equilibration becomes mainly direct : the action of natural selection being restricted to the destruction of those who are constitutionally too feeble

THE CO-OPERATION OF THE FACTORS. 469

to live, even with external aid. As the preservation of in- capables is habitually secured by our social arrangements ; and as very few except criminals are prevented by their in- feriorities from leaving the average number of offspring (in- deed the balance of fertility is probably in favour of the in- ferior) ; it results that survival of the fittest, can scarcely at all act in such way as to produce specialities of nature, either bodily or mental. Here the specialities of nature, chiefly mental, which we see produced, and which are so rapidly produced that a few centuries show a considerable change, must be ascribed almost wholly to direct equilibration.*

* As having an instructive bearing on the question of the varieties of Man, let me here refer to a paper on " The Origin of the Human Races " read before the Anthropological Society, March 1st, 1864, by Mr Alfred Wallace— a gentle- man well known among naturalists, as having independently thought out the hypothesis of natural selection, though at a later date, and less elaborately, than Mr Darwin. In this paper, Mr Wallace shows, very clearly I think, that along with the attainment of that degree of intelligence implied by the use of imple- ments, clothing, &c., there arises a tendency for modifications of brain to take the place of modifications of body — still, however, regarding the natural selection of spontaneous variations, as the cause of the modifications. But if the foregoing arguments be valid, natural selection here plays but the secondary part of fur- thering the adaptations otherwise caused. It is true that, as Mr Wallace argues, and as I have myself briefly indicated (see Westminster Eevieiv, for April, 1852, pp. 496 — 501), the natural selection of races, leads to the survival of the more cerebrally-developed, while the less cerebrally-developed disappear, ."3'it though natural selection acts freely in the struggle of one society with another ; yet, among the units of each society, its action is so interfered with, that there remains no ade- quate cause for the acquirement of mental superiority by one race over another, except the inheritance of functionally-produced modifications. This view, how- ever, agrees equally well with Mr Wallace's conclusion, that at a certain stage of evolution, the brain hep-ins to change much more than the

CHAPTER XIV.

THE CONVERGENCE OF THE EVIDENCES.

§ 171. OF the three classes of evidences that have been assigned, the a priori, which we took first, were partly nega- tive, partly positive.

On considering the " General Aspects of the Special-crea- tion-hypothesis," we discovered it to be worthless. Discredited by its origin, and wholly without any basis of observed fact, we found that it was not even a thinkable hypothesis ; and while thus intellectually illusive, it turned out on examina- tion to have moral implications quite at variance with the professed beliefs of those who hold it.

Contrariwise, the " General Aspects of the Evolution-hy- pothesis," begot the stronger faith in it the more nearly they were considered. By its lineage and its kindred, it was found to be as closely allied with the proved truths of modern science, as is the antagonist hypothesis with the proved errors of ancient ignorance. Instead of being a mere pseud- idea, we saw that it admitted of elaboration into a definite conception — so showing its legitimacy as an hypothesis. In- stead of positing a purely fictitious process, the process which it alleges, we saw to be one that is actually going on around us. To which add, that morally considered, this hypothesis presents no irreconcilable incongruities.

Thus, even were we without further means of judging,

THE CCXNVERGENCE OF 1HE EVIDENCES. 471

there could be no rational hesitation which of the two views should be entertained.

§ 172. Further means of judging, however, we found to be afforded by bringing the two hypotheses face to face with the general truths established by naturalists. These induct- ive evidences were dealt with in four chapters.

" The Arguments from Classification " were these. Organ- isms fall into groups within groups ; and this is the arrange- ment which we see results from evolution, where it is known to take place. Of these groups within groups, the great or primary ones are the most unlike, the sub-groups are less unlike, the sub- sub- groups still less unlike, and -so on; and this, too, is a characteristic of groups demonstrably produced by evolution. Moreover, indefiniteness of equivalence among the groups, is common to these which we know have been evolved, and those here supposed to have been evolved. And then there is the further significant fact, that divergent groups are allied through their lowest rather than their highest members — a truth which the hypothesis of evolution implies.

Of " the Arguments from Embryology," the first and most striking is, that when the developments of embryos are traced from their common starting point, and their divergences and re- divergences symbolized by a genealogical tree, there is manifest a general parallelism between the arrangement of its primary, secondary, and tertiary branches, and the arrangement of the divisions and sub-divisions of our classi- fications— a general parallelism to be anticipated as a result of evolution. Nor do those minor deviations from this general parallelism, which at first sight look like difficulties, tail, on closer observation, to become additional supports ; since those traits of a common ancestry which embryology reveals, are, if modifications have resulted from changed con- ditions, liable to be distorted or disguised in quite different ways and degrees in different lines of descendants

472 THE EVOLUTION OF LIFE.

We next considered " the Arguments from Morphology/' Leaving out those kinships among organisms disclosed by their developmental metamorphoses, the kinships which their adult forms show are profoundly significant. The remarkable unities of type which are found under such different exter- nals, are inexplicable except as results of community of de- scent with non-community of modification. Again, each organism analyzed apart, shows us, in the likenesses obscured by unlikenesses of its component parts, a peculiarity of struc- ture that can be ascribed only to the formation of a more heterogeneous organism out of a more homogeneous one. And once more, the habitual existence of rudimentary organs, homologous with organs that are developed in allied animals or plants, while it admits of no other rational interpretation, has a satisfactory interpretation given to it by the hypo- thesis of evolution.

Last of the inductive evidences, came " the Arguments from Distribution." While the phenomena of distribution in Space, prove to be unaccountable as results of designed adapt- ation of organisms to their habitats, they prove to be accountable as results of the competition of species, and the spread of the superior into the habitats of the inferior, fol- lowed by the changes which new conditions induce. Though the phenomena of distribution in Time, are so fragmentary that no positive conclusion can be drawn from them ; yet all of them are reconcileable with the hypothesis of evolution, and some of them yield it strong support — especially the near relationship that exists between the living and extinct types of each great geographical area.

In each of these four groups, we thus found several argu- ments which point to the same conclusion ; and the con- clusion pointed to by the arguments of any one group, is that pointed to by the arguments of all the other groups. This coincidence of coincidences, would give to the induction a very high degree of probability, even were it not enforced by deduction.

THE CONVERGENCE OF THE EVIDENCES. 473

§ 173. But the conclusion deductively reached, is in har- mony with the inductive conclusion. Passing from the evi- dence that evolution has taken place, to the question — How has it taken place ? we find in known agencies and known processes, adequate causes of its phenomena.

In astronomic, geologic, and meteorologic changes, ever in progress, ever combining in new and more involved ways, we have a set of inorganic factors to which all organisms are exposed ; and in the varying and complicating actions of organisms on one another, we have a set of organic factors that alter with increasing rapidity. Thus, speaking gener- ally, all members of the Earth's Flora and Fauna are con- tinually passing into new environments — experience per- petual re-arrangements of external forces.

Each organic aggregate, whether considered individually or as a continuously- existing species, is modified afresh by each fresh distribution of external forces. To its pre-exist- ing differentiations, new differentiations are added ; and thus that lapse from a more homogeneous to a more heterogeneous state, which would have a fixed limit were the circumstances fixed, has its limit perpetually removed by the perpetual change of the circumstances. Meanwhile, that growing com- plexity of structure thus produced, must, in the average of cases, be accompanied by an increasing definiteness of struc- ture ; since only those organisms can survive which subject themselves to aggregates of forces that are not, in their essen- tials, greatly unlike those with which their structures cor- respond. And at the same time that progression is thus necessitated as a general result ; yet, as change of structure arises only where there is change in the distribution of forces, it will not take place in organisms which elude changes in the distribution of forces, by migration or otherwise.

These modifications upon modifications which result in evolution structurally considered, are the accompaniments of those functional alterations continually required to re- equilibrate inner with outer actions. That moving equi-

21

474 THE EVOLUTION OF LIFE.

librium of inner actions corresponding with outer actions, which constitutes the life of an organism, must either be overthrown by a change in the outer actions, or must undergo perturbations that cannot end until there is a re-adjusted balance of functions and correlative adaptation of structures. Wherever the external changes are such as to be continuously or frequently operative on individuals, this direct equilibra- tion must go on.

But where the external changes are either such as are fatal when experienced by the individuals, or such as act on the individuals in ways that do not affect the equilibrium of their functions ; then the re- adjustment results through the effects produced on the species as a whole — there is indirect equilibration. By natural selection or survival of the fittest — by the preservation in successive generations of those whose moving equilibria happen to be least at variance with the requirements, there is eventually produced a changed equilibrium completely in harmony with the requirements.

And thus it results that those universal laws of the re-dis- tribution of matter and motion, which are conformed to by evolution in general, are conformed to by organic evolution.

§ 174. Even were this the whole of the evidence assign- able for the belief that organisms of all orders have been gradually evolved, this belief would have a warrant much higher than that of very many beliefs that are regarded as established. When we see that there are strong a priori pro- babilities in its favour, and wholly adverse to the antagonist hypothesis — when an examination of the facts which natural- ists have accumulated, leads us to several groups of inductions which unite in supporting it — and when the characteristics which conspire to show that organic evolution has been going on, prove to be deducible from those universal actions known to work evolution of all other kinds ; we have a combination of proofs which might suffice were there no more to be said.

But the evidence is far from exhausted. At the outset of

THE CONVERGENCE OF THE EVIDENCES. 475

the argument, it was remarked that the ensemble of vital phenomena presented by the organic world as a whole, can- not be properly dealt with apart from the ensemble of vital phenomena presented by each organism, in the course of its growth, development, and decay. The interpretation of either implies interpretation of the other ; since the two are in reality parts of one process. Hence, the validity of any hypothesis respecting the one class of phenomena, may be tested by its congruity with phenomena of the other class. We are now about to pass to the more special phenomena of development, as displayed in the structures and functions of individual organisms. If the hypothesis that plants and animals have been progressively evolved, be true, it must furnish us with keys to these phenomena. We shall find that it does this ; and by doing it, gives numberless additional vouchers for its truth.

APPENDIX

[The following letter, originally written, for publication in the North American Review, but declined by the Editor in pursuance of a general rule, and eventually otherwise published in the United States, I have thought well to append to this first volume of the Principles of Biology. / do this because the questions which it discusses are dealt with in this volume ; and because the further ex- planations it furnishes seem needful to prevent misapprehensions.]

ON ALLEGED " SPONTANEOUS GENERATION," AND ON THE HYPOTHESIS OF PHYSIOLOGICAL UNITS.

The Editor of the North American Review. SIR,

It is in most cases unwise to notice adverse criticisms. Either they do not admit of answers or the answers may be left to the penetration of readers. When, however, a critic's allegations touch the fundamental propositions of a book, and especially when they appear in a periodical having the position of the North A merican Review, the case is altered. For these reasons the article on "Philosophical Biology," published in your last number, demands from me an attention which ordinary criticisms do not.

It is the more needful for me to notice it, because its two leading objections have the one an actual fairness and the other an apparant fairness ; and in the absence of explanations from me, they will be considered as substantiated even by many, or perhaps most, of those who have read the work itself — much more by those who have not read it. That to prevent the spread of misapprehensions I ought to say something, is further shown by the fact that the same two objec- tions have already been made in England — the one by Dr. Child, of Oxford, in his Essays on Physiological Subjects, and the other by a writer in the Westminster Reuieiv for July, 1865.

In the note to which your reviewer refers, I have, as he says, tacitly repudiated the belief in "spontaneous generation;" and that I have done this in such a way as to leave open the door for the interpre- tation given by him is true. Indeed the fact that Dr. Child, whose criticism is a sympathetic one, puts the same construction on this note, proves that your reviewer has but drawn what seems to be a necessary inference. Nevertheless, the inference is one which I did not intend to be drawn.

In explanation, let me at the outset remark that I am placed at a disadvantage in having had to omit that part of the System of Philosophy which deals with Inorganic Evolution. In the orginal programme will be found a parenthetic reference to this omitted part, which should, as there stated, precede the Principles of Biology.

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Two volumes are missing. The closing chapter of the second, were it written, would deal with the evolution of organic matter — the step preceding the evolution of living forms. Habitually carrying with me in thought the contents of this unwritten chapter, I have, in some cases, expressed myself as though the reader had it before him ; and have thus rendered some of my statements liable to misconstructions. Apart from this, however, the explanation of the apparent inconsistency is very simple, if not very obvious. In the first place, I do not believe in the "spontaneous generation" commonly alleged, and referred to in the note ; and so little have I associated in thought this alleged " spontaneous generation " which I dis- believe, with the generation by evolution which I do believe, that the repudiation of the one never occurred to me as liable to be taken for re- pudiation of the other. That creatures having quite specific structures are evolved in the course of a few hours, without antecedents calculated to determine their specific forms, is to me incredible. Not only the established truths of Biology, but the established truths of science in general, negative the supposition that organisms having struc- tures definite enough to identify them as belonging to known genera and species, can be produced in the absence of germs derived from antecedent organisms of the same genera and species. If there can suddenly be imposed on simple protoplasm the organization which constitutes it a Paramoecium^ I see no reason why animals of greater complexity, or indeed of any complexity, may not be constituted after the same manner. In brief, I do not accept these alleged facts as exemplifying Evolution, because they imply something immensely beyond that which Evolution, as I understand it, can achieve. In the second place, my disbelief extends not only to the alleged cases of "spontaneous generation," but to every case akin to ihem. The very conception of spontaneity is wholly incongruous with the con- ception of Evolution. For this reason I regard as objectionable Mr. Darwin's phrase " spontaneous variation " (as indeed he does himself) ; and I have sought to show that there are always assignable causes of variation. No form of Evolution, inorganic or organic, can be spontaneous ; but in every instance the antecedent forces must be adequate in their quantities, kinds, and distributions, to work the observed effects. Neither the alleged .cases of " spontaneous gene- ration,'1 nor any imaginable cases in the least allied to them, fulfil this requirement.

If, accepting these alleged cases of " spontaneous generation," I had assumed, as your reviewer seems to do, that the evolution of organic life commenced in an analogous way ; then, indeed, I should have left myself open to a fatal criticism. This supposed " spon- taneous generation" habitually occurs in menstrua that contain either organic matter, or matter originally derived from organisms ; and such organic matter, proceeding in all known cases from organ- isms of a higher kind, implies the pre-cxistence of such highcf

481

By what kind of logic, then, is it inferrible that organic life was initiated after a manner like that in which Infusoria are said to be now spontaneously generated? Where, before life commenced, were the superior organisms from which these lowest organisms obtained their organic matter? Without doubting that there are those who, as the reviewer says, " can penetrate deeper than Mr. Spencer has done into the idea 'of universal evolution," and who. as he contends, prove this by accepting the doctrine of " spontaneous generation " ; I nevertheless think that I can penetrate deep enough to see that a tenable hypothesis respecting the origin of organic life must be reached by some other clue than that fur- nished by experiments on decoction of hay and extract of beef.

From what I do not believe, let me now pass to what I do believe. Granting that the formation of organic matter, and the evolution of life in its lowest forms, may go on under existing cosmical condi- tions ; but believing it more likely that the formation of such matter and such forms, took place at a time when the heat of the Earth's surface was falling through those ranges of temperature at which the higher organic compounds are unstable; I conceive that the moulding of such organic matter into the simplest types, must have commenced with portions of protoplasm more minute, more in- definite, and more inconstant in their characters, than the lowest Rhizopods— less distinguishable from a mere fragment of albumen than even the Protogenes of Professor Haeckel. The evolution of specific shapes must, like all other organic evolution, have resulted from the actions and reactions between such incipient types and their environments, and the continued survival of those which hap- pened to have specialities best fitted to the specialities of their en- vironments. To reach by this process the comparatively well-spe- cialized forms of ordinary Infusoria^ must, I conceive, have taken an enormous period of time.

To prevent, as far as may be, future misapprehension, let me elaborate this conception so as to meet the particular objections raided. The reviewer takes for granted that a " first organism " must be assumed by me, as it is by himself. But the conception of a " first organism," in anything like the current sense of the words, is wholly at variance with conception of evolution ; and scarcely less at variance with the facts revealed by the microscope. The lowest living things are not properly speaking organisms at all ; for they have no distinctions of parts — no traces of organization. It is almost a misuse of language to call them " forms" of life : not only are their outlines, when distinguishable, too unspecific for description, but they change from moment to moment and are never twice alike, either in two individuals or in the same individual. Even the word 4- type " is applicable in but a loose way ; for there is little constancy in their generic characters : according as the surrounding conditions determine, they undergo transformations now of one kind and now of

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another. And the vagueness, the inconstancy, the want of appre- ciable structure, displayed by the simplest of living things as we now see them, are characters (or absences of characters) which, on the hypothesis of Evolution, must have been still mors decided when, asat first, no " forms," no " types," no " specific shapes," had been moulded. That " absolute commencement of organic life on the globe," which the reviewer says I " cannot evade the admission of," I distinctly deny. The affirmation of universal evolution is in itself the negation of an " absolute commencement " of anything. Construed in terms of evolution, every kind of being is conceived as a product of modifications wrought by insensible gradations on a pre-existing kind of being ; and this holds as fully of the supposed "commencement of organic life" as of all subsequent developments of organic life. It is no more needful to suppose an " absolute commencement of organic life " or a " first organism," than it is needful to suppose an absolute commencement of social life and a first social organism. The assumption of such a necessity in this last case, made by early speculators with their theories of " social contracts " and the like, is disproved by the facts ; and the facts, so far as they are ascertained, disprove the assumption of such a necessity in the first case. That organic matter was not produced all at once, but was reached through steps, we are well warranted in believing by the experiences of chemists. Organic matters are produced in the laboratory by what we may literally call artificial evolution. Chemists find themselves unable to form these complex combinations directly from their elements ; but they succeed in form- ing them indirectly, by successive modifications of simpler combina- tions. In some binary compound, one element of which is present in several equivalents, a change is made by substituting for one of these equivalents an equivalent of some other element ; so producing a ternary compound. Then another of, the equivalents is replaced, and so on. For instance, beginning with ammonia, N H3, a higher form is obtained by replacing one of the atoms of hydrogen by an atom of methyl, so producing methyl-amine, N (C H3 H2) 5 and then, under the further action of methyl, ending in a further substitution, there is reached the still more compound sub- stance dimethyl-amine, N (C H0) (C H3) H. And in this manner highly complex substances are eventually built up. Another characteristic of their method is no less significant. Two com- plex compounds are employed to generate, by their action upon one another, a compound of still greater complexity : different heterogeneous molecules of one stage, become parents of a mole- cule a stage higher in heterogeneity. Thus, having built up acetic acid out of its elements, and having by the process of substitution de- scribed above, changed the acetic acid into propionic acid, and propi-

onic into butyric, of which the formula is < Q x /-rr Q\ [ ;

483

this complex compound, by operating on anotner complex compound, such as the dimethyl-amine named above, gene- rates one of still greater complexity, butyrate of dimethyl-amiiie

I c otiity HS) H | N (0 Hs) (C Hs) H- See> then' the

remarkable parallelism. The progress towards higher types of organic molecules is effected by modifications upon modifications ; as throughout Evolution in general. Each of these modifications is a change of the molecule into equilibrium with its environment — an adaptation, as it were, to new surrounding conditions to which it is subjected ; as throughout Evolution in general. Larger, or more integrated, aggregates (for compound molecules are such) are successively generated; as throughout Evolution in general. More complex or heterogeneous aggregates are so made to arise, one out of another ; as throughout Evolution in general. A geometrically- increasing multitude of these larger and more complex aggregates so produced, at the same time results ; as throughout Evolution in general. And it is by the action of the successively higher forms on one another, joined with the action of environing conditions, that the highest forms are reached ; as throughout Evolution in general. "When we thus see the identity of method at the two extremes — when we see that the general laws of evolution, as they are exem- plified in known organisms, have been unconsciously conformed to by chemists in the artificial evolution of organic matter ; we can scarcely doubt that these laws were conformed to in the natural evolution of organic matter, and afterwards in the evolution of the simplest organic forms. In the early world, as in the modern laboratory, inferior types of organic substances, by their mutual actions under fit conditions, evolved the superior types of organic substances, ending in organizable protoplasm. And it can hardly be doubted that the shaping of organizable protoplasm, which is a substance modifiable in multitudinous ways with extreme facility, went on after the same manner. As I learn from one of our first chemists, Prof. Frankland, protein is capable of existing under probably at least a thousand isomeric forms ; and, as we shall presently see, it is capable of forming, with itself and other elements, substances yet more intricate in composi- tion, that are practically infinite in their varieties of kind. Exposed to those innumerable modifications of conditions which the Earth's surface afforded, here in amount of light, there in amount of heat, and elsewhere in the mineral quality of its aqueous medium, this extremely changeable substance must have undergone now one, now another, of its countless metamorphoses. And to the mutual influences of its metamorphic forms under favouring con- ditions, we may ascribe the production of the still more composite, still more sensitive, still more variously-changeable portions of organic matter, which, in masses more minute and simpler than

484

existing Protozoa, displayed actions verging IRtle by little into those called vital — actions which protein itself exhibits in a certain degree, and which the lowest known living things exhibit only in a greater degree. Thus, setting out with inductions from the experiences of organic chemists at the one extreme, and with inductions from the observations of biologists at the other extreme, we are enabled deductively to bridge the interval — • are enabled to conceive how organic compounds were evolved, and how, by a continuance of the process, the nascent life displayed in these became gradually more pronounced. And this it is which has to be explained, and which the alleged cases of " spontaneous generation " would not, were they substantiated, help us in the least to explain.

It is thus manifest, I think, that I have not fallen into the alleged inconsistency. Nevertheless, I admit that your reviewer was justified in inferring this inconsistency ; and I take blame to myself for not having seen that the statement, as I have left it, is open to misconstruction.

I pass now to the second allegation — that in ascribing to certain specific molecules, which I have called " physiological units," the aptitude to build themselves into the structure of the organism to which they are peculiar, I have abandoned my own principle, and have assumed something beyond the re-distribution of Matter and Motion. As put by the reviewer, his case appears to be well made out ; and that he is not altogether unwarranted in so putting it, may be admitted. Nevertheless, there does not in reality exist the supposed incongruity.

Before attempting to make clear the adequacy of the conception which I am said to have tacitly abandoned as insufficient, let me remove that excess of improbability the reviewer gives to it, by the extremely-restricted meaning with which he uses the word mechani- cal. In discussing a proposition of mine he says : —

" He then cites certain remarks of -Mr. Paget on the permanent effects wrought in the blood by the poison of scarlatina and small-pox, as justify- ing the belief that such a 'power' exists, and attributes the repair of a wasted tissue to ' forces analogous to those by which a crystal reproduces its lost apex.' (Neither of which phenomena, however, is explicable by mechanical causes.)"

Were it not for the deliberation with which this last statement is made, I should take it for a slip of the pen. As it is, however, I have no course left but to suppose the reviewer unaware of the fact that molecular actions of all kinds are now not only conceived as mechanical actions, but that calculations based on this conception of them, bring out the results that correspond with observation. There is no kind of re-arrangement among molecules (crystallization being one) which the modern physicist does not think of,

485

and correctly reason upon, in terms of forces and motions like tho<e of sensible masses. Polarity is regarded as a resultant of suck forces arid motions ; and when, as happens in many cases, light changes the molecular structure of a crystal, and alters its polarity, it does this by impressing, in conformity with mechanical taws, new motions on the constituent molecules. That the reviewer should present the mechanical conception under so extremely limited a form, is the more surprising to me because, at the outset of the very work he reviews, 1 have, in various passages, based inferences on those immense extensions of it which he ignores ; indicating, for example, the interpretation it yields of the inorganic chemical changes effected by heat, and the organic chemical changes effected by light (Principles of Biology, § 13.J

Premising, then, that the ordinary idea of mechanical action must be greatly expanded, let us enter upon the question at issue — the sufficiency of the hypothesis that the structure of each organism is determined by the polarities of the special molecules, or physiologi- cal units, peculiar to it as a species, which necessitate tendencies towards special arrangements. My proposition and the reviewer's criticism upon it, will be most conveniently presented if I quote in full a passage of his from which 1 have already extracted some ex- pressions. He says : —

" It will be noticed, however, that Mr. Spencer attributes the possession of these 'tendencies,' or 'proclivities,' to natural inheritance from ancestral organisms ; and it may be argued that he thus saves the mechanist theory and his own consistency at the same time, inasmuch as he derives even the ' tendencies ' themselves ultimately from the environ- ment. To this we reply, that Mr. Spencer, who advocates the nebular hypothesis, cannot evade the admission of an absolute commencement of organic life on the globe, and that the ' formative tendencies,' without which he cannot explain the evolution of a single individual, could not have been inherited by the first organism. Besides, by his virtual denial of spontaneous generation, he denies 1hat the first organism was evolved out of the inorganic, world, and thus shuts himself off from the argument (otherwise plausible) that its ' tendencies ' were ultimately derived f roir the environment."

This assertion is already in great measure disposed of by what has been said above. Holding that, though not " spontaneously generated," those minute portions of protoplasm which first dis- played in the feeblest degree that changeability taken to imply life, were evolved, I am not debarred from the argument that the "ten- dencies " of the physiological units are derived from the inherited effects of environing actions. If the conception of a t: first organ- ism " were a necessary one, the reviewer's objection would be valid. If there were an " absolute commencement " of life, a definite line parting organic matter from the simplest living forms, I should be placed in the predicament he describes. But as the doctrine of Evolution itself tacitly negatives any such distinct separation ; and as the negation is the more confirmed by the facts the more we

436

know of them ; I do not feel that I am entangled in the alleged difficulty. My reply might end here ; but as the hypothesis in ques- tion is one not easily conceived, and very apt to be misunderstood, I will attempt a further elucidation of it.

Much evidence now conspires to show that molecules of the sub- stances we call elementary are in reality compound ; and that, by the combination of these with one another, and re-combinations of the products, there are formed systems of systems of molecules, un- imaginable in their complexity. Step by step as the aggregate molecules so resulting, grow larger and increase in heterogeneity, they become more unstable, more readily transformable by small forces, more capable of assuming various characters. Those com- posing organic matter transcend all others in size and intricacy of structure ; and in them these resulting traits reach their extreme. As implied by its name protein, the essential substance of which organisms are built, is remarkable alike for the variety of its meta- morphoses and the facility with which it undergoes them : it changes from one to another of its thousand isomeric forms on the slightest change of conditions. Now there are facts warranting the belief that though these multitudinous isomeric forms of protein will not unite directly with one another, yet they admit of being linked to- gether by other elements with which they combine. And it is very significant that there are habitually present two other elements, sulphur and phosphorus, which have quite special powers of holding together many equivalents — the one being pentatomic and the other hexatomic. So that it is a legitniate supposition (justified by analo- gies) that an atom of sulphur may be a bond of union among half- a-dozen different isomeric forms of protein; and similarly with phos- phorus. A moment's thought will show that, setting out with the thousand isomeric forms of protein, this makes possible a number of these combinations almost passing the power of figures to express. Molecules so produced, perhaps exceeding in size and complexity those of protein as those of protein exceed those of inorganic matter, may, I conceive, be the special units belonging to special kinds of organisms. By their constitution they must have a plasticity, or sensitiveness to modifying forces, far beyond that of protein ; and bearing in mind not only that their varieties are practically infinite in number, but that closely allied forms of them, chemically in- different to one another as they must be, may coexist in the same aggregate, we shall see that they are fitted for entering into un- limited varieties of organic structures.

The existence of such physiological units, peculiar to each species of organism, is not unaccounted for. They are evolved simul- taneously with the evolution of the organisms they compose — they differentiate as fast as these organisms differentiate; and are made multitudinous in kind by the same actions which make the organism the/ compose multitudinous in kind. This conception is clearly

•16?

representable in terms of the mechanical hypothesis. Every physicist will endorse the proposition that in each aggregate there tends to establish itself an equilibrium between the forces exercised by all the units upon each and by each upon all. Even in masses of substance so rigid as iron and glass, there goes on a molecular re-arrangement, slow or rapid according as circumstances facilitate, which ends only when there is a complete balance between the actions of the parts on the whole and the actions of the whole on the parts : the implication being that every change in the form or size of the whole, necessitates some redistribution of the parts. And though in cases like these, there occurs only a polar re-arrangement of the molecules, without changes in the molecules themselves ; yet where, as often happens, there is a passage from the colloid to the crystalloid state, a change of constitution occurs in the molecules themselves. These truths are not limited to inorganic matter : they unquestionably hold of organic matter. As certainly as molecules of alum have a form of equilibrium, the octahedron, into which they fall when the temperature of their solvent allows them to aggregate, so certainly must organic molecules of each kind, no matter how complex, have a form of equilibrium in which, when they aggregate, their complex forces are balanced — a form far less rigid and definite, for the reason that they have far less definite polarities, are far more un- stable, and have their tendencies more easily modified by environing conditions. Equally certain is it that the special molecules having a special organic structure as their form of equilibrium, must be reacted upon by the total forces of this organic structure ; and that, if environing actions lead to any change in this organic structure, these special molecules, or physiological units, subject to a changed distribution of the total forces acting upon them will undergo modification — modification which their extreme plasticity will render easy. By this action and reaction I conceive the physio- logical units peculiar to each kind of organism, to have been moulded along with the organism itself. Setting out with the stage in which protein in minute aggregates, took on those simplest differentiations which fitted it for differently- conditioned parts of its medium, there must have unceasingly gone on perpetual re-adjustments of balance between aggregates and their units — actions and reactions of the two, in which the units tended ever to establish the typical form produced by actions and reactions in all antecedent generations, while the aggregate, if changed in form by change of surrounding conditions, tended ever to impress on the units a corresponding change of polarity, causing them in the next generation to reproduce the changed form — their new form of equilibrium.

This is the conception which I have sought to convoy, though it seems unsuccessfully, in the Principles of Biology ; and which I have there used to interpret the many involved and mysterious

488

phenomena of Genesis, Heredity, and Variation. In one respect only am I conscious of having so inadequately explained myself, as to give occasion for a misinterpretation — the one made by the Westminster reviewer above referred to. By him, as by your own critic, it is alleged that in the idea of " inherent tendencies " I have introduced, under a disguise, the conception of " the archoeus, vital principle, nisus formativus, and so on." This allegation is in part answered by the foregoing explanation. That which I have here to add, and did not adequately ex- plain in the Principles of Biology, is that the proclivity of units of each order towards the specific arrangement seen in the organism they form, is not to be understood as resulting from their own structures and actions only ; but as the product of these and the environing forces to which they are exposed. Organic evolution takes place only on condition that the masses of protoplasm formed of the physiological units, and of the assimilable materials out of which others like themselves are to be multiplied, are subject to heat of a given degree — are subject, that is, to the unceasing impacts of undulations of a certain strength and period ; and, within limits, the rapidity with which the physiological units pass from their indefinite arrangement to the definite arrangement they presently assume, is proportionate to the strengths of the etherial undulations falling upon them. In its complete form, then, the conception is that these specific molecules, having the immense complexity above described, and having correspondently complex polarities which cannot be mutually balanced by any simple form of aggregation, have, for the form of aggregation in which all their forces are equilibrated, the structure of the adult organism to which they belong ; and that they are compelled to fall into this structure by the co-operation of the en- vironing forces acting on them, and the forces they exercise on one another — the environing forces being the source of the power which effects the re-arrangement, and the polarities of the molecules deter- mining the direction in which that power is turned. Into this con- ception there enters no trace of the hypothesis of an " archanis or vital principle;" and the principles of molecular physics fully justify it. It is, however, objected that "the living body in its develop- ment presents a long succession of differing forms ; a continued series of changes for the whole length of which, according to Mr. Spencer's hypothesis, the physiological units must have an ' inherent tendency.' Could we more truly say of anything, * it is unrepresent- able in thought 1 ' I reply that if there is taken into account an element here overlooked, the process will not be found " unrepre- sentable in thought." This is the element of size or mass. To satisfy or balance the polarities of each order of physiological units, not only a certain structure of organism, but a certain size of organism is needed; for the complexities of that adult structure

439

in which the physiological units are equilibrated, cannot be represented within the small bulk of the embryo. In many minute organisms, where the whole mass of physiological units required for the structure is present, the very thing does take place which it is above implied ought to take place. The mass builds itself directly into the complete form. This is so with Acari, and among the nematoid Entozoa. But among higher animals such direct transformations cannot happen. The mass of physiological units required to produce the size as well as the structure that approximately equilibrates them, is not all present, but has to be formed by successive additions — addi- tions which in viviparous animals are made by absorbing, and transforming into these special molecules, the organizable materials directly supplied by the parent, and which in ovi- parous animals are made by doing the like with the organ- izable materials in the " food-yelk,'' deposited by the parent in the same envelope with the germ. Hence it results that, under such conditions, the physiological units which first aggregate into the rudiment of the future organism, do not form a structure like that of the adult organism, which, when of such small dimensions, does not equilibrate them. They distribute themselves so as partly to satisfy the chief among their complex polarities. The vaguely-dif- ferentiated mass thus produced cannot, however, be in equilibrium. Each increment of physiological units formed and integrated by it, changes the distribution of forces ; and this has a double effect. It tends to modify the differentiations already made, bringing them a step nearer to the equilibrating structure; and the physiological units next integrated, being brought under the aggregate of polar forces exercised by the whole mass, which now approaches a step nearer to that ultimate distribution of polar forces which exists in the adult organism, are coerced more directly into the typical struc- ture. Thus there is necessitated a series of compromises. Each successive form assumed is unstable and transitional : approach to the typical structure going on hand in hand with approach to the typical bulk.

Possibly I have not succeeded by this explanation, any more than by the orig'nal explanation, in making this process " representable in thought." It is manifestly untrue, however, that I have, as alleged, re-introduced under a disguise the conception of a " vital principle." That I interpret embryonic development in terms of Matter and Motion, cannot, I think, be questioned. Whether the interpretation is adequate, must be a matter .of opinion ; but it is clearly a matter of fact, that I have not fallen into the inconsistency asserted by your reviewer. At the same time I willingly admit that, in the absence of certain statements which I have now supplied, he was not unwarranted in representing my conception in the way that he has done.

490

But while I consider that what your reviewer hay said on these two essential points, falls within the limits of legitimate criticism ; I do not consider that he is justified in much that he says by im- plication respecting my general views.

In the first place, he conveys a totally wrong idea of the mode of interpretation he criticizes. He gives his readers no con- ception of the immense extensions which modern science has made of the " mechanical theory," now applied to the solution of all phy- sical phenomena whatever ; but he has deliberately restricted its ap- plications in a way that produces an appearance of difficulty where no difficulty exists. The common uses of the words "mechanical" and " mechanist," are such as inevitably call up in all minds the notions of visible masses of matter acting on one another by mea- surable forces and producing sensible motions. In the absence of explanations or illustrations serving to enlarge the conception thus suggested, so as to bring within it the oscillations of the molecules of matter, and the undulations of the molecules of ether pervading all space, even the cultivated reader must carry with him an ex- tremely crude and narrow idea of the " mechanist theory," and can- not fail to be struck with the seeming absurdity of interpreting vital phenomena in mechanical terms. 13ut the reviewer says nothing to prevent misconceptions so arising. He gives no hint that heat, light, and electricity, are now all recognized as "modes of mo- tion;" and that most of their phenomena are mechanically inter- preted, while the rest are regarded as mechanically interpretable. He does not explain that the " mechanist " theory in its comprehen- sive form embraces actions such as those by which variations in the fcolar spots cause variations in our magnetic needles, and actions such as those through which Sirius tells us what substances are contained in his atmosphere. True he makes a passing reference to chemical changes as being included by me under the conception of mechanical; but he leaves this as a dead statement quite unintelligible to the general reader ; and in the typical example he gives of my mode of in- terpretation (the development of vertebraB by transverse strains) he deliberately excludes the physio-chemical and chemical actions which I imply as co-operating, and describes me as attributing the effects entirely to the pressures and tensions caused by muscular move- ments ! (See p. 408). Instead of the developed ideas of Matter and Motion everywhere implied throughout the Principles of Biology, the reviewer leads everyone to suppose that I bring to bear on biological problems nothing beyond the vulgar ideas of Matter and Motion, and leaves me responsible for the ludicrous incongruity !

That, however, which I regard as most reprehensible in his criti- cism is the way in which he persists in representing the System of Philosophy I am working out as a materialistic system. Already he has once before so represented it, and the injustice of so represent-

491

nig' it has been pointed out. He knows that I have repeatedly nnd emphatically asserted that our conceptions of Matter and Motion are but symbols of an Unknowable Reality ; that this Reality cannot be that which we symbolize it to be ; and that as manifested beyond consciousness under the forms of Matter and Motion, it is the same as that which, in consciousness, is manifested as Feeling and Thought. Yet he continues to describe me as reducing everything to dead mechanism. If his statement on pp. 383-4 has any meaning at all, it means that there exists some "force operating ab extra" some " external power " distinguished by him as " mechanical," which is not included in that immanent force of which the universe is a mani- festation ; though whence it comes he does not tell us. This con- ception he speaks of as though it were mine ; making it seem that I ascribe the moulding of organisms to the action of this " mechani- cal" " external power," which is distinct from the Inscrutable Cause of things. Yet he either knows, or has ample means of knowing, that I deny every such second cause : indeed he has himself classed me as an opponent of dualism. I recognize no forces within the organism, or without the organism, but the variously-conditioned modes of the universal immanent force ; and the whole process of organic evolu- tion is everywhere attributed by me to the co-operation of its vari- ously-conditioned modes, internal and external. That this has been all along my general view, is clearly shown in the closing paragraph of First Principles, where I have said —

"A Power of which the nature remains for ever inconceivable, and to which no limits in Time or Space can be imagined, works in us certain effects. These effects have certain likenesses of kind, the most general of which we class together under the names of Matter, Motion, and Force ; and between these effects there are likenesses of connection, the most constant of which we class as laws of the highest certainty. Analysis reduces these several kinds of effect to one kind of effect ; and these several kinds of uniformity to one kind of uniformity. And the highest achievement of Science is the interpretation of all orders of phenomena, as differently-conditioned manifestations of this one kind of effect, under differently-conditioned modes of this one kind of uniformity. But when Science has done this, it has done nothing more than systematize our experience ; and has in no degree extended the limits of our experience. We can say no more than before, whether the uniformities are as absolutely necessary, as they have become to our thought relatively necessary. The utmost possibility for us, is an interpretation of the pi 3cess of things as it presents itself to our limited consciousness ; but liu\v this process is related to the actual process, we are unable to conceive, much less to know. Similarly, it must be remembered

that while the connection between the phenomenal order and the outological order is for ever inscrutable ; so is the connection between the conditioned forms of being and the unconditioned form of being for ever inscrutable. The interpretation of all phenomena in terms of Matter, Motion, and Force, is nothing more than the reduction of our complex symbols of thought, to the simplest symbols ; and when the equation has been brought to its lowest terms the symbols remain symbols still. Hence the reasonings contained in the foregoing pages, afford no support to either of the antagonist hypotheses respecting the ultimate nature of

492

things. Their implications are no more materialistic than they are spiritualistic ; and no more spiritualistic than they are materialistic. Any argument which is apparently furnished to either hypothesis, is neutralized by as good an argument furnished to the other. The Materialist, seeing it to be a necessary deduction from the law of correla- tion, that what exists in consciousness under the form of feeling, is transformable into an equivalent of mechanical motion, and by consequence into equivalents oE all the other forces which matter exhibits ; may consider it therefore demonstrated that the phenomena of consciousness are material phenomena. But the Spiritualist, setting out with the same data, may argue with equal cogency, that if the forces displayed by matter are cognizable only under the shape of those equivalent amounts of consciousness which they produce, it is to be inferred that these forces, when existing out of consciousness, are of the same intrinsic, nature as when existing in consciousness ; and that so is justified the spiritualistic conception of the external world, as consisting of something essentially identical with what we call mind. Manifestly, the establishment of correlation and equivalence between the forces of the outer and the inner worlds, majr be used to assimilate either to the other ; ac- cording as we set out with one or other term. But he who rightly inter- prets the doctrine contained in this work, will see that neither of these terms can be taken as ultimate. He will see that though the relation of subject and object renders necessary to us these antithetical conceptions of Spirit and Matter ; the one is no less than the other to be regarded as but a sign of the Unknown Eeality which underlies both."

This is the conception which your reviewer continues to speak of as " mechanical " and " mechanist ;" without giving his readers any suspicion of the qualified sense in which only these words can be applied. If he thinks that by doing this he has represented the conception with fairness, or with any approach to fairness, I cannot agree with him.

I am, Sir,

Yours, &c.,

HERBERT SPENCER. London, December 5, 1868.

THE SYNTHETIC PHILOSOPHY

OF

HERBERT SPENCER

FIRST PRINCIPLES.

1 vol. $2.00. CONTENTS.

PART I. — THE UNKNOWABLE.

1. Religion and Science. 4. The Relativity of all Knowl-

2. Ultimate Religious Ideas. edge.

3. Ultimate Scientific Ideas. 5. The Reconciliation.

PART II. — THE KNOW ABLE.

1. Philosophy defined. 13. Simple and Compound Evolu-

2. The Data of Philosophy. tion.

3. Space, Time, Matter, Motion, 14. The Law of Evolution.

and Force. 15. The Law of Evolution (con-

4. The Indestructibility of Matter. tinued).

6. The Continuity of Motion. 16. The Law of Evolution (con-

6. The Persistence of Force. tinued).

7. The Persistence of Relations 17. The Law of Evolution (con-

among Forces. eluded).

8. The Transformation and Equiv- 18. The Interpretation of Evolution.

alencc of Forces. 19. The Instability of the Homoge-

9. The Direction of Motion. neous.

10. The Rhythm of Motion. 20. The Multiplication of Effects.

11. Recapitulation, Criticism, and 21. Segregation.

Recommencement. 22. Equilibration.

12. Evolution and Dissolution. 23. Dissolution.

24. Summary and Conclusion.

THE PRINCIPLES OF BIOLOGY.

2 vols. $4.00. CONTENTS OF VOL. I.

PART I. — THE DATA OF BIOLOGY.

1. Organic Matter. 4. Proximate Definition of Life.

2. The Action of Forces on Or- 5. The Correspondence between

ganic Matter. Life and its Circumstances.

3. The Reactions of Organic Mat- 6. The Degree of Life varies as the

ter on Forces. Degree of Correspondence.

7. The Scope of Biology.

SPENCER S SYNTHETIC PHILOSOPHY.

PART II. — THE INDUCTIONS OF EIOLOGY.

1. Growth.

2. Development.

3. Function.

4. Waste and Repair.

5. Adaptation. G. Individuality.

3.

4.

5.

7. Genesis.

8. Heredity.

9. Variation.

10. Genesis, Heredity, and Varia-

tion.

11. Classification. 12. Distribution.

PART III. — THE EVOLUTION OF LIFE.

1. Preliminary.

2. General Aspects of the Special-

Creation Hypothesis.

3. General Aspects of the Evolu-

tion Hypothesis.

4. The Arguments from Classifica-

tion.

6. The Arguments from Embryol- ogy. "

6. The Arguments from Morphol- ogy.

7.

The Arguments

Organic

Evolution

from Distribu- tion.

8. How is

caused ?

9. External Factors.

10. Internal Factors.

11. Direct Equilibration.

12. Indirect Equilibration.

13. The Cooperation of the Factors.

14. The Convergence of the Evi-

dences.

CONTENTS OF VOL. II. PART IV. — MORPHOLOGICAL DEVELOPMENT.

1. The Problems of Morphology.

2. The Morphological Composition

of Plants. The Morphological Composition

of Plants (continued). The Morphological Composition

of Animals. The Morphological Composition

of Animals (continued).

6. Morphological Differentiation in

Plants.

7. The General Shapes of Plants.

8. The Shapes of Branches.

9. The Shapes of Leaves.

10. The Shapes of Flowers.

11. The Shapes of Vegetal Cells.

12. Changes of Shape otherwise

caused.

13. Morphological Differentiation in

Animals.

14. The General Shapes of Animals.

15. The Shapes of Vertebrate Skele-

tons.

16. The Shapes of Animal Cells.

17. Summary of Morphological De-

velopment.

PART V. — PHYSIOLOGICAL DEVELOPMENT.

1. The Problems of Physiology.

2. Differentiations among the Out-

er and Inner Tissues of Plants.

3. Differentiations among the Out-

er Tissues of Plants.

4. Differentiations among the In-

ner Tissues of Plants.

5. Physiological Integration in

Plants.

6.

among the Out-

Differentiations between the Outer and Inner Tissues of Animals.

Differentiations

er Tissues of Animals.

Differentiations among the In- ner Tissues of Animals. 9. Physiological Integration in Ani mals.

7.

8.

10. Summary of Physiological Development.

SPENCER'S SYNTHETIC PHILOSOPHY. 3

PART VI. — LATVS OF MULTIPLICATION.

1. The Factors. 8. Antagonism between Expendi-

2. A priori Principle. ture and Genesis.

3. Obversj a p, i-jri Principle. 9. Coincidence between High Xu-

4. Difficulties of Inductive YeriS- trition and Genesis.

cation. 10. Specialties of these Rela-

5. Antagonism between Growth tions.

and Asexual Genesis. 11. Interpretation and Qualifica-

6. Antagonism between Growth tion.

and Sexual Genesis. 12. Multiplication of the Human

7. Antagonism between Develop- Race.

ment and Genesis, Asexual 13. Human Evolution in the Fu- and Sexual. ture.

APPENDIX.

A Criticism on Professor Owen's The- On Circulation and the Formation ory of the Vertebrate Skeleton. of Wood in Plants.

THE PRINCIPLES OF PSYCHOLOGY.

2 vols. 84.00. CONTENTS OF VOL. I. PART I. — THE DATA OF PSVCHOLOGY.

1. The Nervous System. 4. The Conditions essential to Ner-

2. The Structure of the Nervous vous Action.

System. 5. Nervous Stimulation and Ner-

3. The Functions of the Nervous vous Discharge.

System. 6. J^stho-Physiology.

PART II. — THE INDUCTIONS OF PSYCHOLOGY.

1. The Substance of Mind. 6. The Revivability of Relations

2. The Composition of Mind. between Feelings.

3. The Relativity of Feelings. 7. The Associability of Feelings.

4. The Relativity of Relations be- 8. The Associability of Relations

twecn Feelings. between Feelings.

5. The Revivability of Feelings. 9. Pleasures and Pains.

PART III. — GENERAL SYNTHESIS.

1. Life and Mind as Correspon- 6. The Correspondence as increas-

dence. ing in Specialty.

2. The Correspondence as Direct 7. The Correspondence as increas-

and Homogeneous. ing in Generality.

3. The Correspondence as Direct 8. The Correspondence as increas-

but Heterogeneous. ing in Complexity.

4. The Correspondence as extend- 9. The Coordination of Correspon-

ing in Space. dences.

5. The Correspondence as extend- 10. The Integration of Correspon-

ing in Time. dences.

11. The Correspondences in their Totality.

BPENOEft S SYNTHETIC PHILOSOPHY.

PART IV. — SPECIAL SYNTHESIS.

1. The Nature of Intelligence.

2. The Law of Intelligence.

CJ

3. The Growth of Intelligence.

4. Reflex Action.

6. Instinct.

6. Memory.

7. Reason.

8. The Feelings.

9. The Will.

PART V. — PHYSICAL SYNTHESIS.

1. A Further Interpretation need- 6. Functions as related to these

ed. Structures.

2. The Genesis of Nerves. 7. Physical Laws as thus inter-

3. The Genesis of Simple Nervous preted.

Systems. 8. Evidence from Normal Varia-

4. The Genesis of Compound Ner- tions.

vous Systems. 9. Evidence from Abnormal Va-

6. The Genesis of Doubly Com- riations.

pound Nervous Systems. 10. Results.

APPENDIX. On the Action of Anaesthetics and Narcotics.

CONTENTS OF VOL. II. PART VI. — SPECIAL ANALYSIS.

1.

2.

Limitation of the Subject. Compound Quantitative Reason-

ing.

3. Compound Quantitative Reason- ing (continued).

Imperfect and Simple Quantita- tive Reasoning.

Quantitative Reasoning in gen- eral.

Perfect Qualitative Reasoning.

Imperfect Qualitative Reason-

4. 5.

6.

7.

ing.

8. Reasoning in general.

9, Classification, Naming, and Rec-

ognition.

10. The Perception of Special Ob-

jects.

11. The Perception of Body as pre-

senting Dynamical, Statico- Dynamical, and Statical Attri- butes.

12. The Perception of Body as pre-

senting Statico-Dynamical and Statical Attributes.

13. The Perception of Body as

presenting Statical Attri- butes.

14. The Perception of Space.

15. The Perception of Time.

16. The Perception of Motion.

17. The Perception of Resist-

ance.

18. Perception in general.

19. The Relations of Similarity and

Dissimilarity.

20. The Relations of Cointension

and Non-Cointension.

21. The Relations of Coextension

and Non-Coextension.

22. The Relations of Coexistence

and Non-Coexistence.

23. The Relations of Connature and

Non-Connature.

24. The Relations of Likeness and

Unlikeness.

25. The Relation of Sequence.

26. Consciousness in general.

27. Results.

SPEXCEE, 8 SYNTHETIC PHILOSOPHY.

PART VII. — GENERAL ANALYSIS.

1. The Final Question.

2. The Assumption of Metaphysi-

cians.

3. The Words of Metaphysicians. The Reasonings of Metaphysi- cians, [ism.

Negative Justification of Real- The Argument from Priority. The Argument from Simplicity. The Argument from Distinct- A Criterion wanted. [ness.

4.

5. C. 7. 8. 9.

4.

10. Propositions qualitatively dis-

tinguished.

11. The Universal Postulate.

12. The Test of Relative Validity.

13. Its Corollaries.

14. Positive Justification of Real-

ism.

15. The Dynamics of Consciousness.

16. Partial Differentiation of Sub-

ject and Object.

17. Completed Differentiation of

Subject and Object.

18. Developed Conception of the

Object.

19. Transfigured Realism.

PART VIII. — COROLLARIES.

1. Special Psychology. 5. Sociality and Sympathy.

2. Classification. 6. Egoistic Sentiments.

3. Development of Conceptions, 7. Ego-Altruistic Sentiments, Language of the Emotions. 8. Altruistic Sentiments.

9. ^Esthetic Sentiments.

THE PRINCIPLES OF SOCIOLOGY,

Vol. I. $2.00.

CONTENTS.

PART I. — THE DATA OF SOCIOLOGY.

1. Super-Organic Evolution.

2. The Factors of Social Phenom-

ena.

3. Original External Factors.

4. Original Internal Factors.

5. The Primitive Man — Physical.

6. The Primitive Man — Emotional.

7. The Primitive Man— Intellect-

ual.

8. Primitive Ideas.

9. The Ideas of the Animate and

the Inanimate.

10. The Ideas of Sleep and Dreams.

11. The Ideas of Swoon, Apoplexy,

Catalepsy, Ecstasy, and other Forms of Insensibility.

12. The Ideas of Death and Resur-

rection.

13. The Ideas of Souls, Ghosts,

Spirits, Demons.

14. The Ideas of Another Life,

15. The Ideas of Another World.

16. The Ideas of Supernatural

Agents.

17. Supernatural Agents as causing

Epilepsy and Convulsive Ac- tions, Delirium and Insanity. Disease and Death.

18. Inspiration, Divination, Exor-

cism, and Sorcery.

19. Sacred Places, Temples, and

Altars ; Sacrifice, Fasting, and Propitiation ; Praise, Prayer.

20. Ancestor- Worship in general.

21. Idol- Worship and Fetich- Wor-

ship.

22. Animal- Worship.

23. Plant-Worship.

24. Naturer Worship.

25. Deities.

26. The Primitive Theory of Thinga

27. The Scope of Sociology.

SPENCER 8 SYNTHETIC PI-1 ILOSOPHT.

PART II. — THE INDUCTIONS OF SOCIOLOGY.

1. What is a Society?

2. A Society is an Organism.

3. Social Growth.

4. Social Structures. 6. Social Functions.

G. Systems of Organs.

7. The Sustaining System.

8. The Distributing System.

9. The Regulating System.

10. Social Types and Constitutions.

11. Social Metamorphoses.

12. Qualifications and Summary.

PART III. — THE DOMESTIC RELATIONS.

1. The Maintenance of Species.

2. The Diverse Interests of the

Species, of the Parents, and of the Offspring.

3. Primitive Relations of the Sexes.

4. Exogamy and Endogamy. 6. Promiscuity.

6. Polyandry.

7. Polygyny.

8. Monogamy.

9. The Family.

10. The Status of Women.

11. The Status of Children.

12. Domestic Retrospect and Pros

pect.

Vol. II.

PART IV. — CEREMONIAL INSTITUTIONS. CONTENTS.

$1.25.

1. Ceremony in general.

2. Trophies.

3. Mutilations.

4. Presents. 6. Visits.

6. Obeisances.

Forms of Address.

Titles.

Badges and Costumes.

7. 8. 9.

10. Further Class-Distinctions.

11. Fashion.

12.

Vol. II.

Ceremonial Prospect.

Retrospect and

PART V. — POLITICAL INSTITUTIONS. $ CONTENTS.

1. Preliminary.

2. Political

eral. 3. 4. 5. 6.

Organization

Political Integration.

in

gen-

Political Differentiation. Political Forms and Forces. Political Heads — Chiefs, Kings, etc.

7. Compound Political Heads.

8. Consultative Bodies.

9. Representative Bodies.

Vol.

10. Ministries.

11. Local Governing Agencies.

12. Military Systems.

13. Judicial and Executive Systems.

14. Laws.

15. Property.

16. Revenue.

17. The Militant Type of Society.

18. The Industrial Type of Society.

19. Political Retrospect and Pros-

pect.

III.

In preparation.

SPENCEK'S SYNTHETIC PHILOSOPHY.

THE PRINCIPLES OF MORALITY.

Vol. I.

PART I. — THE DATA OF ETHICS. §1.25. CONTENTS.

1. Conduct in general. 10. The Relativity of Pains and

2. The Evolution of Conduct. Pleasures.

3 Good and Bad Conduct. 11. Egoism versus Altruism.

4. Ways of judging Conduct. 12. Altruism versus Egoism.

5. The Physical View. 13. Trial and Compromise.

6. The Biological View. 14. Conciliation.

7. The Psychological View. 15. Absolute Ethics and Relative

8. The Sociological View. Ethics.

9. Criticisms and Explanations. 16. The Scope of Ethics.

Vol. II. In preparation.

This philosophical system differs from all its predecessors in being solidly based on the sciences of observation and induction; in repre- senting the order and course of Nature ; in bringing Nature and man, life, mind, and society, under one great law of action ; and in developing a method of thought which may serve for practical guidance in dealing with the affairs of life.

"The only complete and systematic statement of the doctrine of evolution with which I am acquainted is that contained in Mr. Herbert Spencer's 'System of Philosophy,' a work which should be carefully studied by all who desire to know whither scientific thought is tending." — T. H. HUXLEY.

" Mr. Spencer is one of the most vigorous as well as boldest thinkers that English speculation has yet produced." — JOHN STUART MILL.

New York: D. APPLETON & CO., 1, 3, & 5 Bond St.

THE MISCELLANEOUS WORKS

OF

HERBERT SPENCER

EDUCATION:

INTELLECTUAL, MORAL, AND PHYSICAL. 1 vol. $1.25.

CONTENTS.

1. What Knowledge is of most 2. Intellectual Education. Worth ? 3. Moral Education.

4. Physical Education.

SOCIAL STATICS;

OR,

THE CONDITIONS ESSENTIAL TO HUMAN HAPPINESS SPECI- FIED, AND THE FIRST OF THEM DEVELOPED.

1 vol. $2.00.

CONTENTS. INTRODUCTION.

The Doctrine of Expediency. Lemma I. ' The Doctrine of the Moral Sense. Lemma II.

PART I.

1. Definition of Morality. 3. The Divine Idea, and the Con-

2. The Evanescence of Evil. ditions of its Realization.

PART II.

4. Derivation of a First Principle. 10. The Right of Property.

5. Secondary Derivation of a First 11. The Right of Property in Ideas.

Principle. 12. The Right of Property in Char-

6. First Principle. [ciple. acter.

7. Application of this First Prin- 13. The Right of Exchange.

8. The Rights of Life and Per- 14. The Right of Free Speech.

sonal Liberty. 15. Further Rights.

9. The Right to the Use of the 16. The Rights of Women.

Earth. 17. The Rights of Children.

SPENCER S MISCELLANEOUS WORKS.

PART III.

18. Political Rights.

19. The Right to ignore the State.

20. The Constitution of the State.

21. The Duty of the State.

22. The Limit of State-Duty.

23. The Regulation of Commerce.

24. Religious Establishment.

25. Poor-Laws.

26. National Education.

27. Government Colonization.

28. Sanitary Supervision. [etc

29. Currency, Postal Arrangements,

PART IV.

30. General Considerations. 31. Summary.

32. Conclusion.

THE STUDY OF SOCIOLOGY.

1 vol. $1.50.

CONTENTS.

1.

2. 3. 4.

Our Need of it. Is there a Social Science ? Nature of the Social Science. Difficulties of the Social Science.

5. Objective Difficulties.

6. Subjective Difficulties — Intel-

lectual.

7. Subjective Difficulties — Emo-

tional.

8. The Educational Bias.

9. The Bias of Patriotism.

10. The Class-Bias.

11. The Political Bias.

12. The Theological Bias.

13. Discipline.

14. Preparation in Biology.

15. Preparation in Psychology.

16. Conclusion.

ILLUSTRATIONS OF UNIVERSAL PROG- RESS.

1 vol. $2.00. CONTENTS.

1. Progress: its Law and Cause.

2. Manners and Fashion.

3. The Genesis of Science.

4. The Physiology of Laughter.

5. The Origin and Function of Mu-

6. The Nebular Hypothesis. [sic.

7. Bain on the E'motions and the

Will.

8. Illogical Geology.

9. Development Hypothesis.

10. The Social Organism.

1 1 . Use and Beauty.

12. The Sources of Architectural

Types.

13. The Use of Anthropomor-

phism.

ESSAYS:

MORAL, POLITICAL, AND AESTHETIC. 1 vol. $2.00. CONTENTS.

1. The Philosophy of Style.

2. Our Legislation.

3. The Morals of Trade.

4. Personal Beauty.

10 SPENCER'S MISCELLANEOUS WORKS.

6. Representative Government. 9. State Tamperings -with Money

6. Prison Ethics. and Banks.

7. Railway Morals and Railway 10. Parliamentary Reforms: the

Policies. Dangers and the Safeguards,

8. Gracefulness. 11. Mill versus Hamilton — the Test

of Truth.

HECENT DISCUSSIONS

IN SCIENCE, PHILOSOPHY, AND MORALS. 1 vol. $2.00.

CONTENTS.

1. Morals and Moral Sentiments. 6. Of Laws in general and the Or-

2. Origin of Animal-Worship. der of their Discovery.

3. The Classification of the Sci- 7. The Genesis of Science.

ences. 8. Specialized Administrations.

4. Postscript : Replying to Criti- 9. What is Electricity ?

cisms. 10. The Constitution of the Sun.

5. Reasons for dissenting from the 11. The Collective Wisdom.

Philosophy of Comte. 12. Political Fetichism.

13. Mr. Martineau on Evolution.

THE MAN versus THE STATE.

REPRINTED FROM " THE POPULAR SCIENCE MONTHLY," TTITH A POSTSCRIPT.

Small 8vo. Paper, 30 cents.

CONTENTS.

1. The New Toryism. 3. The Sins of Legislators.

2. The Coming Slavery. 4. The Great Political Superstition.

This volume contains all the recent political papers by Herbert Spencer that have appeared in " The Contemporary Review " in England, and (by arrangement) in " The Popular Science Monthly " in this country. These articles, in the course of their publication, have aroused a profound interest, and the present cheap edition is in obedience to a demand for the papers in a form for a wide, popular circulation.

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A LIBRARY

OF THE MOST IMPORTANT

STANDARD WORKS ON EVOLUTION.

i.

Origin of Species by Means of Natural Selection, or the Preser- vation of Favored Races in the Struggle for Life. By CHARLES DAR- WIN, LL. D., F. R. S. New and revised edition, with Additions. 12mo. Cloth, $2.00.

"Personally and practically exercised in zoology, in minute anatomy, in geology, a student of geographical distribution, not in maps and in mu- seums, but by long voyages and laborious collection ; having largely ad- vanced each of these branches of science, and having spent many years in gathering and sifting materials for his present -work, the store of accurately- registered facts upon which the author of the ' Origin of Species' is able to draw at will is prodigious." — Professor T. H. Huxley.

II.

Variation of Animals and Plants under Domestication. By

CHARLES DARWIN, LL. D., F. R. S. With Illustrations. Revised edi- tion. 2 vols., 12ino. Cloth, $5.00.

"We shall learn something of the laws of inheritance, of the effects of crossing different breeds, and on that sterility which often supervenes when organic beings are removed from their natural conditions of life, and Like- wise when they are too closely interbred." — From the Introduction.

III.

Descent of Man, and Selection in Eelation to Sex. By CHARLES DARWIN, LL. D., F. R. S. With many Illustrations. A new edition. 12mo. Cloth, $3.00.

" In these volumes Mr. Darwin has brought forward all the facts and arguments which science has to offer in favor of the doctrine that man has arisen by gradual development from the lowest point of animal life. Aside from the logical purpose which Mr. Darwin had in view, his work is an original and fascinating contribution to the most interesting portion of nat- ural history."

IV.

On the Origin of Species ; or, The Causes of the Phenomena of Or- ganic Nature. By Professor T. H. HUXLEY, F. R. S. 12mo. Cloth, $1.00.

" Those who disencumber Darwinism of its difficulties, simplify its state- ments, relieve it of technicalities, and bring it so distinctly within the hori- zon of ordinary apprehension that persons of common sense may judge for themselves, perform an invaluable service. Such is the character of the present volume."— From the Preface to the American edition.

V.

Darwiniana. Essays and Reviews pertaining to Darwinism. By ASA GRAY, Fisher Professor of Natural History "(Botany) in Harvard Uni- versity. 12ruo, Cloth, $2.00.

" Although Professor Gray is widely known in the world of science for his botanical researches, but few are aware that he is a pronounced and un-

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flinching Darwinian. His contributions to the discussion are varied and valuable, and as collected in the present volume they will be seen to estab- lish a claim upon the thinking world, which will be extensively felt and cordially acknowledged. These papers not only illustrate the "history of the controversy, and the progress of the discussion, but they form perhaps the fullest and most trustworthy exposition of what is to be properly under- stood by ' Darwinism ' that is to be found in our language. To all those timid souls who are worried about the progress of science, and the danger that it will subvert the foundations of their faith, we recommend the dis- passionate perusal of this volume." — The Popular Science Monthly.

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Heredity : A Psychological Study of its Phenomena, Laws, Causes, and Consequences. From the French of TH. RIBOT. 12mo. Cloth, $2.00.

" Heredity is that biological law by which all beings endowed with life tend to repeat themselves in their descendants : it is for the species what personal identity is for the individual. The physiological side of this subject has been diligently studied, but not so its psychological side. We propose to supply this deficiency in the present work." — Irani the Introduction.

VII.

Hereditary Genius : An Inquiry into its Laws and Consequences. By FRANCIS GA I/TON, F. R. S., etc. New and revised edition, with an American Preface. 12mo. Cloth, $2.00.

" The following pages embody the result of the first vigorous and me- thodical effort to treat the question in the true scientific spirit, and place it upon the proper inductive basis. Mr. Galton proves, by overwhelming evidence, that genius, talent, or whatever we term great mental capacity, follows the law of organic transmission — runs in families, and is an affair or blood and breed ; and that a sphere of phenomena hitherto deemed capri- cious and defiant of rule is, nevertheless, within the operation of ascertain- able law." — From the American Preface.

VIII.

The Evolution Of Man. A Popular Exposition of the Principal Points of Human Ontogeny and Phylogeny. From the German of ERNST HAECKEL, Professor in the University of Jena. With numer- ous Illustrations. 2 vols., 12mo. Cloth, $5.00.

"In this excellent translation of Professor Haeckel's work, the English reader has access to the latest doctrines of the Continental school of evolu- tion, in its application to the history of man."

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The History of Creation ; or, the Development of the Earth and its Inhabitants by the Action of Natural Causes. A Popular Exposition of the Doctrine of Evolution in General, and of that of Darwin, Goe- the, and Lamarck in Particular. By ERNST HAECKEL, Professor in the University of Jena. The translation revised by Professor E. RAY LANKESTER. Illustrated with Lithographic Plates. 2 vols., 12mo. Cloth, $5.00.

" The book has been translated into several languages. I hope that it may also find sympathy in the fatherland of Darwin, the more so since it contains special morphological evidence in favor of many of the most impor- tant doctrines with which this greatest naturalist of our century has enriched *cience." — From the Preface.

A STANDARD EVOLUTION LIBRARY.

x.

and Science, A Series of Sunday Lectures on the Rlationo of Natural and Revealed Religion, or the Truths revealed in Nature and Scripture. By JOSEPH LE CONTE, LL. D. 12mo. Cloth, $1.50.

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Winners in Life's Race ; or, The Great Backboned Family. By ARA- BELLA B. BUCKLEY, author of " The Fairy-Land of Science " and " Life and her Children." With numerous Illustrations. 12rno. Cloth, gilt side and back, $1.50.

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The Theory of Descent and Darwinism. By Professor OSCAR SCHMIDT. With 26 Woodcuts. 12mo. $1.50.

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XV.

Outline of the Eyolution Philosophy. By Dr. M. E. GAZELLES. Translated from the French, by the Rev. 0. B. FROTHINGHAM ; with an Appendix, by E. L. YOUMANS, M. D. 12mo. Cloth, $1.00.

" This unpretentious little work will, no doubt, be used by thousands to whom the publications of Mr. Herbert Spencer are inaccessible and those of Auguste Comte repellent, by reason of their prolixity and vagueness. In a short space Dr. Gazelles has managed to compress the whole outline and scope of Mr. Spencer's system, with his views of the doctrine of progress and law of evolution, and a clear view of the principles of positivism." — Mature (London).

XVI.

Principles of Geology ; or, The Modern Changes of the Earth and its Inhabitants, considered as illustrative of Geology. By Sir CHARLES LTELL, Bart. Illustrated with Maps, Plates, and Woodcuts. A new and entirely revised edition. 2 vols. Royal Svo. Cloth, $8.00.

The " Principles of Geology" may be looked upon with pride, not only as a representative of English science, but as without a rival of its kind anywhere. Growing in fullness and accuracy with the growth of experi-

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Elements of Geology. A Text-Book for Colleges and for the General Reader. By JOSEPH LE CONTE, LL. D., Professor of Geology and Natural History in the University of California. Revised and en- larged edition. 12mo. With upward of 900 Illustrations. Cloth, $4.00.

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Animal Life, as affected by the Natural Conditions of Exist- ence. By KARL SEMPER, Professor of the University of Wiirzburg. With Maps and 100 Woodcuts. 12mo. Cloth, $2.00.

XIX.

Crayfish : An Introduction to the Study of Zoology. By Professor T. H. HUXLEY, F. R. S. With 82 Illustrations. 12mo. Cloth, $1.75.

XX.

Anthropology : An Introduction to the Study of Man and Civilization. By EDWARD B. TYLOR, F. R. S. With 78 Illustrations. 12mo. Cloth,

$2.00.

" The students who read Mr. Tylor's book may congratulate themselves upon having obtained so easy, pleasant, and workman-like an introduction to a fascinating and delightful science." — London Athenceum.

XXL

First Principles. By HERBERT SPENCER. Part I. The Unknowable. Part II. The Knowable. 1 vol., 12mo. $2.00.

XXII.

The Principles of Biology. By HERBERT SPENCER. 2 vols., 12mo. $4.00.

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Herbert Spencer's Descriptive Sociology,

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Representing the Constitution of Every Type and Grade of Httman Soci- ety, Past and Present, Stationary and Progressive :

CLASSIFIED AND TABULATED FOR EASY COMPARISON AND CONVENIENT STUDY OF THE DELATIONS OF SOCIAL PHENOMENA.

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LOWEST RACES, NEGRITO RACES, and MALAYO-POLYNESIAN RACES. Compiled and Abstracted by Professor DUNCAN, M. A.

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Snakes.

Comanches.

Iroquois.

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HEBREWS and PHOENICIANS. Compiled and Abstracted by EICHAP.D SCHEP PIG, Ph. D.

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