nih HOM: Creer. ¥ ) Wa RAN ¥ Melee tava Oa ath ; ‘ ; ) 7 RATE ; Disa NODES t4 108 | ‘ ahs NYY wld ¥} , Y WOU OS Sed > — Pinte eee koe a, mi ASE Rea | Lie uN yy qn (a ~. FUT eae CHAPTER II. TRANSFORMATIONS OF ENERGY. aA © already stated, the energy of motion of a moving body is due to its mass multipled into the height from which it must have fallen in order to acquire its velocity.1 And also, its energy of motion is proportionate Energy of to its mass multiplied into the square of its velocity. aoe Energy of motion is thus definitely measurable by quantity. Heat is also definitely measurable by quantity. Equal and heat, quantities of heat are those which equally increase the teasur- temperature of equal quantities of the same substance. able by For instance: equal quantities of heat will raise the tem- Saale perature of equal quantities of water by one degree of the of heat. thermometer. Now, it is found by experiment that equal quantities of energy of motion are capable of transformation into equal quantities of heat; and, conversely, equal quantities of heat are capable of transformation into equal quantities of energy of motion. This is called the law of the dynamical Dynamical equivalent of heat. Its numerical statement is, that the ccna ee energy which is due to the descent of one pound of weight of any substance whatever through 772 feet of height is capable of transformation into so much heat as will raise the temperature of one pound of water by 1°Fahr.? This, 1 See page 20. 2 Tyndall on Heat as a Mode of Motion, p. 40. It would take a fall through 1,390 feet to raise the temperature of a pound of water by 1° Centigrade. 26 Heat is molecular motion. All matter is perfectly elastic. HABIT AND INTELLIGENCE. [CHAP. as might be expected, is alike true, whatever be the process by which the transformation of the energy of motion into the heat is effected: whether it is by collision, as when a projectile flashes fire against the target ;’ or by friction, which may perhaps be regarded as consisting of an infinite number of molecular collisions ; or, as in one of the expe- riments by which Professor Joule ascertained this law, by transforming energy of motion into electricity, and this into heat; or by the compression of air, It is scarcely necessary to say, that when all the energy of motion of a moving body is transformed into heat, the body ceases to move. This fact, that whatever quantity of energy of motion disappears is represented by an equivalent quantity of heat, makes it highly probable that heat is really motion, the motion of the molecules of the heated substance; so that the transformation of energy of motion into heat is really the transfer of the energy of motion from the mass of the body that ceases to move to the molecules of the bodies that become heated. In a word, heat is molecular motion. I do not say that the facts already mentioned are sufficient to prove this: I admit that they only suggest it. But this theory is confirmed by all that we have learned of the action of gases under pressure; and the dynamical theory of heat is now as well established as the undulatory theory of light. I may here remark, though it is a digression, that the fact of the apparently lost energy of motion being trans- formed into heat, or molecular motion, proves the very important and previously unknown truth, that all matter is perfectly elastic. If two balls of a highly elastic sub- stance, such as ivory, come into collision, they rebound with great force, and very little of their energy of motion disappears. But if two balls of lead, or of any other almost inelastic substance, come into collision, they rebound with 1 “Mr, Fairbairn informs me that in the experiments at Shoebury- ness it is a common thing to see a flash of light, even in broad day, when the ball strikes the target.” (Tyndall on Heat as a Mode of Motion, p. 437.) ~I 1. ] TRANSFORMATIONS OF ENERGY. a very little force, and nearly all their energy of motion disappears. We now, however, know that the energy of motion which is lost by the masses is transferred to the molecules, being transformed into heat; so that the elasticity in which the masses are deficient is shown to belong to the molecules. Thus, what appears to be inelasticity is really a form of elasticity. When energy of motion disappears, it is generally trans- Motion formed into heat, but under special circumstances it is rier transformed into electricity. This is done by the common eee electrical machine, in which, by a suitable arrangement, ; a large part of the energy of motion that is usually trans- formed by friction into heat is transformed into electricity instead. The law on this subject appears to be, that similar substances rubbing against each other produce heat, but dissimilar ones produce electricity ; and the more unlike the substances are to each other, the more of the eneroy of motion that disappears is transformed into electricity, and the less into heat." In the electrical machine, artificial means are used to retain the electricity when it is produced. But when these are not used, the electricity nearly always escapes as soon as produced, and, in escaping, is transformed into heat. We may consequently lay it down as a law which is prac- tically true throughout nature, that when energy of motion seems to be lost, it is really transformed into heat. Energy of motion is also transformed into electricity by a totally different process in the magneto-electric machine. The nature of electricity is not nearly so well understood as that of heat and light. I state some experiments and reasonings bearing on the subject in a note to Chapter ITI. Equal quantities of energy of motion, as already stated, Quantita- are capable of transformation into equal quantities of ae heat ; and equal quantities of energy of motion are capa- cee ble of transformation also into equal quantities of electric = = energy.” Equal quantities of electric energy and of heat 1 Grove on the Correlation of Physical Forces. 2 T say that equal quantities of heat or of energy of motion are capable of transformation into equal quantities, not of electricity, but of electric Radiation of heat. Radiant heat, light, and the actinic rays , HABIT AND INTELLIGENCE. [CHAP. are also capable of transformation the one into the other. This relation of quantitative equivalence must indeed neces- sarily exist between all forms of energy, in consequence of the law of the conservation of energy, by which energy is unalterable in quantity through all its transformations : it may be transformed, but its quantity can be neither increased nor diminished. One of the most remarkable properties of heat is its property of transferring itself from one body to another, across vacant space, by radiation. But when heat assumes the radiant form, as it does, for instance, when on its way from the sun to the earth, it obeys laws totally different from those which are properly the laws of heat—it obeys laws which are, so far as experiment can inform us, perfectly identical with those of light—it becomes capable of reflec- tion, refraction, and polarization. Radiant heat is thus a distinct form of energy from heat of temperature, or what is properly called heat ; while, on the other hand, it differs from light only as two differently coloured, rays of light differ from each other. And the same is true of the invisible chemical rays of the solar spectrum, or what have been called the actinic rays; they differ from light or from radiant heat only as two differently coloured rays of light differ from energy ; for the quantity of heat, or of any other form of energy into which the electricity of an electrised body is capable of being transformed, is not simply proportional to what electricians call the quantity of the electricity wherewith it is charged, but to the quantity of the electricity multiplied into its tension : or, what comes to the same thing, supposing the extent of electrised surface to be given, the quantity of heat into which the elec- tricity is capable of being transformed is proportional, not to the quantity, but to the square of the quantity. This may be compared with the law that, supposing the mass of a moving body to be given, the energy due to its motion is proportionate, not to the velocity, but to the square of the velocity. (See p. 24.) The definition of electric intensity and of quantity of electricity is, that if an electrised ball of metal is brought into contact with an unelec- trised one of the same size, the electricity will spread over both; and when it has so spread, its quantity is said to be unchanged, but its intensity reduced to one-half. In this diminution of intensity there is a trans- formation of energy into heat. (See De La Rive’s Electricity, English translation, vol. ii. p. 219, et seq.) bo to) 11.] TRANSFORMATIONS OF ENERGY. each other.! Light, radiant heat, and the actinic rays, all beyond any reasonable doubt consist of vibrations in a medium that fills all space. They are consequently to be classed as one and the same form of energy, which [ classed intend in future to call radiance. ‘The fact that only eee some of the rays of the sun’s radiance are luminous depends not so much on their own nature as on the properties of the nervous apparatus by which we see ; and it is not at all improbable that to some animals rays may be luminous—that is to say, may give the sensation of light—which are obscure to us.’ When radiance is absorbed, it is transformed back into Absorp- heat ; as, for instance, when the sun’s radiance warms an pees object on which it falls. Radiation is the transformation of heat into radiance; absorption is the transformation of radiance back into heat. If radiance is a form of energy, concerning which there can be no doubt whatever, every ray must have some energy. The energy of the luminous and actinic rays, how- ever, when separated from the obscure ones, is very small, as shown by their very small heating power. The rays of the moon are usually believed to have no heating power at all, but Professor Piazzi Smith found that the heat of the Heating moonbeams on the Peak of Teneriffe, as measured by the 10" = thermo-electric multiplier, are equal in heating power to a beams. candle at a distance of twenty-six feet.’ I have now enumerated the principal kinds of actual energy, namely, Energy of Motion, Heat, Electricity, and 1 This is not quite accurate, for all rays are heating rays: if it were not so, there would be radiance incapable of transformation into heat, and we know that all energy is capable of transformation into heat. But the maxima of heating power, of illuminating power, and of chemical power, occur in different rays. 2 It has been suggested that the eyes of cats and other nocturnal animals are more sensitive than ours to the highly refrangible rays which abound in twilight. 3 Piazzi Smith’s Teneriffe, p. 212. Piazzi Smith found that when the moon was at an altitude of about 42° and the weather perfectly serene, its heating power was equal to about a third of that of a candle fifteen feet off. By the law of the inverse square, the heating power of a candle at fifteen feet is three times what it is at twenty-six feet, 30 Trans- formation of motion into heat, and the converse : Motion into elec- tricity, and the converse : Electricity into heat, and the converse : Heat into radiance, and the converse, HABIT AND INTELLIGENCE. [CHAP. Radiance ; and shall proceed to enumerate their principal modes of transformation. I. Motion is directly transformed into heat by collision and friction, and also by the compression of air or any other gas or vapour. Conversely, heat is transformed into motion by the expansion of gases and vapours: under pressure, as in the steam-engine. It is experimentally proved that when any gas or vapour expands into vacant space, so as to do no work in expanding, its temperature is not altered by expanding ; but when it expands in such a way as to do work by expanding (as steam does in the cylinder of a steam-engine), its temperature falls ; and the heat that has disappeared is the equivalent of the work done. II. Motion is transformed into electricity by the common electric machine, and also by the magneto-electric machine. Conversely, the energy of electricity is transformed into that of motion, when a body is moved by electric attraction or repulsion. IIL. Electricity is transformed into heat, when it is dis- charged under such circumstances as to prevent work from being done in any other way. This takes place in the formation of the electric spark. Conversely, heat is transformed into electricity, and an electric current is produced, when two substances that ' conduct heat unequally are placed in contact, and heat applied at the point of contact. Such currents are called thermo-electric currents. IV. Heat is transformed into radiance by radiation. And, conversely, radiance is transformed into heat by absorption. We have seen that by the law of the conservation of energy, the energy that does work always reappears as energy. This is self-evident when the resistance over- come in doing the work consists in raising a weight; the energy reappears as the potential energy due to the raised weight, and is ready to be converted into actual energy again when the weight falls. When the resistance over- a “s — P Lz 11] TRANSFORMATIONS OF ENERGY. ol come consists in friction, we now know that the work done is represented by heat or electricity. But when the work done is of such a nature as to alter the state of the No excep- body on which work is done, as for instance in sawing eee wood or grinding corn, it might be supposed that the ance of the energy that has done the work is represented by the work that'bas done, and will not reappear as energy. This, however, is done work. not the case; it is transformed into heat or electricity, just as when the work done consists simply in overcoming friction. In speaking of the potential energy of gravitation, I have remarked that a foree cannot produce energy, if, like the pressure of the ocean on its bed, it is so placed that it cannot cause motion. But the force that is overcome in sawing wood or grinding corn—namely, the force of solid cohesion—is a force that cannot produce energy, not for Forcesthat want of room to act, but because it is not its nature to do so. Fiat It is mere resistance ; and it would be an improvement in energy. our scientific language if the word /orce were confined to forces that can produce motion, and the word strength always applied to forces of mere resistance. Strength is measured by the force required to overcome Measure of it: the strength of materials, for instance, is measured by sone the weights needed to break or crush the materials. Primary forces. Electric and mag- netic forces not primary. Statice and kinetic energy. Strained elasticity. CHAPTER III. STATIC AND KINETIC ENERGY. HAVE defined potential energy as “energy due to the possible action of a pay force: that is not actually in action.” ? By primary forces I mean forces which do not originate in any other forces, but are to be referred directly to the laws of nature. Gravity and chemical affinity are primary forces: matter has been created with them. But the attractive and repulsive forces of electricity and magnetism are not primary forces, because the electrised and mag- netised states of matter are not parts of its original constitution. Matter may acquire and may lose elec- tricity or magnetism, but it cannot acquire and cannot lose weight. Energy is either potential or actual Energy is also either static or kinetic.” All potential energy is static; but actual energy may become static. The simplest case of static actual energy is that of strained elasticity. If work is done, for instance, by bending a spring or stretching a piece of india-rubber, the work done is represented by the static actual energy due to the strained elasticity ; and when the spring or india- rubber starts back to its original shape, it parts with the energy, which then assumes some other form. A spring 1 See p. 20. 2 Kinetic, from xivéw, to move. Professor Rankine uses the terms potential and actual energy where I speak of static and kinetic. He con- sequently classes as a kind of potential energy what I call static actual energy. The difference is only one of words. thie CHAP. III.] STATIC AND KINETIC ENERGY. 33 that has just been suddenly released from tension, will, if it is elastic enough, vibrate rapidly for some time; the energy of motion due to its vibrations has been pro- duced by transforming the static energy that. was due to the straining of its elasticity: and the vibrations continue until their motion is carried away by being transferred to the air and to the substances surrounding the spring. So, when the string of a musical instrument is drawn Vibrating to one side, the energy that has done work in drawing it *""S* is transformed into the energy due to its tension: and as it vibrates, the energy vibrates back and forward between the static and kinetic forms: being static at the ex- tremities of the vibration, where the string has no velocity, and kinetic in the middle, where its velocity is at the greatest. This is an exactly parallel case to that of the oscillatory transformation of energy between the potential and actual forms in the motion of the pendulum :! for the principle of the transformation is not affected by the fact that, in the case of the pendulum, the energy at the extremity of the stroke assumes the static potential form, and in the case of the vibrating string it assumes the static actual form. Energy of motion, heat, and radiance are all kinetic forms of energy: and so is current, or voltaic, electricity. Static electricity is a form of static actual energy: and in a note to this chapter I shall give reasons for believing that, as heat consists in molecular motion, so magnetism, and probably electricity, consist in the straining of pecu- har molecular tensions. But in the chapters on chemical energies and on vital energy I shall have to describe forms of static actual energy of which we cannot give any such explanation, or indeed any explanation whatever. It ought to be mentioned here, that it is possible for Trans- ; c formations one form of static energy to be transformed into another of static without passing through any intermediate kinetic state. etsy. For instance, the water that descends in the buckets of an 1 See p. 20. D : Electro- dynamic induction. Experi- ment I. Experi- ment IT. Explana- tion. HABIT AND INTELLIGENCE, [CHAP. old-fashioned water-wheel (not a turbine) moves with so small a velocity, that its energy of motion may be regarded as nothing in comparison with the amount of energy of motion that it would attain if it were permitted to fall unobstructed. If, then, the water-wheel is employed in raising weights, the potential energy given out by the descending water is transformed into the potential energy of the raised weights, without any intermediate formation of energy of motion. Of course the same kind of trans- formation is possible between the static energy of a raised weight and the static energy of a compressed spring. NOTE. ELECTRIC AND MAGNETIC ENERGY. In this note I intend to give my reasons for believing that electricity and magnetism consist in the straining of molecular elasticities. Let two conducting wires, a and B, be placed alongside of each other: or, what is better in many experiments, twisted together into a hollow spiral, but kept from metallic contact by some non-conducting substance. Let an electric current be allowed to flow along a for an appreciable time, and then cut off: on the current beginning to flow along a, a momentary current flows along B in the opposite direction to that along 4; and on the current ceasing to flow along a, another momentary current flows along B in the same direction as that along a.? The most probable interpretation of these facts is, that at the moment when the current begins to flow along a, the molecules of B are thrown into a state of elastic tension: the act of the molecules of B, in assuming the state of tension, constitutes the first current along B; and the second current (which, as stated above, is in the opposite direction to the first) is constituted by the act of the molecules of B, on the cessation of the current along 4, falling back into their normal state. 1 De la Rive on Electricity, English translation, vol. i. p. 355. m1. ] ELECTRIC AND MAGNETIC ENERGY. 35 If this interpretation is true, (and it appears to me the only possible one,) it follows that during the flow of the current of a, and between the flow of the two momentary currents of B, a definite quantity of energy has become static in B, due to the tension of its molecules. And, as ought to be the case on this hypothesis, the longer are the wires, the greater will be the force of the current of B ;1 for, supposing the force of the tension to be given, the quantity of energy due to the tension will ob- viously be in proportion to the length of the wire. In other words, the greater is the length of wire that is thrown into a state of tension, the greater will be the quantity of energy due to its tension. But as no process can either create or destroy energy, the energy that has become static in B must have come from some- where ; and its only assignable source is the current of a. This is corroborated by the following facts :— The current of a is the inducing current, and those of B the induced currents. It is possible to leave out B, and to obtain the induced currents in a: in other words, it is possible to obtain the induced currents in the same wire with the inducing current. Let a be sufficiently long, and let B be left out; let Experi- the current along a be stopped by breaking contact suddenly ; in ™°™t HI. the act of breaking contact there will be a momentary increase of the force of the current, producing a spark. This increase of the current is evidently the same as the current that would have been induced in B, had B been there, in the same direction as that of a. The two currents, the inducing and the induced, flow for the moment along the same wire and in the same direction, and the spark is due to the swm of their effects. But the current induced in B, at the commencement of that of A, is opposite in direction to that of a: so that if the inducing and the induced currents can be caused to exist at once in the same wire, the resultant current will be due to their difference. This may be experimentally shown. If B is left out, and a long Experi- enough, the current will be sensibly diminished in force at the ™nt IV. moment of its commencing to flow along a.? 1 De la Rive on Electricity, English translation, vol. i. p. 358. These facts were discovered by Faraday, who at first gave an explanation of them with which mine is substantially identical. He called the state of ten- sion into which wire B is thrown the electro-tonic state. He afterwards, however, changed his opinion on grounds which I do not understand. 2 Ibid. pp. 359, 360. D2 36 Explana- tion. Electro- static nduction, and con- tinuous currents, probably both due to mole- cular tension. Electro- magnetic induction. Experi- ment V. Experi- ment VI. Explana- tion. HABIT AND INTELLIGENCE. [CHAP. According to my explanation, the momentary slackening of the current at its commencement is caused by a portion of its energy being taken up, and becoming static, in throwing the wire into a state of molecular tension; and the momentary increase of the force of the current at the moment when it is cut off is caused by the energy that was static in the wire returning to the state of kinetic, or current, electricity. I have now stated the most important facts of electro-dynamic induction, and, as I believe, given a satisfactory interpretation of them. And I believe, though the evidence is not nearly so strong, that electro-static induction, as in a Leyden jar, will be explained in some similar way ; namely, that the electric charge consists in some kind of molecular tension. The explanation I have given above applies only to those momentary currents which are produced by induction ; it does not apply to continuous currents. But I believe that the hypo- thesis of molecular tension will be found to explain the facts of continuous currents also. The molecules of a conductor along which a current is flowing are, I think, shown to be in a state of molecular tension (of a different kind, however, from that of wire B) by the fact that all the successive portions of the same current repel each other: as is easily proved by suitable experi- mental arrangements.! With respect to the theory of electro-magnetic induction, however, there is no difficulty whatever: it admits of an ex- planation exactly parallel to that which I have given of electro- dynamic induction. In other words, the induction of momentary magnetism in a soft iron bar is an exactly parallel fact to the induction of a momentary current in a conducting wire. Let an electric current be flowing along a wire coiled into a hollow spiral, and let a soft iron bar be put into the spiral ; the iron will be instantaneously magnetised, and the current will for the moment become less forcible. On removing the bar it at once loses its magnetism, while the current, for the moment, becomes more forcible. These facts admit of exactly the same interpretation as those of induced currents. When the iron is put into the spiral, and the current for a moment loses force, the slackening of the current is caused by a portion of its energy being taken up and becoming static in the iron, which it throws into a peculiar state 1 De la Rive on Electricity, English translation, vol. i. p. 231. a os SS .) 7 4a ae ee 111.] ELECTRIC AND MAGNETIC ENERGY. BY of molecular tension, constituting the magnetised state. And when the iron is taken out of the spiral, the energy that was static in the iron becomes kinetic in the wire, and increases for the moment the force of the current. Thus the facts of electro-dynamic and of electro-magnetic induction are not only themselves parallel, but admit of a parallel interpretation ; and this tends to confirm it for both. Besides this, there is other and more direct proof that soft iron, when temporarily magnetised by the passage of an electric current near it, is in a state of tension. Instead of moving the iron bar into and out of the spiral while the electric current is flowing, let the iron remain in the spiral while the current is turned on and of; the act of turning Experi- the current on will magnetise the iron, and the act of turning it oe off will demagnetise the iron. It has been ascertained by Professor Joule, that at the moment Experi- of turning the current on so as to magnetise the iron bar, the Toe bar will be elongated by about a 720,000th of its length ; which tion of increase of length it will lose in the moment of turning the anaes current off so as to demagnetise it.! This appears to be con- magnetisa- clusive evidence that the magnetised iron is in a state of i molecular tension. This is still further confirmed by the very remarkable fact, Experi- that when the iron bar and the wire spiral are placed as in the ne last experiment, and the current alternately turned on and off at the rate of several times in a second (which is easily done by means of a mechanical arrangement), sounds are heard pro- ceeding from the iron.2 By turning the current alternately on and off, the iron is alternately thrown into and out of a state of tension. If a magnet is put into a wire spiral like that used in the Sounds former experiments, while there is no current flowing along the poraes wire, the approximation of the magnet will induce a momentary netisation. current in the wire; and if the magnet is taken out again, another momentary current will be induced in the wire, but in the opposite direction. It is on this principle, although the The mechanical arrangements are different, that the magneto-electric Hee apparatus is constructed for supplying the electric light. machine Whence comes the energy of the induced currents in these 1 De la Rive on Electricity, English translation, vol. i. p. 306. 2 Thid. p. 303. 38 HABIT AND INTELLIGENCE. [cHAP. III. experiments? It cannot be from the static energy of the magnet, for the magnet loses none of its magnetism, and con- sequently cannot have parted with any of the static energy to which its magnetic state is due. The magnets in Mr. Holmes’s electric lighthouse apparatus are stated by him rather to gain than to lose force with use. But if the kinetic energy of the electric current had to be obtained by the transformation of the static energy of the magnets, the whole stock would no doubt be expended in a few seconds at the most. The energy of the transforms current is obtained by transforming the mechanical energy which an is employed in moving the magnets. The transformation takes electric place under the influence of the magnets, and would not take energy —_ place without them, but it does not take place at their expense. eee CHAPTER IV. PRIMARY FORCES. E have seen’ that all energy has its origin in force: for instance, the force of gravity produces the energy Force of motion of a falling body. And as all forms of energy aoa are capable of mutual transformation, it follows that any force may produce any form of energy. If gravity causes a body to fall, and in its descent it produces electricity or heat by friction, there is electricity or heat produced by gravity. But the converse is obviously not true; gravity may produce motion, but motion cannot produce gravity : energy chemical affinity may produce electricity or heat, but cece neither electricity nor heat can produce chemical affinity. ee The ultimate origin of all energy is in primary forces; and ae primary forces are defined as forces which do not rons Primary in any other forces, but are to be directly referred to the ie ae laws of nature, and have their origin directly in creative power. Thus the attractive and repulsive forces of a magnet or Instances of an electrised body are not primary forces, because they aes are not to be referred to the ultimate properties of the mary. bodies. The electrical attractions and repulsions of a body that has been electrised by a common electrical machine are due to the charge of electric energy which the body has taken up; and this electric energy has been obtained by the transformation of the mechanical energy that worked the machine.” And the magnetic attractions and repulsions of an iron bar that is magnetised by the passage P. 19. P30: 40 Three pri- mary forces, gravity, capillarity, and affi- nity. Their pro- perties. HABIT AND INTELLIGENCE, [CHAP. of an electric current are due to the charge of magnetic energy which, as I have endeavoured to prove, is obtained by the transformation of the energy of the electric current, and taken up and, as it were, for the time incorporated with the iron! ‘None of these forces are primary forces, because the bodies have acquired them and may lose them. There are three, and only three, kinds of primary force. These are: 1, Gravity ; 2, Capillary attraction ; and, 3, the attraction of chemical elements that seek to combine (as, for instance, oxygen and hydrogen, or oxygen and carbon), or, to use the common but very infelicitous term, chemical affinity. For the sake of brevity let us call these gravity, capillarity, and affinity. Capillarity is of much less im- portance than the other two, but I mention it in order to make the enumeration complete. Gravity acts at all distances, and is for that reason a force acting on masses—a molar force. Capillarity acts only at insensible distances; in other words, it acts only when bodies are in immediate contact. It is for that reason a force acting on molecules—a molecular force. Affinity also acts only at insensible distances; but there is this difference between capillarity and affinity, that capillarity acts on the molecules into which a body may be broken up by mere mechanical division, while affinity acts on the chemical atoms which are the constituent parts of the molecules. Affinity is consequently an atomic force.? Gravity, acting at all distances, is, probably for that reason, always in action—all matter is always attracting all other matter. Capillarity and affinity, acting only at insensible distances, act, probably for that reason, only under favourable circumstances ; capillarity is chiefly 1 See Note to Chapter III. 2 When I speak of molecules and atoms, I only mean the smallest integrant and constituent parts, without implying any opinion as to whether or not these are infinitely small. I speak of the molecules of a body in the same way that mathematicians speak of infinitely, or inde- finitely, small ares of a curve. a i Iv.] PRIMARY FORCES. contined to the liquid state of matter; and affinity gene- rally, if not always, requires a certain degree of eleva- tion of temperature before it will act. Combustion, for instance, which is the most energetic of all chemical actions, in most cases will only commence at a very high temperature, and never, so far as I know, at a very low one. Gravity is incapable of saturation; that is to say, what- ever be the quantity of matter that any mass of matter is attracting, it is capable of attracting any additional quan- tity with exactly the same force as if it had no other to attract. In briefer language, all matter attracts all other matter. Capillarity, on the contrary, is capable of satu- ration: as capillarity acts between molecules only when they are in contact, the capillarity of a molecule of (for instance) water or mercury for other molecules of the same liquid is in a state of saturation so long as it is surrounded by other molecules of the same liquid; all its capillarity is employed in attracting them, and it cannot attract, or be attracted by, any more. Affinity also is capable of saturation. One volume of hydrogen, for instance, does not attract all oxygen, but only one half-volume of oxygen; and when these are combined to form water, their affinity for each other is saturated; the water has no further affinity for either hydrogen or oxygen. Gravity is not an elective force ; that is to say, it acts on all matter alike: all matter attracts all other matter with a force directly as the mass. Capillarity, on the contrary, is elective; that is to say, some, probably all, liquids have a stronger capillarity for some substances than for others. Mercury, for instance, has a strong capillarity for itself, as is shown by the way it runs into globules ; but it has very little capillarity for other substances, except metals. Oil, on the contrary, has a stronger capillarity for most solids than for itself, as is shown by its rising in a wick and spreading over surfaces, in opposition to gravity. But oil and water, or water and mercury, have hardly any capil- larity for one another, and refuse to mix, or even to adhere. Affinity also is elective: oxygen, for instance, has a very 41 42 Summary of their properties. HABIT AND INTELLIGENCE. [CHAP. strong affinity for hydrogen, carbon, and iron, but hardly any for nitrogen. The potential energy due to gravity becomes actual by the falling of bodies, in which act it is transformed into energy of motion. When the potential energy due to capillarity becomes actual, it also is transformed into energy of motion: this takes place when two drops of mercury or of water rush together. But affinity differs fundamentally from these two, in that its potential energy, when it becomes actual, is not transformed into energy of motion, but into heat or electricity. Combustion is not only the most familiar but the best instance of the trans- formation of chemical potential energy : the heat of com- bustion is the transformed energy due to the affinity of the oxygen and carbon that combine in the act of combustion. But in many cases of combination it is possible, by means of the arrangement of the voltaic battery, to obtain it in the form of electricity instead of heat. When the potential energy of gravity or capillarity becomes actual, the change in the form of the energy does not determine any change in the character of the bodies. Water, for instance, does not change its character in falling, nor do drops of water change their character as water in rushing together by reason of their capillarity. But when the potential energy of affinity becomes actual, the transformation of the energy is always accompanied by a change in the character of the substances; they combine, and the compound has properties unlike those of either of the constituents. To sum up: Gravity acts at all distances and at every moment ; it is incapable of saturation, and is not elective. Capillarity and affinity, on the contrary, act only at in- sensible distances, and under favourable circumstances ; they are capable of saturation, and are elective. Gravity and capillarity, when their potential energy becomes actual, produce it as motion, and the character of the substances remains unchanged. Affinity, on the contrary, produces the energy in the form of either heat or electricity, and a rv.] PRIMARY FORCES. 43 the substances combine, forming a compound with new properties. It is a very important generalization, that all primary All pri- forces are attractive ; there is no such thing in nature as a pat ks primary repulsive force. For this, as for every other ulti- attractive. mate physical law, no cause can be assigned except the Divine will. But its purpose is obvious. The universe is Purpose of held together by attractive forces; and if, as I believe, the oe: nebular, or, as I prefer to call it, the condensation theory of world-formation is true, the universe has been formed by the action of attractive forces. Repulsive forces, on the contrary, it is obvious, could neither form a world nor hold it together. It needs no proof that gravitation and capillarity are attractive forces. In the case of affinity this is not quite so evident. But we know that the action of chemical forces tends to union, and not to separation ; and we know also that it tends to condensation. The proof of this latter assertion is, that (among gases, in which alone it is pos- sible to study chemical forces purely and simply) the volume of a compound is often less than the joint volume of its constituents when uncombined, and never greater From the truth that all primary forces are attractive, it Potential follows that potential energy is never in a body, but always muli: thus, if one of the tentacles of a hydrozoon be irritated, not only that tentacle but the others will contract. In this class, however, the propagation of the stimulus is relatively slow, and, consequently, if one tentacle is irritated the rest will contract but slowly. The dryozoa are a mol- lusean or molluscoidan class, adapted to the same kind of life as the hydrozoa, but with a totally different anatomy, and a discernible, though rudimentary, nervous system; 140 HABIT AND INTELLIGENCE. [CHAP. and among them, if one tentacle is irritated, the rest will contract instantaneously.! As already stated, the parts of organisms in which nutri- tive processes go on (as, for instance, young buds, animal and vegetable embryos, and secreting organs) mostly con- sist of cellular tissue; and vascular structures, among vegetables certainly,” and probably among animals also, consist of modified cells. This is shown both by the Develop- history of their development among the higher vegetables, mentiof , and by the fact that among the higher fungi and lichens of cells. we meet with cells elongated in the direction of the length of the stem, which are evidently a transition from simply cellular to vascular structure.* Cellular and vascular struc- tures are thus probably akin; and the parallel relation Resem- between muscle and nerve is, I think, shown (though I do es Berg not wish to lay any stress on this) by the resemblance of fibre to the minute structure of nervous fibre to that of muscular muscular. fibre4 But the parallelism of the circulatory and nervous systems is far more unmistakeably shown in various other Blood: ways. The nerves and the blood-vessels resemble each vesselsand other in ramifying and inosculating * throughout the entire ramify. body; and also in this, that the action of both is in a great degree (though, as careful researches have shown, not abgo- lutely) controlled by great central organs ; the circulatory The heart system by the heart, and the nervous system by the brain. pnd the Circulatory centralization and nervous centralization, as they may be called, are seen to increase together as we ascend in the animal scale: that is to say, the higher is the organization of any animal, the more complete is the centralization both of its circulatory system, and conse- quently of its nutritive or vegetative life——and of its nervous system, and consequently of its animal life. It is 1 Spencer’s Principles of Biology, vol. ii. p. 368. 2 Thid. p. 264. 3 Carpenter’s Comparative Physiology, p. 671. * For the fact of this resemblance, see Beale’s edition of Todd and Bowman’s Physiology, p. 72. 6 To inosculate means to reunite after ramifying. But though nerve trunks ramify and inosculate, it is believed that the ultimate nerve-fibres remain always distinct. j x11.] THE DIRECTION OF DEVELOPMENT. 141 a most interesting instance of this, that the amphioxus, a Amphi- species of fish which is the lowest of all vertebrates, is at °“"~ once without a brain, though it has a spinal cord, and without a distinct heart, instead of which it has several pulsating enlargements of the blood-vessels." These two characters make it quite unique among vertebrates, and probably show that it is a link between the vertebrates and some now lost class of low organization. The parallelism between the circulatory and nervous Blood- systems is further shown in this, that those parts of the Gass ang body which are the most abundantly supplied with blood- abundant vessels are also the most abundantly supplied with nerves ; Scan and the nails and hair, which have no supply of blood at all, are also without nerves.? The activity of the two is Their | heightened or lowered together. The vessels of inflamed feigned parts are unusually full of blood, and the action of their together. nerves is heightened: this last is proved by increased PES sensibility, and also, as I have shown reason for believing, by elevated temperature.* And, what is a parallel fact to this, the blood-vessels of the brain are comparatively empty during sleep, which essentially consists in a lowered action Sleep. of the brain; and the same is true of the blood-vessels of the retina.t It is possible to lower the nervous activity so Want of as to destroy the sensibility of any part, by tying the ree : arteries so as to deprive it of its supply of fresh blood.> It sensibility. belongs to this class of facts that the heart, which is the Connexion central circulatory organ, is more easily acted on by nervous cena influence than any of the other organs of the vegetative brain. life. The susceptibility of the heart to influences arising from the emotions has caused it, indeed, in popular language, to be regarded as their seat. We have seen how dependent is the activity of the Depend- nervous system on the supply of blood. But the converse ee is not true: the activity of the circulating system does not action on 1 Carpenter’s Comparative Physiology, p. 447. 2 Carpenter's Human Physiology, p. 604. 3 Ibid. p. 99. 4 Thid. p. 590. 5 Carpenter’s Comparative Physiology, p. 175. 6 Claude Bernard, in Revue des Dewx Mondes, 1st March, 1865. 142 circula- tion not reciprocal, Opposite relations of blood and nerve to muscle, HABIT AND INTELLIGENCE. [cHAP. in any similar way depend on nervous agency. Thus, as just stated, the nerves of a part are paralysed by tying the artery that supplies it ; but the circulation of a part is not paralysed by cutting the nerve-trunk that supplies it. Recent experiments on cold-blooded animals have shown a very remarkable contrast between the relations of blood and nerve to muscular fibre. We know that the stimula- tion of a nerve tends to cause the muscle in connexion with it to contract. It is now ascertained that the supply of fresh arterial blood tends to cause the muscles that, it bathes to retain the elongated state; and when the supply of blood is cut off, the muscles tend to contract spon- taneously.1_ In other words, blood tends to keep the muscles elongated and relaxed, and nervous action makes them contract. To state these facts in the language of the dynamic theory :—The blood undergoes oxidation in the lungs: in the process of oxidation energy is liberated, part of which is carried by the blood to the muscles, and supplies them with their charge of static vital energy. So long as they remain elongated, the muscles retain this charge of static vital energy ; but when a nervous stimulus comes and makes them contract, this vital energy is transformed into motor energy. It is to be observed, that the elongated or relaxed state of a muscle is that in which it contains a charge of energy, and the contracted state is that in which the energy has been parted with. This distinction is one on which emphasis must be laid, for it is very important, and by no means obvious. It is totally unlike that which obtains in an elastic body, such as an india-rubber cord; for the india-rubber contains a charge of energy while it is strained, but none while it is relaxed. It is also proved by the foregoing facts that the blood 1 See Dr. Norris’s Report on Muscular Irritability, British Association Report, Nottingham, 1866. This law, I believe, was first stated by Dr. Radcliffe. (Carpenter’s Human Physiology, p. 680.) 2 See p. 97. The theory that the motor energy produced by any muscle is due to the oxidation of that muscle at the very moment, is, I think, conclusively disproved by the experiments of Fick and Wislicenus. (See Philosophical Magazine, June 1866.) oe ee x11] THE DIRECTION OF DEVELOPMENT. 143 supplies the various parts of the body with energy as well Blood - : supplies as with matter. The nerves, of course, transmit energy oieeg me only. well as Va matter. What has been said on the direction of animal develop- ment may be thus summed up :— Every organism transforms both matter and energy, and Summary. it is probable that every living part of an organism always continues to transform them both. But the characteristic of animal development is, that one set of organs—the nutritive—is specially appropriated for the transformation of matter; and another—the nervo-muscular—for the trans- formation of energy. The nutritive, or vegetative, system of organs is for the most part internal, and the charac- teristically animal organs—those of the limbs and mouth, with their muscles—are external to the nutritive. In both vegetables and animals, a vascular, or circulatory, system is developed, the function of which is to minister to the nutritive life by the circulation of nutritive fluid; and in animals a nervous system is developed, the primary function of which is to ensure the harmonious and efficient action of the muscular system, by the transmission of stimuli from one part to another. The foregoing account of the relation of nerve to muscle, Ganglia. however, would be most imperfect were it not mentioned that nerve-fibres never exist without ganglia, which are situated at the junctions of the fibres, and from which the fibres radiate throughout the body. The action of ganglionic tissue is even more mysterious than that of nerve-fibre. It appears, however, to be a universal law that one fibre can communicate a stimulus to another only through a gan- glion. In the higher animals, sensation and thought are Sensation. produced in some altogether inscrutable way by the mutual action of the ganglia and the nerve-fibres: sensation can be produced neither in a ganglion unless it is acted on by a nerve-fibre, nor in a nerve-fibre unless it is in communi- cation with its ganglion. 144 Lewes’s theory of sensation disproved. Impossi- ble to say where sensation begins. Nerve- fibre may act with- out gan- glionic influence. HABIT AND INTELLIGENCE, [CHAP. XII. NOTE. THE FUNCTIONS OF THE NERVOUS SYSTEM. Mr. Lewes, in an admirable treatise on nervous action, which forms part of the second volume of his ‘“‘ Physiology,” has ad- vanced the opinion that all mutual action of ganglia and nerve- fibres is accompanied by sensation. He thinks the action of the visceral nerves, of which we are unconscious, goes to make up the sense of being alive. This theory is plausible, and it might appear impossible to prove it wrong; but I think it is disproved by the fact, that cases of blindness have been observed in which, though there is total insensibility to light, the pupil continues to contract in light and expand in darkness.! There are special nerves for this expansion and contraction; and the fact just quoted, I think, proves their action to be quite independent of sensation. This instance, and many others, though I know of none so striking and conclusive, show how impossible it is to determine where sensation begins ; and if a large part of man’s nervous system is insentient, it does not appear anomalous or improbable that the whole nervous systems of the lower worms and mollusca should be so. I believe the most probable guess we can make is, that in the ascending scale of organization sensation begins with the first appearance of organs of special sense; and the most generally distributed organs of special sense among animals appear to be the eyes. It has been often repeated that the ganglia are generators of nervous energy, and the nerve-fibres its conductors. I agree with Mr. Lewes, however, that this account of the matter is, at least, insufficient. A nerve-fibre is capable of acting even when it is not in communication with a ganglion; as is proved by the well-known experiment of making the cut-off leg of a frog kick by means of exciting its nerve with electricity, and also in the experiment mentioned in the foregoing chapter, by which Helmholtz has measured the velocity of the nervous current. 1 Carpenter’s Human Physiology, p. 583. eo TT a a CHAPTER XITl. ORGANIC SUBORDINATION. W* have seen in the last chapter, that the highest organic development is the most complete physiolo- gical division of labour, and the most perfect physiological centralization. In the lowest organic species, and in the germs of the highest, the parts are all alike and all inde- pendent of each other; in the mature forms of the highest species the parts are all different, and the whole organism Organic _ is bound together into one system, with all its parts Sepa mutually dependent. To speak technically, organic pro- integra- gress consists in increasing differentiation and increasing ae integration.} Besides these, there are within the organism relations of dependence and of subordination, which I have now depend- to describe. Before stating the relation of organic de- suet pendence, I must go back to the inorganic sciences. tion. Whatever exists, so far as is known, or can be known, Space and to us, exists in space ; and whatever acts, acts in time. Bee Consequently the properties of space and time are con- ale ditions of all existence and of all action; the laws under ~*~ which things exist and act cannot be proved, nor even stated, without express or implied reference to the properties of space and time. It results from this, that mathematics, ,,. _.- which is the science of the laws of space and time, is the quently necessary ground of physical science. To take the very ee simplest instances: it would be impossible to prove, or ground of hysical even to state, the law of the parallelogram of forces, unless foince. 1 The word differentiation is now generally used in this sense. Integration is a parallel word to it, and is used in Spencer’s Biology. L 146 HABIT AND INTELLIGENCE. [cuap. the geometrical properties of the parallelogram were known; and it would be impossible to prove, or to state, the law that the energy due to a moving body is proportional to the second power of its velocity, unless the nature of powers and roots were known. Mathematics is thus necessary as a foundation for dynamics. Dynamics, Among physical laws, the most general are those of the basis of force: the laws of force, or, as they are generally called physical De? Af 5 y ’ science. the laws of motion, are the only laws which are true of all action whatever. Consequently, dynamics is necessarily the basis of physical science: or, in other words, the theory of force is necessary as a basis for the sciences of material things.t Secondary The sciences of sound, radiance, heat, electricity, and dynamical ; : . : sciences, magnetism, are merely particular cases of dynamics, being applications of the theory of force to special kinds of actions. The laws of force apply to all the actions of all matter; but there is a great variety of laws that apply only to those actions, or functions, which are characteristic of particular Chemistry. kinds of matter: I mean the laws of chemistry. The laws of chemistry depend on those of heat and electricity in avery great degree ; so decided indeed is the dependence, that it would be impossible so much as to state many of the most important chemical laws, unless the elementary laws of heat and electricity were taken as known. Biology. Finally, the properties of living organisms, and their peculiar actions, in a great degree depend on the general properties of force, and on the special properties of the chemical substances of which the food of the organism, and the organism itself, are composed. Vital properties are certainly more than mere resultants from physical and 1 It may be said that I contradict myself in calling mathematics the ground of physical science, and dynamics its Jasis. There is, however, no contradiction. Dynamics is a part of physical science, but mathematics. is not. Physical science is built on mathematics, as a building on the ground ; the other parts of physical science are based on dynamics, as the higher parts of a building on its base. I attach no importance to these metaphors, but I wish to show that I have not fallen into any inconsistency. r :. i » 7 ~ ’ a q xu. ] ORGANIC SUBORDINATION. 147 chemical ones, but life does not suspend the ordinary physical and chemical properties of the substances in the organism; on the contrary, it works through them. Con- sequently, the action of life depends on the properties of the materials it has to work with; and it is impossible to understand the nutritive functions of organisms, without some previous knowledge of chemistry. It would have been impossible, for instance, to explain the nature of respiration, which is a slow combustion, unless the nature of combustion had first been discovered. So that we have this series :— 1. Mathematics, or the science of the properties of space Series of and time. sciences, 2. Dynamics, or the science of the laws of force in general. 3. The secondary dynamical sciences, being those of sound, radiance, heat, electricity, and magnetism; all of which are particular applications of dynamical theory. 4. Chemistry, or the science of the special properties of particular kinds of matter. 5. Finally, biology, or the science of the properiies of living beings. In this series, each member is ‘dependent on that which each goes before it, but independent of that which comes after Se eee it. Biology is dependent on chemistry, because the actions ceding. of life on the substances in the organism cannot be under- stood, unless the properties of the substances themselves are known first. Chemistry is dependent on the secondary dynamical sciences, because its laws imply those of heat and electricity, and could not be understood without them. The secondary dynamical sciences are dependent on general dynamics, of which they are but particular cases. And lastly, dynamics depends on mathematics, without which it cannot make a single step in reasoning. This dependence is not reciprocal. The truths of mathe- Depend- matics do not in any way depend on those of dynamics tone for their proof. The truths of general dynamics are true, independently of those of the secondary, or special, dyna- mical sciences. The laws of the secondary dynamical L2 148 HABIT AND INTELLIGENCE. [cHap. sciences are true independently of those of chemistry, and can be understood without them. And the laws of che- mistry are true, independently of those of life, and can be understood without them.t Thus the series resembles a building of several stories, each of which rests on that below it. Depend- Although, in order to avoid circumlocution, I have a8 eal spoken of the sciences as depending the one on the other, sciences, yet in reality the dependence is not only of the sciences, ee that is to say of our knowledge of the laws, but of the laws of the things themselves. Not only our knowledge of biological laws, but the biological laws themselves, depend on chemical laws. Not only our knowledge of the chemical laws, but the chemical laws themselves, depend on the laws of heat and electricity. The laws of heat and electricity are but cases of the laws of general dynamics. And not only our knowledge of the laws of dynamics, but the laws of dynamics themselves, depend on the laws of mathematics, which are but the statement of the properties of space and time. I am not here in- sisting, as may perhaps be thought, on a merely identical, or self-evident, proposition. It is quite easy to conceive such a relation between two sciences, that our knowledge of the one shall be dependent on our proficiency in the other, without the subject-matters of the two having any Accidental connexion whatever. Such a relation does exist between Sf histo. OPtics, or the science of light, and histology, or the science logy with of the minute structure of organic tissues. Histology has opes. ‘been created as a science by the microscope, which owes 1 Tt may be objected, that this is wrong in point of fact: it may be said that chemical laws are implied in the theory of electric currents, and biological laws in organic chemistry. I reply, that electro-chemistry does no doubt imply chemical laws, and may be regarded as a branch of chemistry, but the whole theory of electro-statics and electro-dynamics may be stated without any chemical knowledge being needed. And as to organic compounds, chemistry works with them just as if they were mineral substances. Of course the various sciences run into each other, and have many and varied mutual relations. But I think they are more distinct, and stand in simpler relations to each other, than we might have anticipated. E di ’ f ; ; x. ] ORGANIC SUBORDINATION. 149 its existence to the advance of optics ; and yet the subject- matters of optics and histology have no connexion what- ever. But such a connexion is merely accidental and instrumental: the connexion of the sciences in the series which I have drawn above, on the contrary, is logical and real, and is based on the dependence of the things themselves. So far, as the reader will perhaps have perceived, I have Obligation taken these ideas about the dependence of the properties of '° Comte. things, the one on the other, from Comte’s Positive Philo- sophy.! What follows, though I claim no originality for its substance, has not, so far as I am aware, been stated in a systematic form before. We have seen that in inorganic nature, and up to the Depend- laws of life, there is a relation of dependence of the laws (i) ae of one science on those of another, which dependence is one on the not reciprocal. The same relation is continued between ° the different laws of life: animal or motor life depends on Vegeta- vegetative or nutritive life; and mental life depends on bates a animal life. And among ane also, the dependence is not and mental reciprocal: vegetative life may exist without animal life, Me and animal life may exist without mental life. Mental life depends on animal life, and animal life depends on vegetative life, just as vegetative life depends on chemical properties, and chemical properties depend on those of heat and electricity. These are facts of observation. Through- out the whole vegetable kingdom we see vegetative life without animal life; and throughout a great part of the animal kingdom, we see very energetic animal life with scarcely a trace of mental life. But the converse is im- possible ; there is not, nor under the laws of life can there be, any such thing as animal life without vegetative or nutritive life for its basis; or mental life, without animal life as its basis. It is a consequence of this relation, that 1 See Harriet Martineau’s condensed translation of Comte’s Positive Philosophy, vol. i. chap. ii, I have read Comte only in the above-men- tioned translation, which I believe is thoroughly trustworthy. The series T have drawn in the text differs from Comte’s in detail, but is the same in principle. 150 Sleep. Experi- ment. The series continued. HABIT AND INTELLIGENCE. [CHAP. the animal life may be almost, if not totally, suspended in sleep, but the vegetative life cannot be suspended for a moment without death. And another very remarkable consequence of the same relation has been experimentally ascertained ; namely, that it is possible to extinguish the mental life, and in a great degree the animal life of an animal, by removing the parts of the brain that minister thereto, while the organs of the vegetative life continue to perform their functions for a considerable time. Of course in this experiment, as well as in sleep, the involuntary muscles of the heart and lungs continue to act, as on their action that of the vegetative life depends. We may now thus continue the series that we saw to exist from the laws of space and time up to those of life, so as to include the three ascending degrees of life itself; each term of the series being dependent on those which - go before it, but independent of those which come after it :-— 1. The properties of space and time : mathematics. 2. The laws of force: dynamics. 3. Special cases of the laws of forces sound, radiance, heat, electricity, and magnetism. 4. The properties of particular kinds of matter :— chemistry. 5. The laws of vegetative life. 6. The laws of animal life. 7. The laws of mental life. But though the dependence of animal life on vegetative life is of the same kind with the other laws of dependence that I have stated, yet it is not practically possible to treat of them apart, as the subjects of distinct sciences. The old distinction of zoology and botany must no doubt be always necessary in classificatory, or what are called sys- tematic, works; but it would be impossible to treat the physiology of the vegetative life, and that of the animal life, as distinct sciences. With the laws of mental life it is different. They may in a great degree be adequately treated of apart ; and I intend in this work to follow the usual practice, and to keep psychology, or the science of xu. ] ORGANIC SUBORDINATION. 151 mind, as distinct as possible from biology, or the science of life. At the same time I avow my opinion—though it is, perhaps, only a question of words—that psychology is really a branch of biology. We have seen that the relation of dependence of one group of properties, or functions, on another, holds both in inorganic matter and in life. But when we come to vital functions, we find a different though parallel relation, unlike any in the inorganic world. I mean the swbordina- Subordina- tion of one function to another: one function working pe through another. As I have already stated, life acts functions. through the physical and chemical properties of matter ; and it is equally true that the conscious, or vital functions, or those of the mind, act through the unconscious ones. These statements may need explanation. Life, as I have stated before, does not suspend the Matter ordinary laws of matter and energy; life works in accord- Sage ance with those laws and through them, directing their forces to the attainment of ends which they would not have attained of themselves. Thus, though life is so com- pletely dependent.on the ordinary properties of matter that it could not exist, nor even be conceived to exist, without them; yet life makes those properties subordinate to its own purposes. Exactly parallel to this is the relation of the mind to the unconscious life. The mind is dependent for its existence on the unconscious life : mind is a function of the nervous system; and the primary purpose of the nerves, as we have seen in the last chapter, is to enable the muscles to work together. But mind has the power of Uncon- making the unconscious life subordinate to its purposes. sou This last statement will perhaps be scarcely intelligible. uate to Tt may be thought that whether the mind works alone, in as thought,! or through the body, in voluntary muscular action, all mentally directed action is conscious ; and that the only unconscious life is the vegetative life, which is not under the direct control of the mind at all. This, however, would be a mistaken view. Paradoxical as it may sound, it is a 1 This expression is not strictly accurate. I shall have to show farther on, that all mental action is connected with bodily action. 152 Muscular action essentially uncon- scious. Instance in reverie. Summary. HABIT AND INTELLIGENCE. [CHAP. simple truth, that muscular action is itself unconscious. We produce the motion of a particular set of muscles— those of the legs, or hands, or mouth, for instance—by a conscious mental determination; we become aware that they move as we intend, by means of the “muscular sense,” which is produced in muscles by their action. But between the conscious mental determination and the sense of muscular action, there is an intermediate link of which we are utterly unconscious ; namely, the special combina- tion of muscles which is needed to effect the movement we intend. Of this we know nothing whatever except what anatomy teaches us; we effect these combinations by a perfectly unconscious instinct. Were consciousness of the required muscular combinations necessary before we could make the combinations, in the same way that, for instance, consciousness of the meaning of words is neces- sary in order to use the words with accuracy, we could not perform any muscular movement until we had learned the anatomy of the muscles.! It is indeed scarcely a metaphor to say that the brain gives its orders to the muscles with- out knowing the details of the way in which its orders are to be executed. An equally clear proof of the essentially unconscious nature of muscular action is afforded by the fact, that when any set of muscles, especially those used in walking, is set in motion by a determination of the will, and the attention afterwards withdrawn from their action in consequence of the mind falling into a state of abstrac- tion or reverie, the action of the muscles often continues independently of consciousness or will. And, what is a fact of the same kind, if nervous connexion between the brain and the lower extremities is cut off by accidental injury to the spine in man, or by purposely cutting through the spinal cord in an animal, irritation applied to the feet causes no sensation, but produces convulsive move- ments in the legs, of which the patient is unconscious.” To sum up what has been said :—The higher functions are dependent on the lower ones; the vital functions are dependent on the inorganic, and the conscious, or mental 1 Carpenter’s Human Physiology, p. 559. 2 Thid. p. 529. “2igtd Tah '% Fe ee ee eee xu.] ORGANIC SUBORDINATION. 15 vo functions, on the unconscious ; but this dependence is not reciprocal. And the lower functions are subordinate to the higher ones, which work through them ; the unconscious functions are subordinate to the conscious, and the inorganic functions to the organic; and this subordination never becomes reciprocal. It is to be observed that the dependence of functions Depend- one on the other is necessary and constant; the conscious ee functions are always and necessarily dependent on the ordination unconscious ones, and life is always dependent on matter. nae But the subordination of functions one to the other is neither necessary nor constant; the mind often loses its control of the body, and. life often loses its control of matter. When the control of the higher functions over the lower, and the subordination of the lower to the higher, are weakened, the result is disease; when they are destroyed, the result is death, Classifica tion of organic functions. Different classifica- tions for different purposes. CHAPTER XIV. ORGANIC FUNCTIONS. fa the last chapter I have classed the organic functions as vegetative, or nutritive; animal, or motor; and mental. The vegetative and animal functions I have classed together as unconscious, in opposition to the mental, which are conscious. These distinctions may be most conveniently stated in the following tabular form :— Vegetative, or nutritive. Animal, or motor. Conscious functions . . Mental. Unconscious functions In speaking of the same subject, however, it is often necessary to adopt different classifications at different times, according as we regard it from different points of view. That adopted above shows the relations of the dif- ferent vital functions as made known from the common, practical point of view of every-day and every man’s con- sciousness, which begins with the distinction between the body and the mind. This point of view is no doubt in- sufficient, and its results need to be corrected by compa- rison with those obtained from other points of view. We are so constituted and so circumstanced, in the intellectual as well as in the physical world, that the observations taken from any single point of view are necessarily in- complete, and need to be supplemented by others. But, though incomplete, they are true so far as they go, and are untrue only when they are mistaken for the whole truth. In the present chapter, I shall adopt a classification of organic functions which is not based, as the former one aay . ; | CHAP. XTV.] ORGANIC FUNCTIONS. 155 was, on their obvious connexions. That which I am going to use is intended, on the contrary, to explain the way in which one function is developed out of another. The law of the development of organisms, as we have seen, is that they are developed out of simple germs, and that the parts are gradually differentiated the one from the other. The Develop- same is true of functions;—functions also are developed by menor gradual differentiation. I may not, perhaps, be able to pee make this last statement perfectly intelligible, until I come : to the subject of mental science ; it is, however, implied in the views I have to state in this chapter. We have seen that all organisms transform matter and Their clas- energy. The transformation of matter is the peculiarly ee nutritive, or vegetative, function; the transformation of basis. energy is the peculiarly animal function, and is generally motor—that is to say, the energy is generally transformed into motion, though in particular cases it is transformed into heat, electricity, or light. Let us speak of the vegetative functions first. The Vegetative : : 5 . 9 functions primary vegetative function, which is the ground and con- chemical, dition of all other vital functions whatever, is the decom- position, by plants, of water and carbonic acid, and the formation of organic compounds. This function is in its results a purely chemical one, though it produces combina- tions which, as I believe, no chemistry but that of the living vegetable organism can possibly produce." The power of decomposing carbonic acid, and probably of de- composing water also, is peculiar to the vegetable king- dom, though not universal in it.2 Animals cannot decom- pose carbonic acid, and consequently cannot form the primary, or first-formed, organic compounds for them- selves; but they effect various transformations in the organic compounds which they receive in their vegetable food. Some of these transformations may perhaps be due to the ordinary chemical forces, acting as they might act in a laboratory ; but some are certainly due to a peculiar vital action, controlling the chemical forces. This is eminently the case in secretion: one set of secreting cells separates 1 P. 86. 2 See Note A at end of this chapter. 156 and struc- tural. Cellular tissue. Differen- tiation of tissues. Growth and deve- lopment antago- nistic. HABIT AND INTELLIGENCE. [cHap. bile from the blood, another milk, and so on; for these various results are affected by glands, all of which consist of substances of the same chemical constitution ; and con- sequently would all act alike, if their action were merely a chemical one. The next vegetative function consists in the arrangement of the organic compounds so as to form tissue. As already stated, the simplest tissues are cellular, and cell-formation consists in the separation, or differentiation, of the primary structureless germinal matter into consolidated substance, which forms the outside of the cell; and soft, almost fluid, substance which constitutes the cell-contents, and, at least in the simplest organisms, retains the properties of germinal matter. Many organisms, as for instance the lowest Algze, consist of but a single cell, which propagates by spontaneous division. But in others the cells, after dividing, do not separate, but remain together; and thus cellular tissue is formed. The unicellular and multicellular forms of Algze graduate into each other, and the Alege in general consist of a mass of cellular tissue, with little further differentiation. Cells, in the various parts of various organisms, undergo endless modifications, both in form, and by acquiring the power, as stated above, of separating different substances from the sap or blood. Accordingly, the next differentia- tion consists in the acquisition of different characters by different masses of cells, so as to form different tissues: as, for instance, soft leafy substance and hard woody fibre, in plants ; and muscle, nerve, and bone, in animals. The formation of tissues constitutes growth, and the differentiation of tissues the one from the other constitutes development. It is important to observe that growth and development are not the same thing; they do not imply each other, and do not necessarily go on together—indeed, there is frequently an antagonism between them; rapid growth and rapid development appear, at least in certain Leaves and cases, to be incompatible. Thus, flowers are more highly flowers. developed than leaf-bearing branches ; and flower-bearing branches are always found to have lost something of the set he BE ET ee ee ae Ne eee ee es 5 A Lal ala tli i Nic a hd A Moi ey xiv.] ORGANIC FUNCTIONS. 157 indefinite power of growth that belongs to leaf-bearing ones, and if they are supplied with abundant nourish- ment, so as to cause them to grow rapidly, they cease to bear flowers, and are changed back into leaf-bearing branches. A still more remarkable instance of the same kind is that of the worm-like larve of some insects, which Insect at first feed voraciously and grow rapidly, forming com- ais a paratively simple and aeaificrentinted structures: but growth ceases when further development begins; growth ceases when the larva enters into the chrysalis state, and all the vital energies are employed in the work of develop- ment, which consists in transforming the comparatively undifferentiated tissues of the larva into the highly differ- entiated tissues of the perfect insect. And not only so, but the insect becomes inactive: motion ceases as well as growth, in order apparently that no energy may be spared from the work of development. As already remarked,’ it is nearly impossible to doubt that some transformation Transfor- of energy takes place in the act of development. If it is pes a true that a charge of energy is taken up and becomes aes static in the act of unorganized material acquiring organi-" zation, it appears probable that a further charge is taken up in the act of development, which is the acquisition of higher organization.” The tissues which are differentiated from each other Formation combine into organs. In some cases at least, there is no of ee distinction between tissue-formation and organ-formation. The shell of a mollusc, for instance, is at once a peculiar tissue and a peculiar organ. But in the highest organiza- tion, each tissue is found in many organs, and each organ consists of many tissues. Muscle, nerve, and bone, for 1 P. 107. 2 Tf I understand Dr. Beale, he believes rapidly-growing morbid growths, of the cancerous type, to he caused by cellular growth being in such excess as to destroy the power of development. (See Beale’s edition of Todd and Bowman’s Physiology, pp. 92,130.) It is known that cancer consists of “fungous” cellular tissue of very low organization. It is very interesting, and to my mind satisfactory, thus to find this most fearful of all classes of disease traceable, like commoner diseases, to a disturbance in the balance, or harmonious action, of the different vital functions. 158 Classifica- tion of vegetative functions. Animal functions. Four grades of the motor function. Sponta- neous motion. HABIT AND INTELLIGENCE. [CHAP. instance, are found alike in the head, in the limbs, and along the spine of man. It is self-evident that the forma- tion of organic compounds must be anterior to any forma- tion of tissues or organs. But it cannot, I think, be said that the formation either of tissues or of organs is In any sense anterior to the other. It is to be remembered also, that, as already stated, there are some tissues, at least in animals, which do not originate in cells, but are formed by the direct transformation of structureless sarcode." From the point of view which I have taken in the last few paragraphs, the vegetative functions may be clas- sified as Formation of organic compounds ; Formation of tissue; and Formation of organs: of which the first is chemical, and the others may be called structural. We now come to the animal functions, which essentially consist in the transformation of energy. As I aim only at drawing an outline, not at filling it up (which, indeed, in the present state of science, no one, probably, is com- petent to de), I will say nothing of the production of heat, electricity, and light by animals; I will speak, as I did in the last chapter, only of the motor functions, which are the characteristic ones of unconscious animal life: and of the sensory, conscious, and mental functions. In the ascending scale of nature, there are four grades of the motor function, differing from each other according to the circumstances under which the transformation of vital into motor energy is determined. The first of these may be called the spontaneous. To this class belong the circulation, or rather rotation, of the almost fluid contents of vegetable cells, which is often to be seen under the microscope: the motions of the germs of low aquatic organisms, vegetable as well as animal, through the water (which have often caused them to be 1 P, 197. 2 See the chapter on the Dynamics of Life (Chapter X.) for the relation between conscious nervous action and the transformation of energy. xtv.] ORGANIC FUNCTIONS. 159 mistaken for microscopic animals), and that “ciliary” motion, which is the only motor action of sponges, and appears to be universal in the animal kingdom. Motions of this class are found where there is no nervous system, as in plants and sponges: and even where there is a nervous system they are quite independent of nervous agency, as is proved by the fact that the “cilia” in man Cilia. and the higher animals continue in motion long after death, and even when they are detached from the body. So far as has been ascertained, they are also independent of any structure, but are simply due to the primary power of living matter to transform energy. No structure has been as yet discovered in the “ciliated cells” of even the highest animals.+ The next kind of motor action is that which is per- Motion in formed in response to a stimulus, and not accompanied Reeaeie with sensation. This is generally confined to animals, though there are instances of it among plants, as in the in plants: sensitive-plant and in Venus’s fly-trap. The fact that it is found among vegetables at all proves that it cannot be essen- tially dependent on nervous action; and a similar proof is afforded by the Hydrozoa among animals, which have no in animals vestige of a nervous system, yet spontaneously close on Without their food when it touches the tentacles. And all muscular fibre appears to have the power of contracting in response to various kinds of stimuli, such as electrical excitement,? 1 Carpenter’s Comparative Physiology, p. 125. See also Carpenter’s Human Physiology, p. 674. Cilia are minute hair-like projections, which are in constant and rapid motion during life. Their use varies according to position : in animalcules and in the Ciliograda they are organs of motion ; in very many animals they are used to produce currents in the water, sometimes for the purpose of bringing food, sometimes to keep the respiratory organs bathed with fresh supplies of water. In land animals, their only known function is to produce currents of fuid towards the outlets of the body. If cilia are to be called organs, they are organs the formation of which is independent of any differentiation of the tissue. 2 Thus in Helmholtz’s experiment (p. 138) to determine the velocity of the nervous current, electricity is seen to be capable of acting on muscle directly, as well as of acting on it through the means of nerve ; causing the muscle to contract in either case. 160 through nervous agency. Nervous mecha- nism. Two sets of nerves. HABIT AND INTELLIGENCE. [CHAP. the application of some poisons, as well as to the stimulus of a flow of nervous energy. But where there is a nervous system all muscular action appears to be normally pro- duced by nervous agency. This is true even of the action of the heart, which has a nervous system of its own. When motion takes place in response to a stimulus and through nervous agency, the mechanism is as follows :— Every nerve-fibre is connected—at least at one extremity —with a ganglion. Different nerve-fibres have different functions, according to the organs with which they are connected at their owter terminations (their ganglia being called their inner terminations) : some are centripetal, and transmit stimuli from without inwards to their ganglia ; others are centrifugal, and transmit motor impulses from the ganglia outwards to the muscles.!_ All motor action which is determined by nervous agency is a complex fact, in- volving the participation of at least two nerve-fibres and a ganglion, When motion is caused by a stimulus, the stimulus—which may consist, for instance, in the contact of something that irritates the skin, or in the presence of food in the mouth—produces a flow of nervous energy along the nearest centripetal fibre to its ganglion. The ganglionic cell that receives the stimulus communicates it to another cell in the same ganglionic mass, or to another ganglion. Some action takes place among those cells, which determines the flow of a current of nervous energy outwards from the ganglion, along a centrifugal or motor 1 Centripetal and centrifugal are, I think, better words than afferent and efferent, which Dr. Carpenter uses. It is doubly inaccurate to use the words sensory and motor in this sense. All the sensory nerves are no doubt centripetal, but the facts of reflex action stated in the text show that there are centripetal nerves which are not sensory. And though centrifugal nerves are necessarily motor, yet they may be sensory nerves also. I agree with Mr. Lewes in thinking it most probable that the motor nerves are the seat of the muscular sense, or sense of muscular action. Thus some centripetal nerves are sensory, and others are not ; while all centrifugal nerves are motor, but some are sensory and others are not; for the action of a voluntary muscle is accompanied by the muscular sense in its nerves, but the action of the muscles of the heart and stomach, in health at least, produces no sensation. : . ~ 4 xIv.] ORGANIC FUNCTIONS. 161 fibre to the muscle in which the motion is to be produced, in order to make the right response to the stimulus. Such actions as these are called reflex, the nervous Reflex action being, as it were, reflected back from the ganglion. **"°™ There can be scarcely a doubt that this is the only kind of nervous action in those animals which have a nervous system in its most rudimentary form—as, for instance, in the lower mollusca. As we ascend in the animal scale, the proportion of purely reflex actions appears to become con- stantly smaller; but even in man those muscular actions which minister the most directly to the vegetative life are of this kind. The actions of the heart, lungs, and stomach Reflex are reflex, being independent of sensation or will: the pias in stimulus to action is given in the heart by the flowing in lungs, and of the blood; in the lungs, by the flowing in of the air; in eae. the stomach, by the contact of the food. And actions which are normally performed in obedience to sensation or Reflex will may become reflex: thus, if the spinal cord (which is ue ae a vast bundle of nerve-fibres, accompanied with ganglionic abnor- cells) is so injured as to destroy all nervous connexion =": between the lower extremities and the brain, the lower extremities cease to have any sensation, or to be under the ~ control of the will; but the ganglionic masses of the spinal cord act as a “reflex centre” for them; and if the centri- petal nerves are excited, as by tickling the soles of the feet, the spinal cord, on receiving the unfelt stimulus from the centripetal nerves, will reflect it back along the cor- ' responding centrifugal nerves in the form of a motor im- pulse, producing convulsive motions of which the patient is totally unconscious. Next is what Dr. Carpenter calls consensual action: that Consen- is to say, muscular action depending on sensation, but ET involuntary; such as closing the eyes against a flash of light, or shrinking from the contact of anything that cuts or burns. Both reflex and consensual action are in re- sponse to a stimulus; but reflex action, as we have seen, is independent of any sensation, while the stimulus to consensual action consists in sensation. Cause of We do not know, and it is not probable that we ever en M 162 Voluntary action : depending on neryous stimulus. Instinct. Summary. HABIT AND INTELLIGENCE. [ CHAP. shall know, on what the difference depends between the sensory and the merely reflex ganglia: in other words, why itis that some ganglia become sentient when they are acted on by their nerve-fibres, while others are without that wondrous property. The microscope has not revealed any difference between the ganglia, or between the nerve- fibres, which are thus so unlike in their powers. Last and highest is voluntary muscular action. This, also, as well as reflex and consensual action, in all pro- bability depends on the stimulus of currents of nervous energy acting on the ganglia which are in communication with the motor nerves ; but in the case of voluntary action, the exciting currents proceed, not, as in the other two cases, from the outer extremities of nerve-fibres, but from within the brain itself. I shall have to say more on this subject when I come to treat of Mind. It is to be observed that no line can be drawn between consensual and voluntary action. Many actions,.such as closing the eyes or coughing, may be either the one or the other; and an action that was at first voluntary may become consensual from habit; as, for instance, the act of walking, which, though it has to be almost consciously learned by the child, soon comes to be carried on in re- sponse to the sensation of touching the ground with the feet, without needing a fresh determination of the will at every step. It is proved by the facts of instinct that many actions which are voluntary in man are consensual in many, if not all, of the lower animals. Thus, chickens pick up grains, and ducks run to the water, the moment they are out of the egg. We thus enumerate four kinds of motor action in organ- isms, according to the way in which it is produced, as follow :— Ist. Spontaneous. 2d. Produced by an unfelt stimulus, or reflex. 3d. Produced by a felt stimulus, or consensual. 4th. Voluntary. It is interesting to observe how these functions are suc- — cessively added, the one to the other, in the ascending x1v.] ORGANIC FUNCTIONS. 163 organic scale. Thus, all organisms whatever perform spon- taneous motions: all animals, except perhaps some of the very lowest, move in response to a stimulus: all sentient animals move in response to sensations, and probably all animals that have any mental power higher than mere sensation are capable of voluntary motion. We now come to the sensory functions. We have seen Sensory . ¢ : : functions. that the nervous system is, essentially and primarily, a part of the animal apparatus for the transformation of energy; that it is in the highest degree probable that every action of the nerves whatever, as well as of the muscles, is accompanied by a transformation of energy ; and that in sensation and thought the transformation of vital energy is probably into heat. I have enumerated four successive gradations of the motor functions. The gradations of the sensory functions are almost infinite: beginning with simple sensation, and going on through those functions of memory, perception, and thought, which constitute Mind. All these have their Mind. starting-point in sensation: they consist of simple ele- ments, which, however, form endlessly varied combinations. In the logical order, this would be the place for a treatise on mental science; but I prefer to keep to the customary order, and, so far as possible, to treat of mental science apart, and after biology; and for the present I will enumerate all the sensory functions under the two heads of sensation and mind. All the organic functions may consequently be enume- Tabular rated in the following tabular form.:— ahet 7 aa Formative or vegetative func- Chemical Formation of organic tions, essentially consisting ee compounds, in the transformation of | Seragharale Formation of tissue. matter. Formation of organs. ( Spontaneous, Reflex. Animal functions essentially Motor aa consisting in the transforma- : Voluntar tion of energy. ed J Sensory. Mind. M 2 164 Develop- ment of functions by differ- entiation. HABIT AND INTELLIGENCE. [cHAP. I hope I have now said enough to make intelligible the statement at the beginning of this chapter, that vital functions are developed one out of the other by gradual differentiation. Formative, motor, and sensory functions are no doubt too fundamentally distinct to be produced by differentiation the one from the other.t But within each of these three groups there is so perfect a gradation between the various kinds, or rather the various grades, of functions, that it is easily seen how one may be de- veloped out of the other. In the vegetative or formative series, the first and simplest functions are the chemical ones. Above these are the structural functions, the lowest and simplest of which is the formation of cells. Now cells are formed by a chemical differentiation between the con- stituents of the inside and the outside of the cell; so that the chemical function here passes into the structural one. And a gradation is manifestly possible from the formation of the simplest cellular tissue to that of the most complex organ. In the motor series, the gradation is decided: it is impossible to say where the one grade ends and the other begins. The same is equally true of the sensory, though perhaps less obvious: but I defer this part of my subject till I come to treat formally of Mind. In the next chapter I shall have to state the peculiar laws of life on which the possibility of this gradation depends. 1 See Note B (p. 166). XIv.] ORGANIC FUNCTIONS. 165 NOTE A. WE have seen that all organisms whatever have the power of Only vege- effecting chemical transformations in matter, but the power of pee es decomposing carbonic acid and forming the primary organic com- carbonic pounds belongs to vegetables alone. It is not, however, a correct bath account of the matter, to say that vegetables separate carbon from the inorganic world, and that the animals which eat the vegetables give it back to the inorganic world again in the car- bonic acid of their respiration. The truth is that the formation of carbonic acid by respiration, which is a slow combustion, is a function of all organisms whatever,! and probably of every part All or- of every organism that continues to live (not of such tissues as Sara is nails and hair). The opposite function of decomposing carbonic : acid and assimilating the carbon, on the contrary, belongs no doubt to vegetables only ; but it does not belong to every part of a vegetable, nor to every vegetable species, nor to any vege- table at all times (any air-breathing vegetable at least, for it may be different with sea-weeds, the deep-growing species of which are less dependent on light). The power of decomposing carbonic acid does not belong to germinating seeds, for they give off carbonic acid just like respiring animals. It belongs only to the Only the green parts of plants, and to them only when exposed to light; ee: “ and there are tribes of plants that have no green parts, and do tables not decompose carbonic acid at all, but obtain their carbon, like ee animals, by feeding on other plants. Such are the fungi, which only in the obtain their carbon mostly from decaying vegetable matter ; and ae also the Orobanchacez, a tribe of flowering but leafless plants tribes that which are parasitic on other plants, and live on their juices.? ace If then vegetables have motor actions like animals, and if carbonic there are whole tribes of vegetables which, like animals, do not 2°14 decompose carbonic acid, and if the lowest classes of animals Abd gra have no muscles nor nerves, what is the distinction between the tinction kingdoms? I reply, that I do not believe there is any absolute Sey and certain distinction whatever. and ani- mals. 1 A possible exception to this is mentioned in note 2, p. 85. 2 Carpenter’s Comparative Physiology, p. 732. It is to be observed, however, that the copper beech, and other plants with leaves that are not green, decompose carbonic acid in the usual way. 166 HABIT AND INTELLIGENCE. [cHap. XIV. NOTE B. FORMATIVE AND MOTOR FUNCTIONS. I po not know that it ought to be said that there is no gradation . Actions between the formative and the motor functions. The Forami- in Forami- nifera, and some other Rhizopods, put forth projections of the pares, sarcode substance of the body, called pseudopodia, which are, at mativeand Jeast as to function, temporary tentacles. May not these be truly ae homologous with the permanent tentacles of the Hydrozoa? If so, the putting forth and retraction of the pseudopodia, which are manifestly motor actions, are also to be classed as formative, forming the transition from the formative to the motor functions. What supports this conjecture is the fact, that in gromia the pseudopodia are only formed at one end, but in ameba they are formed on any part of the surface of the body ; just as in hydra the tentacles all form a ring round the mouth, but in some of the compound Hydrozoa there are tentacles on various parts. Dr. Wyville Thomson appears to share this view. He says: “I am strongly inclined to regard cilia as locomotive pseudopodia, and to consider them special to the sarcode [living but struc- tureless] element.” (Embryology of the Echinodermata, Natural History Review, Oct. 1864.) Cilia are certainly in some degree permanent organs. CHAPTER XV. THE LAWS OF HABIT. HAVE to begin this chapter by stating in what sense I intend to use the word Habit. We generally use the word with special reference to the Meaning mysterious border-land between the conscious and the un- of conscious functions. Thus we say, that such an action as habit. using some particular tool, for instance, is conscious at Conscious first, and afterwards becomes habitual. This is one of the Se ae most important cases of the law of Habit, and for the habitual. purposes of human education it is all-important: but it is only one case of the law. Among animals in the wild state there is a great variety of instincts to which this explanation will not apply. To mention that which Uncon- Darwin justly calls “the most wonderful of all known scons instincts,” we cannot suppose that the bee, in building its the bee. hexagonal cells, has, or ever had, any conscious knowledge of those geometrical properties of the hexagon which make it the most suitable form at once for convenience and for the economical use of wax. If, as I think we must, we class this and other purely unconscious instincts as cases of habit, the definition of the word habit must be greatly extended. Habitual actions, under any possible definition, include all mental and mentally determined actions which are not purely voluntary. But, if we are to extend the definition of habit so as to include under the denomina- tion of habitual such purely unconscious instincts as that of the bee, we must include under that denomination all motor actions whatever that are characteristic either of organic species or of particular individuals. And this is 168 Motor habits of climbing plants. HABIT AND INTELLIGENCE, [CHAP. true not of the motor actions of animals only, but of those of vegetables as well: for instance, those remarkable motions of some climbing plants that Darwin! has lately described, the tendrils of which swing about until they touch something, and then clasp themselves round it. Here there is no possibility of conscious purpose on the part of the plant itself, and yet the motions of its tendrils are as truly habitual and instinctive as those of a serpent’s body, or of a chameleon’s feet and tail, in grasping the Motor and branches that they climb. Thus all mental and all motor mental habits. Formative habits. Virginian creeper. actions are to be classed as habitual, excepting only those which, in man and some of the most intelligent animals, are directed by a voluntary impulse in pursuit of a con- scious purpose. But a still more extensive use of the word habit is sanctioned by usage, and, in my opinion, with perfect accuracy. Physicians speak of a habit of body; and botanists speak of the habit of a plant, meaning by that expression such characters as whether the stem is herba- ceous or woody, whether the leaves are fleshy or thin, &e. Characters of this kind belong not to the motor but to the formative functions—not to the animal, but to the vegeta- tive life; yet I think it is perfectly accurate to class such characters as habits, and to say that they come under the laws of habit. I believe that all classes whatever of vital functions come under these laws, whether the functions are formative, motor, or sensory; whether vegetative, animal, or mental. Formative and motor actions are inseparably connected. To mention one instance out of an innumerable number :—“ Ampelopsis quinquefolia, or the Virginian creeper, avoids the light, uniformly seeking dark crevices on broad flat surfaces, as a wall, a rock, or the trunk of a tree. The tips of the tendrils, brought into contact with such a surface, swell out, and form in a few days those well-known discs or cushions by which the plant firmly adheres to its support.”? The moving of the tendrils in search of some suitable dark crevice 1 See the Quarterly Journal of Science, April 1866, pp. 257, 258. 2 Ibid. p. 258. ee NS ee ee ae Oe xv.] THE LAWS OF HABIT. 169 in order to fix themselves is a motor action; the for- mation of the cushions is a formative action; yet both are characteristics of the species, and the one is surely as much a habit as the other. I am, however, aware that the mere definition of the right use of a word, however needful and however accu- rate, ought never to be confounded with the ascertainment of a natural law. We ought carefully to guard against the error of making any assertion as to matter of fact, in the disguise of the definition of a term. I now go on to state what I conceive the laws of habit to be. All vital actions whatever come under the laws of habit : All vital and none but vital actions do so. By vital actions I mean (o-"8 all those actions which organisms perform in virtue of habitual, being alive: and when I speak of actions, I include all functions, even those in which the organism is usually said to be passive, as in sensation. The definition of habit, and its primary law, is that all tending to vital actions tend to repeat themselves ; or, if they are not eae such as can repeat themselves, they tend to become easier S¢!ves- on repetition. It may appear that this law is in no way peculiar to the actions of living beings: for there are many inorganic Apparent actions that tend to repeat themselves, and to become ieee easier on repetition. For instance: flowing water gene- rally makes a channel for itself, and tends to flow after- wards in the same channel; and if a piece of paper has been once folded, it is easier to fold it again in the same folds than in new ones. But there is a fundamental differ- funda- ence between such cases and all true cases of vital habit. nen The cases just mentioned are cases in which the direction of action is determined by mere change of form : the water tends to flow in the channels, because their form is suit- able; the paper tends to lie in particular folds, because it has acquired their form. But let the channels be filled up, or let the folds be taken out of the paper by hot pres- sure, and these tendencies will be utterly lost. But, it may be said, may not organic habits be the result ey of changes of the same kind? May not the formative, organic 170 habits depend on structure, contra- dicted by embry- ology. Habits become hereditary. Habit is myste- rious. HABIT AND INTELLIGENCE. [CHAP. motor, and mental characteristics of every living species and individual be due to peculiarities of structure so minute and subtle as to elude the microscope ?! I reply, that this would be a most plausible view if” habitual characters were confined to the individuals in which they are formed. But this is not the case: all habits (that is to say, according to my definition of the word habit, all characters whatever) become, or tend to become, hereditary. This is as certain as any proposition can be which cannot be proved by experiment, but rests for its proof on cumulative evidence. Now, we have seen that an embryo consists, not of a miniature of the parent form, but of a small mass of germinal matter, without structure or form, but having an inherited tendency to re- produce the structure, form, and all the habitual characters of its parents. This truth can be expressed in the lan- guage of the theory of habit only by saying that every habitual tendency passes, or tends to pass, from the organ which is its seat (as, for instance, the brain is the seat of mental habits) into the germinal matter of the body: and when a portion of that germinal matter is thrown off in order to produce a new individual, it imparts its habitual tendencies to the new individual. It is, no doubt, con- ceivable that if our microscopes were powerful enough, they might reveal some peculiarity of structure corre- sponding to every habitual character of the fully formed organism. But it is not conceivable that the microscope should reveal peculiarities of structure corresponding to peculiarities of habitual tendency in the embryo, which at its first formation has no structure whatever. I therefore conclude that in all habitual tendency there is something quite inscrutable and mysterious: as there certainly is in the tendency of the germinal matter of the embryo to develop into a new individual of its own species; which, indeed, is only a particular case of habitual tendency. 1 This view of mental habits as depending on acquired peculiarities of nervous structure, has been lately maintained, most ingeniously and elaborately, by Professor Bain, (See the Fortnightly Review, Ist February, 1866.) xv.] THE LAWS OF HABIT. 171 The law that all habits tend to become hereditary is subject to a very important limitation, of which I shall have to speak when I come to treat of mind. When any peculiar tendency is inherited, it sometimes Inherited . : : -, characters appears in the offspring at the same age at which it appear appeared in the parent, but sometimes earlier:' never, aie . a e probably, or only in the rarest cases, at a later age. Here- same age ditary diseases afford many instances of both kinds of eae parent, eases: of the peculiarity reappearing in the child, in some sometimes cases at the same age at which it was acquired by the ue parent, and in some cases at an earlier age. A remarkable instance of the habit showing itself at an earlier age, is the fact of young dogs, the parents of which have been taught to point, themselves sometimes beginning to point the first time they are taken out.? I agree with Darwin in attaching great importance to this class of facts, re- specting the age at which variations occur, in throwing light on the origin of species. There can be no doubt that even when a habit does not Hereditary become hereditary, a tendency to it, or a facility for ac- pas ea quiring it, does become hereditary. In those cases, for aoa instance, in which a young pointer has not inherited the Ear habit of pointing, that habit is nevertheless more easily acquired by him than it would be by a dog whose ances- tors had not been taught to point. The case of a dog 1 Darwin on the Origin of Species, 4th ed. p. 14. (It is from the fourth edition I shall always quote.) 2 The following is a very striking instance of the same kind: “Sir C. Lyell mentions that some Englishmen, engaged in conducting the operations of the Real del Monte Company in Mexico, carried out with them some greyhounds of the best breed to hunt the hares which abound in that country. It was found that the greyhounds could not support the fatigues of a long chase in this attenuated atmosphere, and before they could come up with their prey they lay down gasping for breath ; but these same animals have produced whelps, which have grown up, and are not in the least degree incommoded by the want of density in the air, but run down the hares with as much ease as do the fleetest of their race in this country.” (Carpenter's Comparative Physiology, p. 987.) In this case the power of breathing with facility in a rare atmosphere, which only had a tendency to be produced in the parents, was congenital in the offspring. Habit is change- able, and spon- taneously variable. HABIT AND INTELLIGENCE. [orar. which is easily taught to point because his ancestors have been taught before him, is similar to that of a man who once learned to practise an art or to speak a language, and, though he has forgotten it, can learn it again much more easily than he could if he had never known it. The fact of the dog’s ancestors having learned to point gives the same facility to the dog himself in learning it, which the fact of the man having once learned an art gives him in learning it again.? Another most important law of habit must be formally stated, though it is implied in what has been said about the acquisition of new habits. It is, that all habits are in some degree changeable. New habits are constantly produced by change of circumstances, and by education, which indeed is only a special and artificial set of cireum- stances: and this could not be the case if habits were not in some degree changeable. But besides the changeability of habit as the result of changing circumstances, there is a certain amount of spon- taneous variability, which does not depend—at least not directly—on change of circumstances. No child is exactly like either of its parents, and no two children of the same parents are exactly alike. These differences might be attributed to differences of circumstances acting on the offspring through the parents; but such an explanation is shown to be at least insufficient, by the fact that the same differences are found to exist between twins, though, in general, in a somewhat less degree than between other children of the same parents; and it is obvious that twins have been subjected to precisely the same influences. The same is very generally true of those domesticated races of animals which produce several young at a birth. It is a most important, and a much debated question, whether there is any limit to spontaneous variation. Variation in a single generation is beyond doubt confined within very narrow limits: no such variations appear to be possible (among the higher animals and vegetables at least) as would be implied in “ gathering grapes of thorns 1 See Bain on the Emotions and the Will, Appendix C. o Celis | xv. | THE LAWS OF HABIT. 173 and figs of thistles.” But I agree with Darwin in be- I believe in no limit lieving that there is no limit to the possible extent of t6 varia. variation acting cumulatively, if only a sufficient number of generations is allowed;—no limit, I mean, as to the possible extent of change: there are, I believe, definite laws as to its direction. Of these laws we know but litle; it is, however, a very important truth, that variation does not go on equally in all directions at once, but takes place in particular directions at particular times: in other words, organisms acquire habits of varying in particular direc- tions; and these habits of varying are characteristic not only of individuals but of species and genera; perhaps we may say, of whole classes. As an instance of this— not by any means the strongest instance I can think of, but the most familiar—may be mentioned the well-known fact that the acquisition of any power that depends on habit makes it easier to acquire other powers of the same kind: thus, the mastering of one language makes it easier to master other languages. This is not simply a case of a - habit perpetuating itself. The knowledge of any language consists in the habitual connexion in the mind between the words of the language and the ideas they represent, so that the one will recall the other without effort ; and these - connexions are different for every different language. But though the habitual connexions are different, the habit which is cultivated in acquiring them for one language facilitates their acquisition for another: a habit has been acquired of acquiring a particular kind of habits. This law, that organisms acquire a habit of varying, or, in other words, of altering their habits in particular directions, is shown by Darwin to be true of the formative functions as well as of the motor and mental ones; and he has clearly perceived its great importance in accounting for the origin of species. I shall have more to say on this subject in a future chapter. Habits, as we have seen, are formed and strengthened by repetition of the acts. This, indeed, is only a state- ment of the elementary law of habit. The converse is tion. Habits of varying. Instance of acquir- ing lan- guages. Habits are also true: habits are weakened, and may at last be weakened 174 HABIT AND INTELLIGENCE. [crap. andde- destroyed, by discontinuance of the acts ; as, for instance, siroyed PY when we forget how to speak a language, or to practise an art, which we once knew but have discontinued. From these two laws—that habits are strengthened by repetition of the acts, and are weakened by their dis- Strength continuance—it follows that the strength of any particular ofa habit habit, other things being equal, depends on two different depends on time factors: one, the length of time during which the habit durt E : Cehih at has been exercised; the other, the shorter or longer time has been that has elapsed since it has been exercised. The effect of and on ” these two factors, however, is not the same in kind. The Hee. present strength of any particular habit depends chiefly on exercised. its having been recently exercised; but the tenacity of a Present habit, or, in other words, the difficulty of weakening or strength ? : : : ; of a habit. destroying it by disuse, is a different thing from its Tenacity present strength, and the two do not stand in any constant of a habit. : : : proportion to each other. The tenacity of a habit depends on the length of time during which it has been exercised : that is to say, the longer a habit has been in forming and strengthening by exercise, the longer time it will take for it to be weakened or destroyed by disuse. These facts are familiar. Every one knows that habits of long standing are not easily lost ; and the most tenacious habits are those which belong to the species, and have been exercised not merely through a lifetime but through an unknown number Hereditary of generations.1 Hereditary characters, indeed, are seldom eens —I believe never—destroyed by disuse during a single most generation, though they may be destroyed by disuse tenacious. : - ° during many generations: the domestic fowl and duck, for instance, have nearly lost the power of flight by long-continued disuse. Thus the law of the hereditary transmission of habit is equally true of its destruction as of its formation. 1 T am surprised that Darwin should say, “T do not wish to dispute the truth of the proposition that inheritance gains strength simply through long continuance, but I doubt whether it can be proved.” (Variation under Domestication, vol. ii. p. 26.) That inheritance should so gain strength appears to me at once a necessary consequence of the laws of habit, and a necessary inference from the general truth that the characters of the variety are more variable than those of the species. 175 xv. ] THE LAWS OF HABIT. ( For the sake of clearness, I have stated the law of the Weaken- weakening and destruction of habits by disuse as if it eee by were an independent law. But in reality it is not so: disuse isa it is a mere case of the elementary law of habit. The be the elementary law is, that by acting in any way a habit is w. formed of acting in that way: and it is a mere case of that law, that by ceasing so to act, a habit is formed of not so acting; or, what is the same thing in other words, the All actions habit of so acting is lost. The only really elementary Pecgue laws of habit are these three: that all actions, whether all habits formative, motor, or mental, tend to become habitual ; that ree all habits tend to become hereditary ; and that all habits ee are in some degree variable. able. As I have stated, the present strength, or what may be called the prominence of a habit, depends on its having been recently exercised ; but its tenacity depends on what is quite different, namely, the length of time (millions of generations, it may be) during which it has been exercised. These stmple and well-known truths are little more than obvious corollaries from the elementary laws of habit; but on them depend some very remarkable and rather intricate interactions between different habitual characteristics. A habit which has been much exercised during a compara- tively short time may be very prominent, but it cannot be very tenacious; and it may be lost by disuse during a period of time which is too short to produce any per- ceptible effect in destroying a more tenacious, though perhaps less prominent habit. Cases of this kind are no doubt difficult to identify, but it certainly is possible that new mental and moral habits, amounting to a change of character, may be acquired as a result of education and circumstances, and may afterwards disappear with ad- vancing age and under new circumstances, while the Reappear- original, perhaps hereditary, character reappears. ene ot olf A tenacious habit may appear to be lost when it is in Latent reality only latent. A latent habit is one which, though b*>i* not obvious, may at any time reappear; sometimes spon- taneously, sometimes by placing the organism in the same circumstances as those which produced the habit at first. 176 Reversion to ances- tral cha- racters. Laws of habit are elemen- tary and universal laws of life, Active habits strengthen, passive impres- sions weaken, by repe- tition. HABIT AND INTELLIGENCE. [CHAP. It is a well-known instance of this, that when the use of an art, or of a language, has been laid aside so long that, at the first attempt to recommence it, it appears to be totally lost, a little practice will often prove sufficient to regain it in a mere fraction of the time that would be necessary to learn it if it were really new. This is a case of the rapid reappearance of a latent habit under favouring cireum- stances. The most remarkable instance of the spontaneous - reappearance of a habit is the reversion of individuals, and, as I believe, of species, to ancestral characters after the lapse of many generations ; which, according to general belief, sometimes occurs in the human race, and beyond all question does occur among domesticated breeds of animals.! The characters of the breed which have arisen under domestication, and consequently are of later date than those of the species, are prominent habits: those of the species which reappear in these cases of reversion are tenacious habits, which may, as it were, be overlaid and concealed by the later acquired ones for a great number of generations, and yet reappear at last. I shall have to speak, further on, of the importance of this class of facts in accounting for the characters of species. In the chapter on the Dynamics of Life I have stated my belief that the differentia of life consists in certain powers, which all living beings possess, of transforming matter and energy. Except the laws of those transformations, I believe the elementary laws of habit are the only laws of life which are at once elementary and universal. I regard these as ultimate laws, like the laws of gravitation and of the affinities of the chemical elements, and, like them, incapable of being referred to any others. It is an important result of the laws of habit, that while active habits are strengthened by the repetition of the act, passive impressions are weakened by the repetition of the impression.” Both of these facts are perfectly familiar: 1 Darwin’s Origin of Species, pp. 15, 190. The most remarkable instances, both of variation and of reversion, are those of the domestic pigeon. * So far as I am aware, this remark was first made in Butler’s “ Analogy of Religion.” | | . xv.] THE LAWS OF HABIT. Vie every one knows that being habituated, or accustomed, is an explanation alike of being able to do what an unaccus- tomed person could not do—as, for instance, to execute a difficult piece of music ; and of being able to resist what an unaccustomed person would have great difficulty in resisting, such as great heat or cold, and impressions of par- ticular kinds of horror or fear. These effects are opposite, and it might appear that the weakening of impressions by repetition is the result of a distinct law, opposite in its character to the general law of habit; but it is in reality a Both are ease of that law. A passive impression becomes weaker by pein repetition, because the organism acquires the habit of not responding to it. A passive impression means one which is not followed by action. An impression which is not followed by action differs from one which is followed by action, not in the nature of the impression, but only in the response the organism makes to it. The same impression, acting on two similar organisms, may, according to circum- stances, remain a merely passive impression on the one, and may become an active stimulus to the other. To mention a familiar instance: two men hear the same loud Instance bell in the morning ; the one is accustomed to awake and cae an get up at the sound, and he awakes; the other is accus- accus- tomed to disregard it, and he disregards it and sleeps a through it. This view is supported by the fact, that it is possible to increase the strength of merely passive feelings —feelings, that is, which do not lead, and are not meant to lead, to action—by the habit of brooding over them; and, without so much mental action as is implied in brooding, it is possible to give a mastery over the mind to the passive - emotions, especially to fear, merely by acquiring a habit of yielding to it.” All this is familiar; but, so far as I know, it has not yet been clearly pointed out that the law of passive impressions weakening by repetition, while active habits strengthen 1 [ have met with this illustration of the law somewhere in Whately’s writings. 2 See Bishop Fitzgerald’s Note B to Chap. V. of Butler’s “ Analogy of Religion.” N 178 The same is true of the uncon- scious life, Effect of medicines and stimu- Jants. Action of the heart under a stimulus. General law re- specting passive im- pressions. Instance of climbing plants. HABIT AND INTELLIGENCE. [CHAP. from the same cause, is not confined to mental and volun- tary actions, but has its foundation far down in the uncon- scious life. One instance of this is the well-known fact, that the power of medicines and stimulants is diminished by constant use. Another and very remarkable instance of the law is the way in which the heart responds to a stimulus; such as a blow, or a sudden fright, or an electric shock. The first effect of a stimulus on the heart is to cause a momentary cessation, or at least slackening, of its action. If the shock is violent enough, it causes death ; but otherwise the effect passes away, and is followed by a quickening of the heart’s action—the well-known “ beating of the heart” produced by a shock. If the stimulus is repeated, supposing its intensity to be the same, its effect will become less with every repetition,! showing that the heart is acquiring the habit of not making any response to it—just like the sleeper who acquires the habit of making no response to the bell. From such instances as these— which are clearly not exceptional, but normal—I think we may infer, not only that, as already stated, organisms are capable of acquiring a habit of not responding to stimuli, but also that they always do form such a habit, unless there is some cause to determine them to form the opposite habit ; namely, the habit of making a response. A still more remarkable instance of this law, and one where neither voluntary determination nor nervous action of any kind can come into play, is afforded by the motions of those climbing plants which have been already referred to. It is stated by Mr. Darwin, that a thread weighing no more than the 32d of a grain, if placed on a tendril of the Passijflora gracilis, will cause it to bend; and merely to 1 Claude Bernard, in Revue des Deux Mondes, March 1, 1865. The stimulus used in ‘such experiments is that of an electric current sent through the pneumogastric nerve. The heart, in relaxing under a stimu- lus, acts differently from other muscles, which contract under the same; the arteries, which have a muscular coat, contract under a stimulus, such | as drawing the point of a needle over the skin without making a seratch ; but though this effect is opposite in kind to that produced by a stimulus on the heart, yet, like the latter, it is weakened by repetition. (Carpenter’s Human Physiology, p. 231.) xv.] THE LAWS OF HABIT. 179 touch the tendril with a twig causes it to bend; but if the twig is at once removed, the tendril soon straightens itself. But the contact of other tendrils of the plant, or the falling of drops of rain, do not produce these effects—proving, apparently, that the tendrils have acquired the habit of disregarding these:1 a wonderful instance of vegetable instinct. It is.a most important fact that organs increase with Organs exercise, not only in functional power, but also in size; ee while, conversely, organs that are disused, in whole or in part, diminish, not only in functional power, but also in size: and such modifications, like all others, are capable of becoming hereditary. It is difficult to prove that this con- nexion between the habitual exercise of an organ and its magnitude is true of the organs of the nutritive life, because most of them are incapable of any excessive stimulation without producing disease; but I think the “expansion of the chest ” which properly-directed exercise produces, shows it to be true of the lungs. It is well known to be true of Lungs, the muscles; and though the evidence is less direct, I eee think it is scarcely possible to doubt that it is so of the organs belonging to the nervous system—that the brain, for instance, is increased in functional power and in size by successive generations of mental cultivation.” _ It will be noticed by the reader that I have taken the Laws of instances of habit which I have quoted, indifferently from ces among mental and bodily habits, or, as I prefer to say, both mind from among conscious and unconscious habits; showing pas eee how the same laws of habit govern both the conscious and the unconscious life. It is to be observed that the laws of habit do not account Laws of for the origin of every particular habit. This, however, is ae not because of any imperfection in our knowledge of the nie ee subject: it is because the laws of habit, by the definition particular of the word, have to do only with the repetition of actions }#>t and the perpetuation of tendencies ; but they do not neces- sarily throw any light on the cause of the first of a series 1 Quarterly Journal of Science, April 1866; * See Note at end of this chapter. N 2 180 Voluntary actions may become habitual. This will not ac- count for all habits. Question of the origin of species. Summary, Habit. Heredi- tary trans- mission, Variation. Disuse. Promi- nence. Tenacity. HABIT AND INTELLIGENCE. [ CHAP. of actions that has become habitual—just as the laws of motion, though they are perfectly well understood, throw no light on the origin of force. We know that in man, and in a less degree among the more intelligent animals, a great variety of actions are capable of becoming habitual that were voluntary in their origin. On this possibility the whole art of education is founded. But this explana- tion will evidently not apply to the facts of what I have called spontaneous variation; nor will it apply to any formative habit whatever, nor to such motor habits as the cell-building instinct of the bee, or the turning and twining instinct of the Passi/lora gracilis, mentioned above. By the definition of habit that I have adopted, all specific characters are habits; and, in this sense, the question of the origin of particular habits includes the whole vast and enigmatic subject of the origin of species. But, little as that subject is understood, recent research and speculation have let a few rays of light into the darkness. We may thus sum up the laws of habit :-— All vital actions—formative, motor, and mental—tend to become habitual. All characters tend to become hereditary. An acquired character, when transmitted to offspring, appears some- times at the same age at which it appeared in the parent, sometimes earlier. All characters are in some degree variable, and particular characters may acquire a habit of varying. The foregoing three are the elementary laws of habit; the following are derived as corollaries from them :— Habits, being formed by use, are weakened and destroyed by disuse. The prominence of a habit, or its present strength, depends on its having been recently exercised. The tenacity of a habit, or the difficulty of destroying it, depends on its having been Jong exercised. Consequently, a prominent habit may disappear, while a tenacious, perhaps a hereditary one, survives it. A habit may become latent, and reappear. The re- Ve Ree ees ee ae ae xv.] THE LAWS OF HABIT. 181 appearance of habits is sometimes the result of favour- ing circumstances, sometimes spontaneous. Leversion to Reversion. ancestral characters is a case of the reappearance of habits. When a stimulus is responded to, it strengthens in force eet with repetition ; when it is not responded to, it weakens. sions. Organs strengthen and enlarge with exercise ; and, con- pitch of versely, they weaken and diminish with disuse. organs. It is now time to consider, in more detail than I have yet done, the manner in which the characters of a race will be modified by changes in the circumstances under which it has to live. It is a universal law, that the health, and ultimately the Great life, of any organism whatever will be destroyed by any aa very great change in external circumstances. The most sttuctive. obvious instances of this law are the familiar facts, that air-breathing animals will die in the water, and water- breathing ones will die in the air. These facts, however, do not throw much light on any law of life, for they admit of a purely physical explanation. It is physically impos- sible, quite irrespective of any law of life, that a man’s lungs should breathe water, or that a fish’s gills should breathe air. But, independently of physical reasons like this, all great changes are destructive of health and life. Cold regions and warm ones, moist places and dry ones, have all their own peculiar races of animal and vegetable inhabitants ; and those species which are native to one kind of abode will, as a general rule, be destroyed by transplanting to a totally different one. Were it not so, differences of climate would be no barrier to the migrations of species, instead of being, as they often are, the most impassable of all barriers. In many cases we cannot say what is the reason of this inability of organisms to adapt themselves to new circumstances. Sometimes, in all pro- bability, it is in part merely physical: for instance, animals with a coat only of hair may be unable to endure the cold of those countries where most of the native quadrupeds are clothed with fur. But this kind of reason cannot be given in every case. It is, I think, quite impossible to 182 HABIT AND INTELLIGENCE. [cHar. This isnot assign any such merely physical reason for the fact that Feels the European race of man is unable to perpetuate itself in explicable. the climate of Bengal. I believe such facts are to be referred to the laws of habit. We have seen that every organism has a certain power of becoming habituated to impressions. This it does in two different ways: if the impression demands a response, such as to close on its prey or to run away from its enemy, the organism acquires the habit of making the right response; if it does not demand nor admit of any response, the organism acquires the habit of disregarding it. Now, exposure to a different climate from that to which an organism has been accus- tomed, in some cases, no doubt, produces a response in the vegetative life—as, for instance, in those animals which acquire a coat of hair better suited to their new abode ; and sometimes it produces a response in the motor life, as when it determines a species to acquire the habit of peri- odical migration. But in many cases—probably in the vast majority—no appropriate response is possible. To use familiar language, nothing can be done, and the change— the unaccustomed heat or cold—must be endured. The organism must become habituated to the climate—that is to say, must acquire the habit of disregarding the change ; Organisms and if it cannot do this, the change will destroy its health, scan and ultimately its life. It may not be sufficient to kill the by changes individual; but if its health is at all injured, and this is that iY not recovered in future generations, the race will die out of eee its new abode. This kind of adaptability is very different ated to. in different species: thus the horse has been successfully introduced by man into every climate, from the equator to Iceland and Siberia: the ass would perish in a very cold climate. The reasoning in the foregoing paragraph may seem Creat and Vague. I am, however, convinced that there must be sudden some profound connexion between the two facts, that changes of oreat, and sudden changes in the circumstances of their stances are lives are destructive to organisms, and that organisms ea are unable to effect great changes in their habits, except very gradually. The proof that there is such a connexion ee xv. THE LAWS OF HABIT. 183 is strengthened by the converse facts which I have next to state. As just stated, great and sudden changes of circum- Corre. stances are destructive, and great and sudden changes of Ana habits are impossible. I believe we might say that great a eS and sudden changes of circumstances are destructive, because great and sudden changes of habits are impossible.* And what confirms the belief that these two laws really stand in the mutual relation of cause and effect is this further pair of laws, which evidently are similarly related to each other,—that great changes of circumstances are Great often not destructive, provided they are not sudden; and ee aele that great changes of habit are often possible, provided ®re often they are not sudden. I believe we may say, as before, that eee great and gradual changes of circumstances often are not uate destructive, because great and gradual changes of habit are changes in possible. I do not yet wish to speak of those changes which, ee as I believe, have occurred in geological time ; but, as an instance of a wonderful change that has occurred in histo- rical time, I may mention the dog, which, though naturally carnivorous, has in his domestic state gradually become in great part a vegetable feeder, and has been taught to tend sheep. These changes must have taken many generations to bring about. A carnivorous animal would perish if sud- denly put on a vegetable diet: not that it would disagree with him—he would die of hunger sooner than touch it. The process of adaptation—or, in other words, the effects Adapta- of changes of circumstances in producing new habitual cha- eee, racteristics— may now be stated ; not, indeed, in detail, but with some degree of precision. External changes, if of any importance, will either destroy the organism, or cause the organism to acquire new habits, so as to adapt itself to the changes. The new habits will be either active or passive. Active and An animal may, for instance, be placed in a severer climate rere than that to which it is native: this may take place either 1 Herbert Spencer would probably say, that if the organism, or the race of organisms, is unable to readjust its internal relations to the new set of external relations, it will perish. This would no doubt be true, but I do not see that it would be in any sense an explanation. 184 Change o. climate. Change of food. HABIT AND INTELLIGENCE. [CHAP. from such a change of climate as we know from geological evidence to have taken place in past ages,’ or from the animal being transported by man and becoming wild in its new abode, or from spontaneous migration; and it is sometimes impossible to say what determines the migra- tions of animals. In such a case, as already remarked, the animal, if it becomes adapted to the new climate at all, and is not destroyed by it, may become adapted by acquiring either the passive habit of disregarding the cold, or the active habit of producing warm fur on its skin. Of passive habits I need say no more; but the subject of the forma- tion of active habits, including formative ones, to meet new circumstances of life, is practically an infinite one. Suppose another instance of the same kind. In conse- quence of the migrations of the animals that serve as its food, a beast or bird of prey is compelled to change its mode of hunting. It may need keener sight, in order to obtain its new prey: in this case, its sight will be more exercised, and will become stronger; and in the course of some generations, probably, its eyes will be enlarged. Or it may need a keener sense of smell: in this case the same changes will be effected in its olfactory organs. Or it may need greater fleetness: in that case the muscles of its legs will become stronger and larger; and, what is most im- portant to observe, such a change as this will directly or indirectly affect the form of every part of the body—partly 1 Ido not mean that there is any proof of a glacial period being one of extreme cold. A cold suinmer would be enough to produce it. 2 Migrations sometimes occur in very unexpected ways. I extract the following from the Quarterly Journal of Science, October 1864 :— “The sudden occurrence of Pallas’s sand-grouse (Syrrhaptes paradoxus) over the greater part of Europe has attracted the attention of ornitholo- gists, and Mr. Alfred Newton has collected information which shows that this remarkable bird, hitherto almost unknown to the European fauna, has been met with during the year 1863 in no less than 148 localities in Europe and Great Britain, tracing the invading host through 33° of longi- tude, from Galicia to Donegal. He regards the proximate cause of this wonderful movement as the natural overflow of the population of Syrrhaptes, resulting from its ordinary increase, being a bird which has comparatively few enemies, while its time of incubation is short in eos aioe with what it is in most ground-feeding birds.” The Surrhapies is a native of the steppes of Central Asia. = ST xv.] GROWTH OF ORGANS WITH EXERCISE. 185 by the direct action of the pressure of the enlarged muscles, modifying the form and position of the other muscles and of the bones—partly also, no doubt, by the increased nutri- tion demanded by the enlarged muscles diminishing the supply of nutrition to the other parts of the body, and so compelling a diminution of their size. It is also to be observed that, besides these secondary changes, as they may be called, it is quite possible for change of habit, as the result of new circumstances, to take place primarily in two or more directions at once. Thus, for instance, the necessity of pursuing a new kind of prey, or of pursuing the same kind of prey in a different manner, as will occur when forests are destroyed, may have the effect of causing an animal to improve both in keenness of sight and in Improve- fleetness. This appears to be not only a possible, but a ae nae probable case: keenness of sight and fleetness are often feetness. united in animals, as, for example, in birds of prey. In this chapter I have considered the laws of the forma- tion and perpetuation of habit; in the next I shall have to consider the laws of its variation. NOTE. GROWTH OF ORGANS WITH EXERCISE. Iv appears uncertain whether the increase in size of organs that Why do are much exercised can be accounted for by any physical cause ; arma a or whether, like the law of habit, it is an ultimate law of life, exercise? and as such inexplicable. Herbert Spencer has made a most coe elaborate and ingenious attempt to prove that it is entirely due theory. to the increased flow of blood that always takes place to and through an organ in activity.1 He makes out an exceedingly strong argument for believing that the deposit of woody sub- stance in the vascular tissue of plants, which is the process- by which woody fibre appears to be formed, is originally due to the Woody accelerated flow of the sap in the vessels near the surface of fibre. 1 Principles of Biology, Part V. Chaps. iv., vii., and viii. Animal tissues. Possible nervous action in increasing nutrition in exer- cised parts. Increased flow of blood to exercised parts, possibly due to re- laxation of the nerves of the arteries. HABIT AND INTELLIGENCE. [CHAP. XV. trunks and branches that are agitated by the wind. But it ought not to be taken for granted that the case of muscular and other animal tissue is parallel to this. In plants, the waste of the tissues is very trifling, and it is probably null in vascular tissue which is filling up and hardening into woody fibre; so that an increased flow of sap may very well fill up the vessels with the substance it brings, just as drains are silted up. But in animals, especially warm-blooded animals, the waste is great and rapid ; and the more any organ is exercised, the more sub- stance it loses by waste: and it is not easy to understand why the increased flow of blood through an organ should not only increase its nutrition (which it certainly will do), but cause the nutrition to exceed the waste, so as to produce growth. Perhaps the excess of nutrition to which the growth is due may be in some way caused by nervous agency, which we know to be called into play by every vital action whatever among those classes of animals that have a well-developed nervous system. Such action of the nervous system would, no doubt, be inexplicable ; but it would not be more so than its action in stimulating secretion,! or, indeed, than any strictly vital action whatever. The cause of the increased flow of blood to and through parts that are in exercise does not appear to be fully understood. With respect to the muscles, I am inclined to think that Herbert Spencer has assigned an adequate cause, namely, the varying pressure on the blood-vessels during muscular action. But this, obviously, will not apply to the flow of blood to the brain being greater during the waking state than during sleep, or any other flow that is directly produced by nervous action. Possibly the increased flow of blood in this class of cases may be due to the calibre of the small arteries being increased by the relaxation of the nerves that control them ; but I only offer this as a suggestion. 1 Carpenter’s Human Physiology, p. 738. | 4 | : ; . 1 CHAPTER XVI. THE LAWS OF VARIATION. ie the last chapter we have seen that the habitual Changes characters of organisms are subject to two quite dis- pated tinct classes of changes, which, following Herbert Spencer, ally pro- ; duced, and I shall call functionally produced changes and spontaneous spon.’ variations. I shall have to speak, farther on, of a kind of taneous. changes concerning which it is difficult to say how they are to be classed. I have to some extent, though in ex- treme outline, traced in the last chapter the laws according to which changes are functionally produced, and I now go on to state the conditions which are favourable to variation. I must, however, begin by stating a set of laws that have no very obvious connexion with the subject. We have seen that great and sudden changes of habit are impossible, and that great and sudden changes of external circumstances are destructive to an organism. Whatever may be the connexion of these two laws with each other (and I believe it is very close), their opposites are also true: slight changes of habit are possible, and Benefit of shght changes of external circumstances are beneficial to eee organisms. Concerning the variability of habit, I need not =~ say anything more at present; but, as an instance of the beneficial effect of shght changes of external circumstances, may be mentioned the proverbial benefit of “ change of air” Change of —that is to say, in reality, change of external circumstances “” —in renewing the bodily and mental health, especially of sufferers from monotonous, depressing, or exhausting oc- cupation ; and the equally well-known benefit of “changing Change of the seed” of cultivated plants—that is to say, bringing the *°* 188 HABIT AND INTELLIGENCE. [cHar. seed from a distance, instead of sowing that which has been raised in the same farm or garden. Benefit | I agree with Darwin! in believing that there is a pro- eee found connexion between this last-mentioned law and the ofrace. general law, that slight mixtures of race, or “ crossings of the breed,” tends to promote the health and vigour of the race. I say slight mixtures, because very different races will not mix at all: the pollen of a rose on the stigma of a foxglove, for instance, would produce no more effect than if it were so much dust blown off the road. And between peo these two extremes of kindred races which are benefited races, by mixture, and totally distinct races which will not mix at all, there is a wide class of intermediate cases. Some- times, when two distinct species of plants are hybridised, seed is produced, but in less abundance than if the plant bearing it had been fertilized with pollen of its own species. Sometimes, among animals, the offspring is vigorous, but infertile, and cannot give origin to a hybrid race: the mule, between the horse and the ass, is a well-known instance of this. Sometimes offspring is produced, but is weak, and dies early; sometimes, in the case of birds, without being able to break through the egg.? Slight There is another set of facts which I will mention here, aoe though they are not relevant to the subject of the present great ones Chapter, because I believe they stand in the closest con- ee: nexion with the laws of habit and of the effects of changing circumstances. I mean that, among conscious organisms, slight changes are agreeable, but great changes painful, or at least disagreeable. This law is a most important oue in mental science. Summary. I have now enumerated four pairs of laws which, it is scarcely possible to doubt, stand in the closest relation to each other. They are as follow :— 1 Origin of Species, p. 318. ° Ibid. p. 315. See the whole chapter on Hybridism. Darwin says, that the impossibility of obtaining hybrid offspring at all in some cases, and the infertility of such offspring in other cases, are very distinct facts : but I think it needs no proof that they are facts of the same class. In the former, we cannot obtain hybrid individuals: in the latter, we cannot obtain a hybrid race. : | | XVI. ] THE LAWS OF VARIATION. 189 Habits are capable of change; but only a slight change is possible in a-short time. Changes of external circumstances are beneficial to organisms, if they are slight; but injurious if they are great, unless made gradually. Changes of external circumstances are agreeable to the mind when slight, but disagreeable when great. Mixture of different races is beneficial to the vigour of the offspring, if the races mixed are but slightly different ; but very different races will produce either weak offspring, or infertile offspring, or none at all. It is a very remarkable fact, that confinement to a small Effect of area appears to have a tendency to diminish the size of mime | a race of animals, even when there is no other evidence of small area. diminished health and vigour. Thus, animals found on islands are frequently smaller than those of the same species found on continents; and animals bred in an aquarium are often observed to degenerate in size, even while the race continues vigorous.! It is scarcely possible to doubt that such diminution is caused by want of room for sufficient variety in the conditions of life, or by want of sufficient mixture of the race—in other words, by the interbreeding of too small a number of individuals—or most probably by both; though the second reason will not apply in cases where the diminution is visible in a few generations among species of organisms that are able to propagate for a long time without union of the sexes.” What may be called the law of sexuality in organisms— Sexuality. that is to say, the necessity of the union of two unlike individuals of the same species for the purpose of genera- 1 T state these facts on the authority of Mr. (now Sir John) Lubbock’s paper on the reproduetion of Daphnia. (Philosophical Transactions, 1857.) The Daphnia is a small fresh-water entomostracous crustacean. * Ability to propagate without union of the sexes is not the same thing with hermaphroditism. The land mollusca (snails and slugs) have both the sexes on the same individual, and yet they cannot fertilize themselves. On the other hand, no winged insect is a hermaphrodite, and yet many species produce fertile eggs without impregnation. 190 Genera- tion is onlv a modifica- tion of the general vital process. Reproduc- tion of Algee. HABIT AND INTELLIGENCE. [CHAP. tion—appears to stand in the closest connexion with the law of the beneficial effect of slight mixtures of race: indeed, the former law is probably only a case of the latter. This connexion, and the whole subject of the real nature of generation, can be made manifest only by study- ing it among the simplest and lowest organisms. When the generative process is studied only among the highest organisms, among which it is always sexual, the inference is natural and inevitable that generation is altogether a special function; but its phenomena among the lowest organisms show that it is only a modification of the general vital process of growth and development. I will here briefly recapitulate what I have said on its true nature in the chapter on Organic Development. Any por- tion of the structureless germinal matter of any organic species is capable of developing into a perfect individual of the species, when placed under suitable circumstances ; but the higher the organization, the more special the cir- cumstances are required to be in order that it may so develop. In hydra, a form in which there is little dis- tinction between germinal matter and formed material, a single small fragment, if detached, and permited to remain in any place where a perfect hydra can live, will soon develop into a perfect hydra. In the vast class of the Protozoa, which are the simplest of all animals, and among vegetable tribes of equal simplicity; propagation very gene- rally takes place by means of spontaneous division into parts. But among the higher forms, in which the life is more centralized, and the parts more mutually dependent, a portion of germinal matter, if simply detached, will not develop into a new organism, but will die; and, conse- quently, special generative organs are set apart, in which the germinal matter is prepared and supplied with nourish- ment in a particular form, for the purpose of its developing into a new organism. Among the lowest organic forms, reproduction can scarcely be distinguished from growth. Among all Algee—and, indeed, throughout the vegetable and animal kingdoms—cells give birth to cells. Among the lowest Algze, in which each individual consists of but — a 72 eee ee. iw eee ~ OTe xv1.] THE LAWS OF VARIATION. 191 a single cell, cells divide into cells; and when they have divided, they separate. Among forms a little higher, they adhere together after dividing, and constitute cellular tissue. When they separate, we call the result propagation ; when they adhere, we call it growth: but there is evidently no fundamental difference between the two cases; they are found in nearly allied forms, and, indeed, they graduate into each other through species in which the adhesion of the cells is very slight. The reproduction just described is non-sexual; but the simplest first and simplest form of sexual reproduction is presented aes by those same unicellular Alge. There is no distinction reprodue- of sex, but reproduction takes place by means of the fusion 7°77) a> into one mass of the germinal matter that forms the con- Alge. tents of two cells. Among the Diatomacesz and Desmidex, two individuals, each of them consisting of but a single cell, place theinselves together and burst: the contents of the two mix, and from their union a fresh brood arises.2 Although there is here no visible distinction of sex, yet Its esscn- what appears to be the essential condition of sexual repro- {2; Cot duction is fulfilled; namely, the fusion of the germinal mixture of matter derived from two different organisms, or, at least, seal two different cells. In the case just described, the cell- from two fe e sources, walls, after they burst and liberate their contents, are as useless and dead as the cast skin of a snake. But in the zygnema—an Alga of rather higher organization, which Zygne- consists of cells united into filaments, somewhat like strung lost a beads—we meet with the first and simplest form of the of sexual distinction between the sexes. Two of the filaments—that aoa is to say, two distinct plants—approach and lay themselves alongside each other, and the contents of the cells of one filament pass over into the cells of the other, where the union is formed which is to give origin to a new brood The essential matter in sexual generation is the union of Purpose germinal matter from two distinct sources; and the pri- ie eae mary purpose of the distinction between the sexes appears tion, to in- to be to increase this difference. It would be premature ee 1 Spencer’s Principles of Biology, vol. ii. pp. 15, 16. 2 Carpenter’s Comparative Physiology, p. 878. 3 Thid. p. 881. 192 of the two sources. Herma- phrodite animals, and plants, not always self-ferti- lizing. Agency of insects in fertilizing flowers. HABIT AND INTELLIGENCE. [CHAP. to say that this last statement is proved; but it certainly receives strong confirmation from the facts that Darwin has collected, and on which he justly lays great stress, concerning the fertilization of hermaphrodite organisms. Many of these, as land-mollusca and earth-worms, have the male and the female organs so placed with respect to each other, that they cannot fertilize themselves, but pair, like animals with the sexes on distinct individuals. And though many hermaphrodite animals do habitually fertilize themselves, yet Darwin states, on Huxley’s authority as well as his own, that among known animals there is not a single instance of the female organs being out of the reach of the spermatozoa of another individual occasion- ally entering. No terrestrial animal is known to fertilize itself, and among aquatic species the spermatozoa may be carried in currents of water.' With respect to plants, the evidence is even more remarkable and more decided. Most flowers are hermaphrodites ; and when the remarkable dis- covery of the sexuality of flowers was first made, it was naturally supposed that each flower was fertilized by its own pollen. But Darwin has shown that in many cases this is impossible, in consequence of the stamens and pistils coming to maturity at different times ;? and there are some instances of reciprocally dimorphic species of plants—that is to say, plants bearing two kinds of flowers, each of which contains both stamens and pistils; but the pistils of each form can be properly fertilized only by the pollen of the other.* Insects are the principal agents in carrying pollen from one flower to another; and it is Darwin’s belief that flowers have been endowed with their bright colours (and, if so, no doubt with their nectar also) for the purpose of attracting insects.* Insects are useful, 1 Origin of Species, p. 113. 2 Thid. pp. 111, 325. 3 Ibid. p. 320. 4 Tbid. p. 239. “I have come to this conclusion,” he says, “from finding it an invariable rule that when a flower is fertilized by the wind, it never has a brightly-coloured corolla. Again, several plants habitually produce two kinds of flowers ; one kind open and coloured, so as to attract insects, the other kind closed and not coloured, destitute of nectar, and never visited by insects.” : ieee dati ee xv1.] THE LAWS OF VARIATION. 193 no doubt, not only by bringing pollen from other flowers, but also, in those flowers which are capable of being fertilized by their own pollen, by brushing it on to the stigma; and it is probably impossible in all cases to separate these two functions; but, after what has been said, we cannot doubt that the occasional introduction of pollen from another flower must be beneficial. Pollen will, no doubt, be brought from flowers of other species ; but this will not be injurious—it will have no effect whatever. Sexuality, and the existence of distinct generative organs, Sexuality are not the same thing, and do not imply each other. The ae existence of separate generative organs is only a case of existence the “ physiological division of labour,” in virtue of which Se law every separate function in the ascending scale of °'S*PS: organization tends to acquire a separate organ for itself. There are many cases of the action of the reproductive function through distinct reproductive organs without the sexual relation coming into play: the females of some butterflies, for instance, produce true eggs from true ovaries, which are fertile without being fertilized by the male.’ And, on the other hand, the instance of the lower Algz, mentioned above, shows that sexuality may exist without separate reproductive organs, or even distinction of sexes ; and that it essentially consists in the union and fusion of germinal matter from two distinct sources. It con- sequently appears a highly probable conclusion, that sex- uality, though a universal law of life, is not an ultimate depends one, but is to be referred to the necessity for slight changes, eee in order to keep up the vigour of life. Many of the lower of slight organisms, no doubt, propagate by spontaneous division, as Stn the simpler Algz; others, by throwing off buds, as the Hydra; others, which are not so lowly organized, by true eges that have not been fertilized by the male, as the butterflies just mentioned; and others are capable of fer- tilizmg themselves by a truly sexual process, hke many flowers. But it appears probable that no organic species ? 1 Spencer’s Principles of Biology, vol. ii. p. 214. O 194 HABIT AND INTELLIGENCE. [cHAP. No species is able to go on doing so for an indefinite number of eae ae generations! It appears probable that any race whatever time will at last lose its vigour, and die out, if its life is not, as without : : 2 wexnal. it were, renewed by the mixture of germinal matter from eeounes another source. Sa But, whatever may be the ground of the law of sexuality, bility pro- there can be no doubt of the beneficial effect both of slight ted E aye : 5 slight y changes in the conditions of life, and of mixture of nearly pene. allied though distinct races; and it is also true, that changes ditions, in the circumstances of life, and mixture of races, tend to and by OP ey (EER mixture Promote variability. rae of races. I am now speaking of spontaneous variation. This, as I have mentioned at the beginning of this chapter, is a distinct thing from functionally-produced change; and I propose to call the tendency of a race to vary spontane- Varia. ously, variability, and its aptitude for acquiring function- end ally-produced changes, modifiability. These two properties ability. are different; but there is, I think, a good deal of reason for believing that they are, to a great extent, found together —at least among the highest animals. The wonderful modifiability of the instinctive faculties of the dog, which has enabled him to acquire totally new instincts under domestication, must, I think, be in some way connected with the variability of his form, which has given origin to such different races as the greyhound and the terrier. Sponta- Spontaneous variations may be defined as those differ- nas ‘how ences of offspring from parent, and of the offspring of the eee same parents from each other, for which no reason can be with the assigned. These do not come under the law of habit, in douse that they do not originate, in virtue of that law, by the repetition of actions; but they do come under the law of habit, in that they are instances of the variability of habit, 2 Darwin’s Origin of Species, p. 109. 2 See Darwin’s Origin of Species, p. 21. The statement that changes in the conditions of life tend to promote variability is repeated several times throughout the work, and is illustrated by the examples of various domestic animals and cultivated plants. —— ee ee - ee Xvi. THE LAWS OF VARIATION. 19 Or and also in their tendency to become hereditary, and in their greater tenacity the longer they have been inherited through successive generations. There is this important difference between functionally- produced changes and spontaneous variations, that func- tionally-produced changes are necessarily produced during the lifetime of individuals, though they may be transmitted to the offspring ; but spontaneous variations appear to take their rise in the act of originating a new individual. They originates sometimes arise in the production of a new individual only ae without sexual generation, as in the case of plants that dividuals. produce “sports” from buds; for every branch produced from a bud may in some sense be regarded as a new indi- vidual. A long list could easily be given of “ sporting ” « SEE = plants: by this term gardeners mean a single bud or offset, lariat which suddenly assumes a new and sometimes very dif- ferent character from that of the rest of the plant. Such buds can be propagated by grafting, &c., and sometimes by seed. These sports are extremely rare under nature, but far from rare under cultivation.2 Variations, or new varie- Variations ties, are, however, oftener obtained by sexual generation. 7° ™ost abundant There are some species—as, for instance, the potato— in cases of which have been rendered go variable by cultivation, that Bene ie new varieties are produced whenever they are raised from seed. Reproduction from seed is reproduction by sexual generation ; and, as I have shown, there is some degree of mixture of race in all sexual generation. This is probably to be regarded as true even of a flower that fertilizes itself ; for the anthers and the pistils, though in the same flower, are distinct elements of the organism. But of course the mixture of race is greater when the pollen is brought from another flower on the same plant, and greater still when it is brought from another plant. And, as already stated, the greater the mixture of races, the greater is the pro- bability of variation in the offspring—provided, of course, that they are races that breed freely together, and produce fertile offspring; for the tendency to variation does not 1 See Note at end of chapter. ? Darwin’s Origin of Species, p. 9. 02 196 Inter- mediate breeds are ditficult to obtain. Reversion common in mixed breeds. Otter sheep. Variation usually slow among animals: often sud- den among plants. HABIT AND INTELLIGENCE. [cHAP. wear out, but appears to increase in the first few genera- tions. When the breeds that are crossed are very different, the variation is so great that it is nearly impossible to obtain a permanent breed of character intermediate between the two parent breeds.! Mixture of race, which is thus a stimulus to variation, also facilitates reversion to ancestral characteristics. Thus, in all the domestic varieties of the pigeon, birds are occa- sionally found that have reverted to the blue colour, with black bars on the wings, and other characters of the wild rock-pigeon, from which the domestic breeds are descended ; and such specimens are particularly abundant where the breeds have been mixed.? Sometimes, where very unlike breeds have been mixed, the offspring are neither inter- mediate between the parents nor irregularly variable ; but some of them resemble one parent, and some the other. Something of this is very common in families of children, where some generally resemble the father’s family, and some the mother’s. But the most remarkable instance I know is that of the otter, or Ancon sheep, an accidental variety that appeared at one time in North America, and was preserved by the farmers, in consequence of its being unable to leap fences. It began with a single lamb; and when the otter sheep were crossed with the common kind, some of the lambs resembled the ram, and others the ewe; but no mixed breed was ever produced. Variation among animals is usually gradual, being but slight in a single generation, though, under favourable cir- cumstances, it may accumulate through successive genera- tions. When it is considerable, as in the case of the otter sheep just mentioned, it is usually in some degree monstrous.* Sudden variation is much commoner among plants, as in the case of “sporting” plants. The following cases of the sudden origin of what appear to be permanent 1 Darwin’s Origin of Species, p. 21. 2 Thid. p. 188. 3 It is impossible to draw the line where mere variation ends and monstrosity begins. A variation of a degree that would be called a monstrosity in an animal, would be called only a “sport,” or a singular variety, in a plant. te Bid hi XvI.] THE LAWS OF VARIATION. 197 varieties of plants are mentioned on the authority of M. C. Naudin! :—“ The first case mentioned is that of a poppy, which took on a remarkable variation in its fruit— Poppy. a crown of secondary capsules being added to the normal central capsule. A field of such poppies was grown, and M. Goppert, with seed from this field, obtained still this monstrous form in great quantity. Deformities of ferns Ferns. are sometimes sought after by fern-growers. They are now always obtained by taking spores [seeds] from the abnormal parts of a monstrous fern; from which spores ferns presenting the same peculiarities invariably grow. .. . The most remarkable case is that observed by Dr. Godron, of Nancy. In 1861 that botanist observed, amongst a sowing of Datura tatula, the fruits of which are very Datura spinous, a single individual of which the capsule was per- aie fectly smooth. The seeds taken from this plant all fur- nished plants having the character of this individual. The fifth and sixth generations are now growing without ex- hibiting the least tendency to revert to the spinous form. More remarkable still, when crossed with the normal Datura tatula, hybrids were produced, which, in the second generation, reverted to the two original types, as true hybrids do.” The last statement may be compared with what has been mentioned above, as to the impossibility of obtaining a breed intermediate between the common and the otter sheep. Very little is known about the laws of variation. We may, however, make the following statements with con- fidence :— Variation is not always going on, nor does it go on in Only some 7 ' jf rs Taces are every part of an organism at once. Many races produce yiapie, hardly any variations ; aud, among the variable ones, some and some c characters characters are more variable than others; and when a 6f those. character has recently varied, it is apt to continue variable, 1 Quoted from the Comptes Rendus, in the Quarterly Journal of Science, October 1867, p. 527. 198 Correla- tion of variations. Homo- logous parts vary toyether. Mental and motor characters are more variable than for- mative ones. HABIT AND INTELLIGENCE. [CHAP. having, as I have already expressed it, acquired a habit of varying, Thus, among the domestic races of animals, the most variable characters are those in which each breed differs from the other breeds of the same species; and these are characters which, having arisen by variation since the races were first domesticated, must have varied conspicuously in a time which is very short in comparison with the lifetime of a species. Certain variations are habitually found together, or, as Darwin expresses it, are correlated. The nature of the correlation is sometimes quite unintelligible: thus, per- fectly white cats frequently have blue eyes instead of green, and are deaf.! But in many cases it is to be referred to the simple and intelligible law, that homologous parts tend to vary together. Thus, similar characters on the opposite sides of the body are so much a matter of course, that any exception to the law is regarded as a monstrosity —as, for instance, when the hands are of unequal length ; and, in the same way, the hands and the feet, which are homologous parts, usually have similar characters. It would be very difficult to find a person with large hands and small feet, or the reverse. Throughout the organic creation some classes of func- tions appear to be much more variable than others. Mental and motor habits are much more variable than formative ones: thus, in man, mental characters are incomparably more variable than bodily ones as between different indi- viduals, and also more modifiable by education and cir- cumstances. The same appears to be true of the domestic races of animals, at least of the more intelligent ones. As instances of the changeableness of purely motor functions may be mentioned the cases of the domestic hen, duck, and goose, which from disuse have nearly lost the power of flight, without the muscles of their wings being diminished in size in any proportionate degree. These are cases of - functionally-changed habit. The habit from which the tumbler variety of the pigeon derives its name is a far more extraordinary case, and is certainly unique among 1 Darwin's Origin of Species, p. 12. Se xvi] THE LAWS OF VARIATION, 199 flying animals: it must have originated in some spon- taneous variation. Among formative functions it appears to be a law that The the minutest structures are the least variable. Thus, the pues form of the leaves of any species of tree is in general #s the least ; ; . variable. tolerably constant, while the form of the entire tree 1s much more variable; and the form of organs appears to be Organs much more variable than the structure of tissues. Very 4tissues. little is yet known on this subject. Darwin has made a vast collection of facts showing the variability of organs ; but no one has yet made a similar microscopic examination of various tissues, for the purpose of ascertaining the degree and the limits of their variability. There is, how- ever, a remarkable constancy of microscopic structure throughout the same species of organisms: thus, a small fragment of a tooth, even when in a fossil state, is often Teeth. 0 / ) enough for the identification of a species;* and the same a as is true of shells, even when their forms are subject to Shells. considerable variation.2 Dr. Beale makes the following striking remark on the constancy of histological characters throughout the same species, and their distinctness from those of other species :— “The anatomical differences between corresponding tis- sues of closely allied species are often so distinct that the anatomist familiar with them could distinguish the one from the other. For example, it would be difficult to state in few words the difference between the unstriped muscular Muscular fibres of the bladder of the hyla [or tree-frog], of the olsen: common frog, and of the newt, and yet there is a recog- nisable difference; and corresponding differences can be demonstrated in other textures, if a comparison be care- fully instituted.” ? The subject, however, needs systematic investigation. Investi- The variability of external form is visible to the eye in Sion, 1 Carpenter's Comparative Physiology, p, 152. 2 My friend the Rev. John Grainger, of Dublin, informs me that the structure of shells presents little that is characteristic of particular species, but the scwlptwre, or external markings, often enables the species to be determined from very small fragments. 3 Beale’s edition of Todd and Bowman’s Physiology, p. 41. 200 HABIT AND INTELLIGENCE, [CHAP. The dog many instances, especially in the races of the dog and of ay the pigeon, which are so unlike each other that no one, on merely seeing them, would guess that they belonged to the same species ; and yet it is proved that they do belong to the same species, in both cases, by the fact that they breed freely together, and, in the case of the pigeon, by satis- factory evidence that all the domestic races have originated since the bird was first domesticated. If similar varieties exist in the microscopic characters of any species, they will not be obvious to the eye, but must be sought for. The most It is to be observed that those characters which are constant most constant and least liable to variation, as between characters in species individuals and between mere varieties of the same species, are also the most appear, as a general rule, to be also the most constant romenees throughout whole wide classes. Thus, the structure of asses. i nerve, muscle, and bone is much more constant between species of the same order, and between orders of the same class, than is the distribution of the nerves and the form © of the muscles and bones. To mention a very remarkable Bones of instance: the skeleton of a pterodactyle, which was a pterodac- : : - Bate tyle. reptile organized for flight, has a strong resemblance in its general outline to that of a bird ; but a microscopic exami- nation of a fragment of one of its bones shows it to have the structure belonging to the bone of a reptile? This law, that the characters which are most constant throughout the species are also the most constant throughout the group to which the species belongs, is, as I agree with Darwin in thinking, of great importance in explaining the origin of species. I only refer to it here as affording a strong presumption in favour of the truth of the opinion ‘TI have advanced, that the minutest structures are thie least variable. The lowest - The lowest organisms, on the whole, are the most cremost Variable. Vegetables are, on the whole, much more lowly variable. organized than animals,.and they are much more variable in form. The Algz, which are among the lowest vece- tables, are peculiarly variable; and among the Foramini- fera, which are among the lowest animals, the variability 1 Carpenter's Comparative Physiology, p. 140. ee — se SO eo xvI.] THE LAWS OF VARIATION. 201 is so great, and the intermediate forms present such innumerable gradations, that it is utterly impossible to fix the species.’ “ When any part or organ is repeated many times in the Parts structure of the same individual, as the vertebree in snakes aes and the stamens in polyandrous flowers, the number is times are variable ; whereas the number of the same part or organ, pacaels: when it occurs in lesser numbers, is constant.” 2 Between these two laws—that low organisms are vari- able, and that the number of often-repeated parts is variable—there is this connexion, that multitude of similar parts is a mark of low organization. And we may assign Reason of this very obvious reason for both, that the lower the organ- fee ye ism, the more will its form be a matter of indifference . a and the greater the number of parts, the more will their exact number be a matter of indifference. The form of a sea-weed must be nearly a matter of indifference to its health and life; but any great deviation from the normal form in one of the higher animals constitutes deformity, and is destructive. And, similarly, a pair of legs more or ‘less may make no difference to a centipede, but it is an affair of life or death to a quadruped. The laws of ordinary spontaneous variation, as stated Summary. above, may be thus summed up :— Spontaneous variations are quite distinct from func- tionally-produced modifications. Spontaneous variation occurs only when a new individual comes into existence. It occurs oftener in cases of sexual than of non-sexual propagation, and is stimulated by mixture of races and by changes in the external circumstances of life. Some races are more variable than others; and when a race has become variable, some characters are more variable than others. When a race, or a character, begins to vary, it will continue to vary for an indefinite time. . Homologous parts tend to vary together, and there are other correlations which cannot be reduced to any law. 1 Carpenter on the Foraminifera, published by the Ray Society, 1862. 2 Darwin’s Origin of Species, p. 176. 202 HABIT AND INTELLIGENCE. [cnap. Mental and motor characters are more variable, as well as more modifiable, than formative ones. Among formative characters, the minutest structures are the least variable. The lowest organisms vary most; and when there are many similar parts, their number is variable. I believe that very little more than what I have stated is known on the subject of ordinary spontaneous variation. But, as I have mentioned at the beginning of this chapter, besides functionally-produced modifications and sponta- Third class neous variations, there is a third kind which it is difficult of changes. £5 class, These take place only in organisms of low type. Crystals It has been mentioned in the chapter on Crystallization, gee a that crystals of the same species—that is to say, of the es same chemical composition and the same “ crystallographic they are elements”—often differ very much in form, according to deposited. the character of the medium from which they have crystal- lized. This variability is regular; that is to say, the same alteration in the medium determines the same peculiarity in the form of the crystal. Thus, “common salt crystal- lizing from pure water forms cubes; but if the water con- tains a little boracic acid, the angles of the cubes are truncated.” “Carbonate of copper, crystallizing from a solution containing sulphuric acid, forms hexagonal tabular prisms ; but if a little ammonia is added, the form changes to that of a long rectangular prism with secondary planes on the angles; if a little more ammonia is added, several varieties of rhombic octahedra appear.” ? Similar Similar variations from similar causes appear to take ee place among many low organisms, especially among those Fungi which constitute mould. We have not what can be called direct evidence of this; but there is evidence that the forms of the inferior Fungi are extremely variable; ? and it is scarcely possible to doubt that the various forms of Fungi which are characteristic of particular situations 1 Pe fo. 2 “Tt is asserted by Fries, that out of a single species of Thelephora more than eight genera have been constructed by various authors.” (Carpenter’s Comparative Physiology, p. 214.) Compare what has been said above on the variability of the Foraminifera. — =) | | ? xv1.] THE LAWS OF VARIATION. 203 are not all really distinct species, but that the same germ will develop into different forms, according to the soil on which it falls. ‘Thus, no Puccinia but the Puccinia rose is found upon rose-bushes, and this is seen nowhere else ; Omygena exigua is said to be never seen but on the hoof of a dead horse; and Jsaria felina has only been observed upon the dung of cats deposited in humid and obscure situations.”! We can scarcely believe that the air is full of the germs of distinct species of Fungi, of which one never vegetates until it falls on the hoof of a dead horse, and another till it falls on cat’s dung in a damp and dark place. It is probable—indeed, in my mind, certain—that among the lower Fungi, as among crystals, the same species assumes different forms according to the medium in which the development takes place. Variations of this kind are clearly not of the class of spontaneous variations, for the form is determined by the medium in which the development takes place. It is difficult to say whether they are functionally produced. The inferior Fungi appear to be very susceptible of func- Function- tionally-produced modifications: thus, they are sometimes a Oe developed in liquids, and then they assume very much of difications the character of Alge.? But it is difficult to see how any” ae change of form could make one of those low Fungi which constitute mould better suited to a new habitat; and I incline to think that the variations by which the same germ of mould develops into different forms, in different situations, is rather analogous to the variations of crystal- line form of which I have spoken than to any result of the laws of habit. The power of some of the lower organisms to develop Origin of into different forms in different situations is probably the nie es 1 Carpenter’s Comparative Physiology, p. 214, note. 2 Tbid. p. 198. A remarkable instance of the change caused by removing a fungus to a new medium is mentioned in a communication from Mr. Varley to the Microscopical Society of London (Zoologist, 1850, p. 2674). After describing a fungus that often destroys the common house-fly, the report goes on :—“ By immersing the fly in water, Mr. Varley found that the mode of growth of the fungus was altered, the heads being no longer produced, and the whole plant becoming long, crooked, and filamentous.” Their meta- genesis. Indi- viduality, difficult to define among the lower organisms. Morpho- logical units of different orders. HABIT AND INTELLIGENCE. [CHAP. XVI. explanation of what formerly was the most perplexing question in biological science—I mean the origin of the internal parasites of animals. The germs of these, I have no doubt, have been in all cases originally introduced from without, either in the food or through the skin, and have been developed into different forms from those which they would have assumed had their development taken place elsewhere. This conjecture is almost proved by the dis- covery of the fact that many of these internal parasites, or Entozoa, alternate by metagenesis* with animal forms that inhabit the earth or the water. NOTE. INDIVIDUALITY. Amone the lower organisms, it is nearly impossible to define what an individual is. The single cell of one of the lowest Algze can scarcely be described as other than an individual, and yet it is homologous with the cells that combine to form the filaments of zygnema, or the fronds of the higher Alge. The separate hydra is homologous with each polypite of the com- pound Hydrozoa ; yet the latter are united into one compound organism, in much the same way that the leaves of a tree are united. In fact, the word “individual” is scarcely applicable ; the expression, “morphological unit,” proposed by Herbert Spencer, is better, as we can speak of morphological units of: different orders. Among the higher orders of plants, the mor- phological units of the first order are the cells ; these, by com- bination, form the units of the second order, which are the leaves with their stalks ; these combine into units of the third order, that is to say, into branches, or axes (understanding by this term the product of a single bud), and the branches into trees : just as the unit of organization in an army may be suc- cessively taken to be the individual soldier, the company, and the regiment. 1 Metagenesis, a parallel word to metamorphosis, and signifying that two forms produce each other alternately. Thus, to take an instance from a different class, many hydra-like forms produce meduse, and the meduse again produce hydra-like forms. ee en CHAPTER XVII. THE PROBLEM OF THE ORIGIN OF SPECIES. AVING in the last two chapters considered the laws of habit and variation, we are now in a position, not indeed to solve, but to state, the question of the origin of species. In the chapter on the Chemistry of Life, we have seen that vital properties are not a resultant from the ordinary properties of matter; and that the origin of life, as much as the origin of matter and energy, must, in all probability, be directly referred to Creative Power. But the question of the origin of the distinct species and classes of living beings is quite another question, and it may be thus stated :— We have seen, in the chapter on Organization, that organization is not the cause, but the effect of life: vital- ized matter has a tendency to produce organization. We have also seen, in the last two chapters, that the characters of species are not absolutely invariable, but that modi- fications may he functionally produced in the lifetime of the individual, and variations, sometimes of very percep- tible magnitude, may arise spontaneously in the production of new individuals; and also that, in all probability, the germs of many of the lower organisms, if developed in Have all a new habitat, will produce new forms. Does not all this ae been make it probable that distinct species, and even classes, rare have not been separately created, but that they have been derived all derived by descent, with modification and variation, eas from one, or at most a small number, of germs—mere gems ? 206 T believe the latter. Where I dissent from Darwin. T believe in a guid- ing Intel- ligence. Develop- ment theory, not con- trary to ex- perience. Changes in lan- guage. Geological changes. HABIT AND INTELLIGENCE. [CHAP, minute masses of germinal matter that were once vitalized by Creative Power ?1 I believe they have been so derived, and in this I agree with Darwin. But I must again repeat here that I am not a believer in what is usually, and quite accurately, called “ Darwin's theory.” According to Darwin and his disciple Herbert Spencer,? the laws of habit and variation are sufficient to account for the whole process of modification by which the most highly organized vegetables and animals have been derived by descent from their first vitalized but unorganized germs. I altogether differ from this: I think the process of modification proves the agency of an Intel- ligent Power, acting through and controlling the laws of habit and variation, just as all the vital forces act through and control the inorganic ones. But before we discuss the agency by which the modi- fication has taken place, let us consider the proofs—which, in my opinion, are overwhelmingly strong—that the modi- fication has taken place. Several chapters, however, will be required to state the proofs even in outline ; and in this chapter I intend only to reply to some of the more obvious and important objections. It is often said that any theory of the origin of species by descent from other species is contrary to experience, because all experience shows that every species produces its own kind, and not some other. This objection is not of much force. What is true within the limits of a very short experience is not necessarily true in a much longer time. Thus, languages do not, at least in general, change per- ceptibly in a lifetime; but we know that they change in historical time: thus, Latin has given origin to the entire group of modern Romanic languages in less. than two thousand years. The outlines of continents and the heights of mountains are not perceptibly changeable in historical times ; but we know that in geological time they are as fleeting as the outlines of a cloud. So, if it is proved, as it 1 See Note at end of chapter. 2 In calling H. Spencer a disciple of Darwin, I do not mean to disparage the great originality, or the wonderful ingenuity, of his work on Biology. ae xvu.] THE PROBLEM OF THE ORIGIN OF SPECIES. 207 is, that organisms are in any degree susceptible of variation, that variation may become indefinite in amount, if inde- finite time is granted for the variations to take place, and to be added together. I cannot admit that there is any intrinsic improbability No in- in the derivation of the highest organisms by descent from eer. the lowest: I think the improbability is all the other way. bility. It would be contrary to all the analogy of nature to sup- pose that any of the highest works were ever produced at once and by a single creative act; but it is in accordance with all that we know of the ways of nature to suppose that species, like individuals, have been developed out of simple germs. When we know that every individual anajooy of organism, vegetable and animal, has been developed out Pn cl of a simple structureless germ, and that there is no test, ae chemical or microscopic, by which the germ of one organism ean be distinguished from that of another, no matter how unlike may be their mature forms; and when we know that every organic species must have had an origin, is it not most natural to believe that the origin of the species has been parallel to that of the individual? The reason why the theory of the origin of species by development is so often regarded as improbable, and as having at least a strong presumption against it, is chiefly, I believe, that it Subject has not yet had time to become familiar. It was impos- pele sible that the development theory could be entertained at all until the very great antiquity of the earth was known ; for, so long as it was believed that the order of nature was only coeval with human history, it was a necessary inference that the organic creation came into existence all at once, at the beginning of things. And it was impossible that the development theory could be argued on a right basis until it was proved that every individual is developed from a perfectly simple and unorganized germ, and until the old notion that the germ was a fulded-up miniature of _ the perfect form was disproved. A really scientific objection to the development theory is Separation the general fact that the union of distinct species will not ohspectes : F _ by mutual produce offspring, or at least not fertile offspring ; and it sterility, not abso- lute. Reason of mutual sterility unknown. HABIT AND INTELLIGENCE. [ CHAP. has been a very generally received opinion that this cri- terion of distinction between species is absolute; that the distinction of species thus made and thus marked is a per- fectly impassable barrier ; and that, consequently, not only the first origin of life, but also the origin of every distinct species, needed a distinct act of Creative Power. In my opinion, however, Darwin has fully answered this objection by the great mass of facts brought together on this subject in the chapter on Hybridism, in his great work on the “Origin of Species.” As that work is very well known and very readable, I will only state his conclusions in the barest outline ; some of them have been stated already in the last chapter, but I will repeat them here. It is true that similar organisms produce offspring together, and that unlike organisms will not produce offspring together. But it is not true that this distinction is rigid and absolute ; on the contrary, it admits of grada- tion. Between perfect mutual fertility and perfect mutual sterility there are an indefinite number of gradations. Sometimes forms that are regarded as distinct species are perfectly fertile with each other, taking the production of the average number of seeds as the test of perfect fertility ; sometimes forms that are regarded as only varieties of the same species are not perfectly fertile with each other. The fertility of different varieties and species with each other admits of various degrees, as estimated by the number of seeds produced. Sometimes hybrids are perfectly normal organisms; sometimes they are healthy, but infertile ; sometimes they are weak, and perish early. It is important to observe that we do not know on what the mutual sterility of distinct species depends. It is cer- tainly not on mere visible unlikeness: the various domestic breeds of pigeons are all perfectly fertile together, and yet they are more unlike in external appearance than many animals are to each other which are universally classed as distinct species, and most probably are not fertile together." 1 I say most probably, because the fact is very difficult to ascertain, as few animals breed freely in confinement. The case of the mule between the horse and the ass, however, is an important one on this subject ; xvi] THE PROBLEM OF THE ORIGIN OF SPECIES. 209 The same is true of the various breeds of the dog: no one, merely by seeing them, would think that a greyhound and a terrier were of the same species. It is quite possible— though I only offer this as a suggestion for which it may Sugges- never be possible to find proof—that mutual sterility ee depends on the length of time that has elapsed since the ject. two stocks separated. The various races of pigeons have all been produced under domestication, and consequently during the historical period; but the origin of wild races may go back into geological time: so that, although the most unlike races of pigeons are more visibly unlike than the horse and the ass, the stocks of the horse and the ass may have diverged from each other hundreds of thousands of years before the stocks of the various races of pigeons began to diverge; and this may be the sole reason why all breeds of pigeons are mutually fertile, while the horse and the ass are not perfectly so. Another very obvious objection to the development Transi- theory is, that if species have been formed by slow transi- a tion, by descent, from one form to another, we ought to find innumerable transitional forms ; but instead of finding these, we find that each species is quite distinct. This is partly answered by the statement that in very many cases each species is not quite distinct, and that we often still do find a great variety of intermediate forms ; so that—what oom appears a strange paradox and yet is true—it often happens that the more thoroughly a genus is known, the more difficult it is to determine which of its members are species and which only varieties! But this answer is utterly insufficient. If species have been formed by slow transi- but mostly tion, only a very small proportion of all the forms that ae must have existed are now living. But why do we not find their remains entombed among the rocks ? To this objection Darwin has, I think, given a conclusive it is well known to be healthy, but not fertile. With cultivated plants experiments of this kind are in general easy. . 1 Darwin’s Origin of Species, p. 58. P 210 Imperfec- tion of the geological record. Destruc- tion of fossils. Denuda- tion. Metamor- phism. HABIT AND INTELLIGENCE, [cHar. reply in his chapter on “the Imperfection of the Geological Record.” On this subject, as on that of Hybridism, it is only necessary for me to state his conclusions in the barest outline. Soft-bodied animals, Jike the naked worms and mollusca, are not preserved at all; they perish without leaving any record of their existence. The same is mostly true of land animals, though for a different reason. It is only in the rarest cases that land animals can die under such circum- stances as to be buried, and afterwards fossilized. Asa rule, it is only the hard parts of aquatic organisms that will be fossilized and preserved ; and when they are so preserved, the older the fossiliferous beds, the less will be the chance of their preservation to our age. “The stir of the forces whence issued the world”? is not quiet yet, and never has been quiet. The deposition of new strata never ceases ; and it must be remembered that, as the quantity of matter in the world is unchangeable, if there is deposition going on in one place, there must be an equivalent amount of denudation somewhere else—that is to say, an equivalent amount of destruction of old strata; though we habitually forget this, because we see the deposit, and do not see the denudation. Besides the effect of denudation in destroying old fossili- ferous beds, there is the effect of metamorphism—that is to say, the chemical effect of heat. It was first pointed out, I believe, by Sir John Herschel,? that (granting, what is unquestionable, the theory of the earth’s central heat) any deposition of strata must raise the temperature of the, strata underneath, exactly in the same way that putting on a warm coat raises the temperature of the skin. Such a rise of temperature, if sufficiently great, will cause a meta- morphic change in the strata so covered, and will destroy their fossils. As metamorphism from this cause, as well as from the intrusion of igneous rocks, is not an exceptional but a normal action, and as the same is true of denudation, 1 Matthew Arnold. 2 See his letter to Sir C. Lyell, in the Appendix to Babbage’s Ninth Bridgewater Treatise. Darwin has made no use of this argument. xvi] THE PROBLEM OF THE ORIGIN OF SPECIES. 211 it follows that metamorphism and denudation—fire and water—are incessantly destroying the ancient records of creation; so that all that geologists can ever hope to re- cover are the latest leaves of the volume, and these in but a fragmentary state, because a large proportion are covered by the sea, and others buried inaccessibly deep. With all this, it remains true that many intermediate Discovery forms have been discovered ; and, what is a most important er fact, no class—no type of form—has been found among forms. fossil species fundamentally unlike all living classes. As has been truly remarked, all fossil forms can be arranged either in or between living groups. To mention what is, perhaps, the most striking instance of this: till very lately, the class of birds appeared to be isolated from all others ; but recent discoveries have proved—or at least all but proved—the former existence of an entire order connecting Reptilian the birds with the reptiles. uns Another objection to the development theory is that Varia- raised by Dr. Beale, and alluded to in the last chapter ; po namely, that although the forms and external characters gical of species are variable, there is no proof that their histo- mE logical or minute structural characters are so. On this subject we are as yet, I believe, quite too ignorant to base any argument, either the one way or the other. My own opinion, however, based purely on the analogy of the case, is this: that.as structure differs between species and between classes, but differs less than form, so it will be found that structure varies within the limits of the same species, but varies less than form. Having thus briefly touched on the principal arguments against the development theory, I will go on with those in its favour. I cannot as yet enter on the subject of the process by which the development of new species has been brought about, and its causes; but I will here state in outline some reasons for believing in the development theory, which are to me of great weight, though they are so far from obvious that they appear to have been unnoticed until the publication of Darwin’s great work. P2 212 Species are permanent varieties. Varieties are most numerous where species are so. Aberrant genera are poor in species. HABIT AND INTELLIGENCE. [cHAP. If the development theory is true, and if all species whatever, or all the species of any wide group, are de- scended from the same original ancestor, then, as Darwin has expressed it, “species are only well-marked and per- manent varieties ;” and the species of a genus stand to each other in the same relation as do the varieties of a species. Such is shown to be the case by a great amount of evidence which Darwin has collected, and arranged in a way that could only have been done by one combining his vast knowledge of the details of natural history with his power of seizing on the essential relations of a subject. As his “ Origin of Species” is a well-known and very readable book, it is only necessary to state the following facts in the barest outline. “Where many large trees grow we expect to find sap- lings;” and when a form has been varying much within recent geological time, so as to form many species belong- ing to one genus, we may expect to find that it is still varying. Such is the fact: if large genera (that is to say, genera containing many species) are compared with small ones, it will be found that, on the average, a greater number of the species of the large genera than of the small ones present varieties; and those which do present varieties, present a greater number of them. These results, of course, are not uniform, only average ones." What is a fact of exactly the same order, aberrant genera (that is to say, genera that are very peculiar, and not nearly akin to any others), are nearly always small genera.? On the theory of development, an indefinite number of intermediate forms must have existed formerly. A genus has become aberrant by the extinction of all the species and genera that formerly connected it with other known forms; and as the same causes, whatever they are, that determine the increase or the extinction of a species or of a genus usually act nearly alike on kindred species and genera, the causes that make a genus aberrant by destroying the kindred genera will also make it poor in species by destroying many of its species. Such genera are Ornithorhynchus and Lepidosiren. 1 Darwin’s Origin of Species, p. 65. * Ibid. p. 508. ae ener ae ys? Byte say | xvit.] THE PROBLEM OF THE ORIGIN OF SPECIES. 913 These may be compared to trees which have been left alone by the gradual destruction of their neighbours, and have at the same time lost many of their own branches. A different fact, poiting in the same direction, is the following :—In the chapters on Habit and Variation we have seen that if a character begins to vary, it will con- tinue to vary for an indefinite time. It is a case of this law, that when variations have arisen under domestication, and consequently a very short time ago, geologically speak- ing, the part that has varied, and is characteristic of the variety, continues to be the most variable part of the organism. The varieties of the domestic pigeon are the best instance of this But what I wish to remark is, that what I have just stated of the varieties of a species is equally true of the species of a genus. When any part in one species of a genus is unlike the corresponding part ee in the other species, whether in size or in shape, or in any as between other way, the same part is also usually variable as between ov) 0° the different varieties, and between the different individuals thespecies. of the species; or, what is saying the same thing in more concise language, specific characters are more variable than generic ones.” For instance, in some groups of beetles the Wings of presence or absence of wings is of no importance in classi- Restless fication: there are genera in which some species have wings, and others have none; and sometimes this cha- racter is variable even between individuals of the same species. This fact is what might be expected on the sup- position, that species are only “ well-marked and permanent varieties :” on the supposition of the separate creation of every species, it is utterly inexplicable. Reversion to ancestral characters is closely connected Reversion with variation, and is sometimes due to the same cause; jin namely, mixture of races. In all the breeds of pigeons, individuals are sometimes found that have reverted to the plumage of the rock-pigeon, from which the domestic breeds‘ are descended ; but these are most numerous when two breeds have been crossed. Crossing of the breeds is also a cause of general variation. ¥ PDarwin’s Origin of Species, p. 180. 2 Ibid. pp. 177, 182. 214 and in species. qa Laws of variation and of reversion. HABIT AND INTELLIGENCE. [cHar. That which is true of the varieties of the pigeon is also true of the species of the genus Equus. The zebra and Be oes of other species of Equus are striped: the ass usually has stripes on the back and shoulders only, though sometimes on the legs also: the horse, in general, has no stripes at all; but horses are occasionally found with stripes on the back and shoulder, and in the Kattywar breed in India these are usual, and bars on the legs are common. The fact of these stripes being characteristic of other species of the genus, and occasionally appearing in the horse, suggests that their appearance in the horse is a case of reversion ; that all the species of Equus, like all the varieties of the pigeon, are the descendants of one ancestral species, and that the appearance of these stripes is a case of reversion to its characters. What greatly strengthens this presump- tion is, that mixture of race between different species of Equus, as between different varieties of the pigeon, in- creases the tendency to the production of these characters. Bars on the legs are much more common in the mule between the horse and the ass than in either the horse or the ass. We thus find this law of variation, that characters which are variable as between the species of a genus are also apt to be variable as between the varieties and between the individuals of a species. And we also find this law of reversion, that individuals are sometimes found which present what appears to be the character of the original form from which all the varieties of a species, or all the species of a genus, have been descended. I believe that these laws are true on a much wider scale? and will elucidate relations between species, not only of different genera, but of different classes. The law, that those characters which are variable as between the species of a genus are also variable as between 1 Darwin’s Origin of Species, p. 191. 2 Darwin thinks that the occasional formation of a sixth finger in man may be due to reversion to the characters of a remote ancestor near the bottom of the vertebrate scale. No living species of air-breathing vertebrate has more than five digits, but that number is exceeded in fishes and some extinct reptiles. (Darwin’s Variation under Domestication, vol. ii. p, 16.) Te ee xvu.] THE PROBLEM OF THE ORIGIN OF SPECIES. 215 the varieties of those species, is, I think, only a particular Extension case of a much wider law, which may be thus stated :— nines The wider a group to which any character belongs, the less 2s stated law that character liable to exceptions. Thus, for instance, jointed eo legs are a character of all the higher Articulata, and they are found in every species of all their classes, without a single exception. Wings, on the contrary, are found only in Wings of the class of true or hexapod insects ; and, though general, mies they are by no means constant in their class. Another instance of this kind is that of the respiratory organs in the Branchix three allied molluscan classes of Gasteropoda, Heteropoda, etc and Pteropoda. Most members of these classes have dis- tinct respiratory organs; but their occurrence is not uni- versal, as some of them respire through the general surface of the body. When they do occur, their position is very variable as between the various families. In Firoloides Desmarestiz, a heteropod, there are none, though they occur in some nearly allied species; and in one of these, by name Atlanta Lesuertt, some individuals have them, and others not.2, Another instance of the same kind is that of Develop- the extraordinary larval forms presented by some of the "nino. Echinodermata in the course of their development. This dermata. peculiar mode of development, of which I shall have to speak in a future chapter, “in each order appears to be exceptional ; and in certain cases it is known to be carried to its most abnormal degree in one species, while in a closely allied species of the same genus the mode of repro- duction differs but slightly from the ordinary invertebrate type. It seems highly probable that even in the same species the development and independence of the first zooid may be carried to a greater or less degree according to circumstances.” * The other law which we have seen to be established, rete. that varieties tend to revert to the characters of the of the 1 Except, perhaps, among the lowest of the acari, or mites. 2 Huxley on the Morphology of the Cephalous Mollusca (Philosophical Transactions, 1853). 3 Dr. Wyville Thomson on the Ktymology of the Echinodermata (Natural History Review, July 1863). 216 above- stated law of rever- sion. Circular and bilate- ral flowers. Peloria. Non- sexual generation in worms, HABIT AND INTELLIGENCE. [CHAP. ancestor of the species, and species to the character of the ancestor of the genus, is, I believe, only a case of a principle which is true far beyond the limits of species and genera. I believe that reversion sometimes occurs to the characters of very remote ancestors indeed, from which the species in which the reversion occurs is separated, not only by many myriads of generations, but by very profound changes. One instance of this is among flowers. It can scarcely be questioned that the normal form of flowers is circular, and that (supposing the development theory to be true) flowers of bilateral! forms have been derived from circular ones by descent, with modification. It is to be observed, that flowers cannot be divided into two classes according as they are circular or bilateral: all the rose tribe have circular flowers, and all the foxglove tribe have bilateral ones; but among the geraniums this character is variable within. the genus—indeed, I believe, within the species. Individual flowers are sometimes found which are cireular, though belonging to bilateral species. This variation is common enough to have received a name, “ peloria,” and, on my view, it is a case of reversion. Another instance of what I believe to be reversion is far more extraordinary. Non-sexual generation is usually a proof, or at least a concomitant, of comparatively low organization: it is universal among the lowest animal forms ; it does not occur among the true mollusca, at least in any ordinary form ;” but it is common among the marine worms, which are about on a par with the Gasteropodous mollusea in grade of organization, though very unlike them in plan. I shall also show reason for believing that all the 1 Bilateral may sound pedantic, but I prefer it to irregular, which is an inaccurate expression. 2 The only known instance of non-sexual generation among the mollusca is a very extraordinary one, which occurs in their highest class, the Cephalopoda. The male of the cuttle-fish tribe bears no resemblance to the female ; it is produced by one of the arms of the female being detached and undergoing a peculiar development, which transforms it into a male animal, or at least a male organ. baie a ee Te 5 pear. ad 2. % a ee “Ab er there are some very small breeds of dogs. But the possible hee xvi} THE PROBLEM OF THE ORIGIN OF SPECIES. 217 higher Articulata are descended from worms; and I regard as an instance of reversion to the characteristics of worms, the fact that non-sexual generation occurs among some of the lower Crustacea, and also among some genera of winged € OnE insects. The larvee of Aphis, and of some Diptera, produce ;” other larvee by non-sexual generation ; and some butterflies produce fertile eggs without being fertilized by the male. Having in this chapter brought forward evidence in favour of the theory of the production of species by a process of development, I intend in the next to begin the consideration of the laws of that process. insects. NOTE. Jn the chapter on the Laws of Habit, I have stated my belief Is there a that there is no limit to the possible extent of variation, pends if only a sufficient number of generations is allowed. This is I maintain contrary to the general view, which is that the possible extent as of variation is rigidly confined within the limits of species. It opposition has been generally believed till now that variation can only form eeeeter varieties—as, for instance, the domestic varieties of the dog and of the pigeon have been formed by variation from their original wild stocks ; but that no amount of variation, even though acting through geological time, could possibly derive two such species as the dog and the cat by descent from a single ancestor. This view has been maintained by a singularly able writer on Aroument Darwin’s theory in the North British Review for June 1867, North : > British who makes the following appeal to the test of fact :— REET The dog varies much in size, as in all other characters, and ibe the Tma- smallness of the dog appears to have reached its limit ; for very Limit of smallness small dogs are prized by fanciers, who are willing to pay highly in dogs for them, so that there is an inducement to breed smaller dogs bas s been than the smallest yet bred. The method of doing so would be, ae of course, to select the smallest dogs to breed from, and then select the smallest of the offspring. But this is not done, and apparently cannot be done. The small breeds of dogs, though they were originally produced by variation from a much larger 218 HABIT AND INTELLIGENCE. [cHAP. XVII. stock, have ceased to vary in the direction of smaller size. The reviewer infers from this that the dog has a specific limit as to size, beyond which it is incapable of varying. Variation I reply to this, that every form has a tendency to revert to Bede bs ancestral forms ; and the limit of smallness in the dog, or the reversion. limit of any particular variation in any species, is at the point where the tendency to further variation is balanced by the tendency to reversion. But, though the dog has at present attained a limit at which the tendency to variation in the direction of smallness is held in check, there is no proof that such a limit is-so grounded in the laws of life as to be abso- lutely impassable. All tendencies come under the laws of habit, 7 and consequently may wear out and disappear with mere lapse ; of time. If, consequently, the smallest breed of dogs now in 3 existence were kept separate and not permitted to mix with any other for a sufficiently long time—say for a thousand years— during which time all individuals that showed any tendency to revert to the larger size of the original stock should be destroyed, Tendency it is probable that the tendency to revert to the larger size to revert s : z : * : : Sune gee would disappear : the race might then begin again to vary in + out with the direction of smaller size; smaller individual dogs might be Pe of produced than any now in existence; and, if selection were > applied to them, they would become the parents of a race of and limit dogs which would continually grow smaller, until the ten- a dency to variation was once more checked by the tendency recede. to reversion. The reviewer shows that he is aware of this argument, but thinks that the assumption on which it rests—namely, that mere lapse of time will obliterate the tendency to reversion—is without any proof. Of course direct proof is out of the question. Geology gives no evidence on such subjects ; history is generally silent ; tradi- tion has forgotten them ; and experiment is impossible in cases where an experiment might be only beginning to yield any Reason of result at the end of a thousand years. But I think it is as Be oe certain as such a thing can be, that all tendencies come under this. the laws of habit, and that all unused habits and unmanifested tendencies become weaker by mere lapse of time; though I admit we can scarcely ever be certain that the tendency to reversion has altogether died out. CHAPTER XVIII. DISTRIBUTION. EFORE we go any further in discussing the problem of the origin of species, it is necessary to consider the doctrine, which is chiefly associated with the great name of Cuvier, that all the facts of organization are to be Cuvier’s directly referred to the conditions of that existence for (cme which the organism has been created. This brief and somewhat technical statement of the theory needs to be further explained. I have already defined organization as the adaptation of structure to function ;1 and I have stated the law of the physiological division of labour, which consists in setting apart parti- cular tissues and organs for particular functions. Cuvier’s doctrine is no more than an extension of this: it is, in truth, no mere theory, but a generalized statement of unquestionable and most important facts. He taught that of organic every part of an organism? is directly and specially adapted orden to every other part, and all to the conditions of that exist- ence for which it has been made; and this doctrine, so far from being in his hands a mere barren truism, enabled him, with the aid of his vast anatomical knowledge, to restore many extinct organisms, which were known only by a few fragments of bone: to restore them, I say, to the mind’s eye and on paper, as an architect can restore a ruin. Thus, ee ales 2 So far as I am aware, Cuvier made no attempt to apply this principle to vegetables ; but no one who understands the rudiments of the subject can doubt that all such general principles are applicable to animals and vegetables alike, 220 Func- tional and structural adapta- tions. The form of each part is deter- mined by the rest, and all by the ani- mal’s life. This is true : but will it explain all the facts ? We must admit a further principle. HABIT AND INTELLIGENCE. [CHAP. for instance, carnivorous teeth are necessarily united with a carnivorous stomach; extremities modified for swimming, as in the seal or whale, are necessarily united with a form of body also suited for swimming. Such as these may be called functional adaptations. Besides these are what may be called structural adaptations: as, for instance, the adap- tation. of the form of the bones to the pressure of the muscles upon them; or the adaptation of the forms of all the organs in a serpent’s body to the elongated form of its body. Functional and structural adaptations, of course, run into each other, and cannot be rigidly separated: the forms and characters of the various parts of an organ- ism are determined by each other, and they are all col- lectively determined by the nature of the animal’s life. Thus, a carnivorous life determines the existence of a carnivorous organization; a herbivorous life determines the existence of a herbivorous organization: so of a ter- restrial or aquatic life, and of endless other classes of modifications. Concerning this doctrine, the question is not whether it is true; for there is no doubt whatever that every organ in an organism must be adapted to all the rest, and all to its mode of life: these truths are implied in all the facts of organization, and no fact of organization can be under- stood without bearing them in mind. The question is, whether Cuvier’s doctrine alone will suffice to explain the facts of organization: whether the organization of a species can be referred to the conditions of its existence alone, or whether it is a resultant from the conditions of its exist- ence jointly with some law, or principle, of a totally different and quite peculiar kind. Cuvier maintained the former: he always asserted that the whole organization of a species was directly referable to the conditions of its existence. But the necessity is now universally recognised of admitting a modifying principle of a distinct kind from this. The two, laws—for I admit them to be true, though I deny that they explain everything—the law, I say, that every part of an organism has its form and character deter- ey ee ee COR ee ee ne ey < Di cs i aaa Svar Lo bo e xvu.] DISTRIBUTION. mined by those of the other parts, and the law that the form and character of the whole organism are determined _ by the mode of its life, are inseparable from each other. But, though logically in the closest connexion, they have bearings on two very distinct parts of biological science. The first—the law that the characters of all the parts are Bearingsof mutually connected and mutually determined—has special payers bearings on the problems of morphology ; the second—the on mor- law that they are collectively determined by the mode of eee its life—has special bearings on the problems of distri- bution. bution. If those two laws were not only true, which they _ are, but all-explaining, which they are not, all the facts of morphology and distribution ought to be explicable by them. The distribution and habitats of every species ought to be explicable by the relation of its organization to its mode of life; and its morphology ought to be explicable by the relation, both functional and structural, of its organs to each other. Now, is it so? These questions are, in their nature, capable of being answered by an appeal to facts. I shall consider the facts of distribution first, not as being the most important, but as being the simplest, and the easiest to make intelligible. It is to be observed, that even if Cuvier’s doctrine were all-explaining, it would not contradict the doctrine of the modifiability of species; it would rather give support to that doctrine ; because we know, from geological evidence, that the physical circumstances of localities as to climate, vegetation, &c. are subject to vast though slow changes ; and if we believe that the characters of species are deter- mined by the external conditions of their existence, it is a reasonable inference. that those characters may be modified by a change in the conditions of existence. But the strongest evidence that we have of such changes in the characters of species is afforded by facts that cannot be in any sense referred to Cuvier’s principle, though they are quite consistent with it. If Cuvier’s doctrine, that the organization of an animal is determined by the conditions and mode of its life, were 222 External circum- stances do not deter- mine dis- tribution. Mountain species. HABIT AND INTELLIGENCE. [CHAP. not only true, but true without any other modifying cause —in other words, if organization depended on the con- ditions and mode of life alone—it would be almost a necessary inference, that if in any two regions the conditions of life as to climate, food, and other external circumstances are nearly similar, the organic populations of the two ought also to be nearly similar; and the degree of similarity between the organic populations of any two regions ought to be in some sort of proportion to the similarity between the conditions of life in the two. But there is no such relation—not even an approximation to it. The degree of resemblance between the organic populations of any two regions depends in scarcely any degree on the similarity of climate and other conditions, but almost entirely on their contiguity or distance, and the facility with which they may have been colonised the one from the other, or both from the same source. For instance, mountain regions, when distant from each other, though having similar climates, are not found to have similar organic populations: on the contrary, the peculiar species of each mountain region are, for the most part, nearly related to the species inhabiting the neighbouring low country, but modified, as if to suit the mountain life. The inference is obvious, and I believe true, that the mountain species are, not metaphorically, but literally kindred with those of the plain: either the one group of species has been descended from the other, or both have been descended from the same source. I will mention another special and very remarkable instance of the way in which species are distributed. The three regions of Australia, South Africa, and the southern part of South America are situated between nearly the same parallels, and have nearly the same climate (including, of course, in the word climate, moisture as well as temperature); and climate is much more important in determining the conditions of life than all other influences put together. On the other hand, the three old continents, Europe, Asia, and Africa, contain every variety of climate, from the coldest to the hottest, from the driest to the wettest. But Australia, South Africa, eve cer eo | d ee ee ee ee ee tptiihiies i On| ET ae Ace 2 Iga eT ee ee 9145-381 y » CaO ee ee - mide oa XVIII. | DISTRIBUTION, bo bo oO and South America are separated, and probably have been so for long geological ages, by wide, impassable oceans ; while the lands of Europe, Asia, and Africa are continuous, and possibly have recently been more completely con- tinuous than they are now. Accordingly we find, if we Distribu- look to the distribution of the mammalia, that there is eel this strong resemblance between the zoological character in the old of Europe, Asia, Africa, and also North America, notwith- ew standing their vast diversities of climate, that the most conspicuous and characteristic mammalian order in them all is that of the Ungulata, or hoofed animals. In Aus- Australia, tralia, on the contrary, notwithstanding its similarity in climate to that of South Africa, there is not a single indi- genous Ungulate animal: all the indigenous mammalia belong to the two orders which stand lowest in the class, and are represented respectively by the kangaroo and the ornithorhynchus.!_ The mammalia which are characteristic of the southern part of South America, again, belong to South the order Edentata, which are represented by the sloth and ae the armadillo, and are utterly unlike either the Ungulata or the characteristic Australian orders. It belongs to this class of facts, that Madagascar, notwithstanding its almost Madagas- perfect similarity of climate to the neighbouring African “” continent, has a totally different mammalian population, chiefly consisting of lemurs, a tribe allied to the monkeys.? Wingless And New Zealand has no indigenous mammalia at all, ae ce their place being occupied by a remarkable tribe of wing- Zealand. less birds. The facts revealed in the recent geological history of the same countries are equally remarkable and important. Concerning Madagascar-we have, I believe, no information as yet about the remains of extinct animals; but in the Extinct old continents, in Australia, and in South America, the UN extinct mammalia of the geological periods nearest to the 7sion re- : : semble the present were, in each region, generally of the same order, livingones. 1 The so-called wild dog of Australia is no real exception, as it is a half- domestic animal with the native savages. 2 See Dr. Sclater on the Mammals of Madagascar, Quarterly Journal of Science, April 1864. 224 Resem- blance of species in conform- able strata. Connexion by descent, but modi- fied. HABIT AND INTELLIGENCE. [CHAP. though different in species, from those which now inhabit the same region. The same is true of the wingless birds of New Zealand. And it is a general fact in geology, that the fossil species of two strata, lying conformably one on the other, present a strong resemblance even where they are not identical. The inference is obvious, though I do not say certain, that when we thus find the same area to have been inhabited, in two successive geological periods, by similar though different species, the connexion between them is that of descent. I say that, at the present stage of the argument, I do not regard this conclusion as proved. I believe I shall give ample proof of it before I have com- pleted this work. I must, however, mention that when I speak of the newer set of species as connected with the older by the bond of descent, I do not mean direct descent like that by which we are connected with the races of men that inhabited the British Islands a thousand years ago: I mean descent modified in a peculiar way, which I shall have to explain in another chapter. But it may be urged, that Cuvier’s principle of the perfect adaptation of the organism to the conditions of its existence may, after all, account for the facts of both the geographical and the geological distribution of species. It may be urged that we do not really know enough on the subject of the relation of the structure and characters of any species to the conditions of its life to assert that the one cannot be determined by the other. In a great variety of cases, the connexion is obvious enough ; as, for instance, in the adaptation of the monkey’s hand or the woodpecker’s foot to a life spent in climbing trees. But where the con- nexion is not obvious, we have no right to infer that it does not exist. The more we know concerning the subject of the complex relations between organisms and the con- ditions of their lives, the more intricate do these relations appear. Darwin has insisted on this with great force, and with all his accustomed lucidity. I reply to this argument, that it is refuted by evidence identical in kind with the evidence of experiment. There are a great many instances of organisms being introduced — xvi] DISTRIBUTION. 225 by the agency of man into countries to which they Foreign were not native, and thriving so well in their new abode sane ea that they have gained on, and partly expelled, the indi- ale genous species. Thus, European thistles and clover have ~~ covered great part of the Pampas of Buenos Ayres, and European clover and other plants are rapidly superseding the indigenous herbage of New Zealand.! And there is evidence which, if possible, is still more direct and conclusive. No one can doubt the purpose of the webbed feet of water-fowl; yet there are geese that Upland inhabit dry places, and make no use of their webbed feet. 2°°S* Nor can any one doubt that our woodpeckers’ feet are Ground adapted to climb trees; yet there is a woodpecker inha- setae biting the Pampas of South America, where trees are unknown.? The inference is obvious, and I think certain, that the “upland geese” are a colony of geese which have abandoned an aquatic life; and that the wood- peckers of the Pampas are a colony of woodpeckers which have strayed away from their aboriginal forests, or perhaps have been expelled by the increase of some other species of animal that preyed on them. In both these cases the species have become modified in their po ae i. soap epe Tie: = Vier oo a new habitats. Another most significant fact is, that small islands at a Bats on distance from any continent usually contain no indigenous ee mammalia whatever, except bats, and many peculiar species of these are found on such islands. How have those species originated? The obvious answer is, that they are the descendants of bats that were once blown across the sea, and have become modified into new species in their new abodes. A fact which at first sight is not quite so intelligible is, Birds on that among the small number of known species of land- ihe sme birds that are unable to fly, a large proportion are found to fly. on islands. Thus, the Mauritius, before it was colonized __ by man, was inhabited by the dodo, an enormous sluggish The dodo. _ bird; and the neighbouring island of Rodriguez was inha- 1 Darwin’s Origin of Species, p. 242. 2 Ibid. p, 212. 3 Ibid. p. 469. Q 226 HABIT AND INTELLIGENCE. [cHAP. bited by a bird, less sluggish than the dodo, but also The , Unable to fly, called the solitaire: these two species are stated to be allied to the pigeons.t But the most remark- able group in the world of such birds is that which is now becoming extinct in New Zealand. These are not allied to the pigeon, but are of the same order with the ostrich. The | The extinct dinornis, the largest of all birds, was of this dinornis. : : : ; family, which is now represented by the comparatively ee small apteryx. As indicated in the name of the apteryx, apteryx. these birds not only are unable to fly, but are without — external wings. I believe the origin of these remarkable species is to be accounted for in the same way as that of the species of bats mentioned in the last paragraph. It is to be observed that the islands to which those birds belong contain no native mammalia whatever, except, I suppose, Origin of bats. The only assignable origin of those species of birds such races. j. that they are descended from flying birds, which were blown across the ocean long ages ago, and, finding the country uninhabited, became the ancestors of birds which, having plenty to eat and few enemies, in successive gene- rations grew to an enormous size, and lost the power of . flight. As the islands are without native mammalia, they could have few if any enemies on the ground; and if there were birds of prey, these could perhaps be best eluded by keeping on the ground among the dense vegetation of those islands, and not endeavouring to use the power of flight. Cuvier’s We may conclude, that although the principle of the Eee adaptation of every organic species to its mode of life, and aap the to the external conditions of its life, is generally true, and distribu. of great importance, yet it utterly fails to account for the Hee facts of the geographical and geological distribution of These facts species ; while the facts of distribution give some support — fhe theory to the theory of the origin of species by descent with eh Sine modification. Especially is this true of the facts, that fication, geology shows the same area to have been usually inha- bited during two successive geological periods by similar though different species; and that geographical zoology — 1 See “The Dodo and its Kindred,” by H. E. Strickland, F.G.S. _ xviii.) DISTRIBUTION. and botany show extensive contiguous areas to be usually inhabited by similar though different species, even when _ the climates of the two are very unlike. In the next chapter I intend to consider how far the _ facts of morphology can be referred to the principle of the _ adaptation of every part of an organism to all the rest. 227 CHAPTER XIX. MORPHOLOGY. ie the last chapter the question was asked, whether the truth that every organism has been specially adapted to the conditions of its life will suffice to account for all the facts of the distribution of the various species of organisms through the regions they inhabit ; and we have seen that although every organism is adapted to its habitat, yet this law of adaptation will not account for the facts of distribution. We have now to consider another and a parallel question. We know that the form and structure of every organ in an organism is adapted to its function: thus, for instance, every carnivorous animal has carnivorous teeth. It neces- Fune- __ sarily follows from this, that the organs are all functionally ee adapted to each other: thus, every carnivorous animal has tions. not only carnivorous teeth, but a carnivorous stomach, and such an organization of the muscles of the legs and jaws as enables it to seize, kill, and tear its prey. So that these - three—teeth, digestive organs, and muscular organs—are each adapted to the carnivorous life, and to each other. And besides these purely functional adaptations, there are Structural structural adaptations ; such as—to mention what is, per- oe haps, the most obvious instance—the adaptation of the forms of the bones to the forms and pressures of the mus- Is all mor- cles. Now, the question we have to consider is, whether Seale these laws of structural and functional adaptation will by the law account for all the facts of morphology, or whether the of adapta- ; ae eer tion? morphology of any particular species is a resultant from a a a es CHAP. XIX. ] MORPHOLOGY. ade the law of adaptation, jointly with some other law of a totally different and quite peculiar kind. The law of adaptation may be briefly stated thus :— Statement Every organ is adapted, structurally and functionally, to cbthat law. all the rest ; and the whole organism, and every organ in it, is adapted to its mode of life. This is true; every fact of organic morphology is consistent with it, subject to some small though remarkable exceptions, of which I shall have to speak further on: but it is not true, as we shall find, that every fact of morphology can be referred to it alone. Crystalline morphology and organic morphology are so Crystalline remote from each other, that no analogical reasoning from aes the one to the other can be in the slightest degree conclu- logy: sive. But such analogies may be very suggestive. We have seen that crystals of the same species are subject to great variations in form, due to chemical differences in the medium from which they have been formed; and that there is reason to believe in similar variations among the lower species of organisms, due to the soil, or medium, in which they are developed. We know that in crystals, notwithstanding the variability of form within the limits of the same species, there are definite and very peculiar formative laws, which Formative cannot possibly depend on anything like organic functions, ne because crystals have no such functions ; and it ought not zation to surprise us if there are similar formative, or morpho- eae logical, laws among organisms, which, like the formative 0! fine- laws of crystallization, cannot be referred to any relation of form or structure to function. Especially, I think, is The same this true of the lowest organisms, many of which show Pested great beauty of form, of a kind that appears to be altogether part, of due to symmetry of growth; as (to mention the best in- Hickey stance that occurs to me)! the beautiful star-like rayed 1 I quote the following instance of the same kind of beauty resulting from regularity, from the Duke of Argyll’s “ Reign of Law,” pp. 199, 200: [The Diatomacee, a group of the lowest Alge,] “have shells of pure silex, and these, each after its own kind, are all covered with the most elaborate ornament—striated, or fluted, or punctured, or dotted, in patterns which are mere patterns, but patterns of perfect, and sometimes of most complex 230 HABIT AND INTELLIGENCE. [owar. Acantho- forms of the Acanthometre, which are low animal organisins metre. not very remote from the Foraminifera. These appear to consist of nothing but structureless sarcode, with a skeleton of siliceous spicules; they are allied to the Thalassicollide, of which, perhaps, they ought to be regarded as a genus. But the Acanthometre are very much more definite in form than most of the 7halassicollide ; yet this definite- ness of form does not appear to be accompanied by any corresponding differentiation of function between different parts—or, in other words, by any physiological division of labour; and, so far as I can see, the beautiful regularity and symmetry of their radiated forms are altogether due to unknown laws of symmetry of growth, just like the equally beautiful and somewhat similar forms of the compound six-rayed, star-shaped crystals of snow. The adaptation of structure to function is the law, and indeed the definition, of organization; and the higher we ascend in the organic scale, the more definite and Correla- complete is this adaptation. But even in the very highest eee forms there are homologies and correlations which cannot not refer- be regarded as simply instances of adaptation. Before able to : ane . : adapta. © Stating any of these, it is necessary to explain with more tion. definiteness what is meant by homological relations. There are homologies between different species, and there are homologies within the limits of the same species. I shall consider first those which are within the limits of the same species. Dlasses of | Of such homologies there are four distinct cases :— logies. 1. Between different parts of the same organism ; 2. Between the same organism at different stages of its development ; 3. Between the sexes of the same species ; and 4. Between forms that mutually produce each other by metagenesis. The first three cases occur among all organisms what- beauty. ... In the same drop of moisture there may be some dozen or twenty forms, each with its own distinctive pattern, yet all as constant as they are distinctive, yet having all the same habits, and without any perceptible difference of function.” _ oa + a ont _ occurs among flowering plants, but it is common among xIx.] MORPHOLOGY. 231 ever, except those very low ones that have no distinction of parts or of sexes. The fourth occurs only among com- paratively low organisms. Among the higher organisms, the different cases are perfectly distinct ; but among vege- table and comparatively low animal organisms, they gra- duate into each other: indeed, where individuality is very indefinite, it may be said that all the four are particular cases of homology between parts of the same organism. To make this last statement intelligible, it is necessary Homo- to take an instance; and the most obvious and familiar as instance is that of a tree in flower. The tree at once bears of a tree in young buds and branches with fully developed leaves. peer These may be regarded either as different parts of the same organism, or as distinct organisms of the same species in different stages of development. The tree also, in most cases, bears flowers of both sexes ; and if the male elements are in one set of flowers and the female in another (which is a very common arrangement), the two sets of flowers may be regarded either as different organs belonging to the same organism, or as distinct organisms of the same species, but of opposite sexes. Metagenesis, in the usual ‘sense of the word, seldom Meta- genesis of & Hydrozoa. the Hydrozoa,! which, though unmistakeably animal, have a very remarkable resemblance to plants, both in the way in which single individuals or “morphological units” are united into compound ones, and in the relation of the generative organs to the whole compound organism. Among some Hydrozoa the generative organs consist of flower-like expansions, which never become detached from Hes the body of the Hydrozoon which has produced them, and | OF aad which are precisely analogous to flowers. But in other cases these flower-like expansions acquire a mouth and tentacles so as to be fitted for living alone. They then break off, swim away, and in many cases grow many times larger than the stock they have left, before they mature the gene- 1 It is, I think, to be regretted that the unmeaning word hydrozoa has been substituted for the beautiful and most appropriate word zoophytes, or animal plants. 232 Medusz, In what sense meta- genesis occurs in flowering plants. HABIT AND INTELLIGENCE. [CHAP. rative products. Such detached freely-swimming flowers of the Hydrozoa, as they may be called without metaphor, are known as Medusz, or jelly-fish:1 but it is a most important fact, to which we shall have to return again, that there are Medusze which have not this origin. It will appear at least probable, after what has been said, that the freely-swimming Medusz which are detached from some Hydrozoa are homologous with the flower-like organs that remain permanently attached to others: and what makes this probability a certainty is, that both have been ob- served in the same species. The reproductive organs in the same species have been observed in some cases to break off and swim away, while in other cases they grow, mature their products, and die like a flower, without quit- ting the parent stock. A clearer case could not be ima- gined of gradation between the production of a distinct organ, and the production of a distinct organism: for, according to any common and obvious use of words, the flower-like body is only an organ if it matures its products while in connexion with its parent organism, but is a distinct organism if it is detached; and yet the fact that the two cases occur in the same species conclusively proves that they cannot be fundamentally distinct. I have said that metagenesis does not usually occur among flowering plants. But if we are to regard, not the tree, but the product of each bud, as an individual, then metagenesis takes place in all trees whatever: the leaf-bearing branches give origin to flower-buds by non-sexual generation, and the flowers give origin by sexual generation to new leaf-bearing buds, which are developed out of their seeds. Among the homological relations within the limits of the same species, there are many facts that cannot be accounted for by the principle of the adaptation o structures and functions to each other. To use a concise technical mode of expression; there are many correlations which are not adaptations. Thus the flower- like generative organs of the Hydrozoa are essentially 1 It ought perhaps to be mentioned that Thalassicollide is not Greek for jelly-fish. xIx.] MORPHOLOGY. 23% polypites, differing from the ordinary polypites very much Homo- as flower-buds differ from leaf-buds.! (The polypite is the poe morphological unit of the Hydrozoon, as the leaf-bud of Hydro- is of the tree.) And in those cases where the generative Ce organ breaks off and swims away as a Medusa, it still continues to be essentially a polypite, though greatly modified, and as it were disguised. The relation between the leaf-buds and the flower-buds of flower- of a tree is exactly the same as this. It is now universally ™ Pants. recognised that the flower-bud does not fundamentally differ from the leaf-bud; and that the flower consists of parts which are essentially leaves, but modified to do other work than that of the leaves.?, Thus the calyx-leaves, the petals, the stamens, and the pistils, are all homologous with the leaves and with each other. This is shown by the history of their development from the bud, and is confirmed by the fact that in monstrous or abnormal flowers they all graduate the one into the other. The best instance with which I am acquainted of correlation be- tween leaf-bearing and flower-bearing axes—that is to say, between the products of leaf-buds and of flower-buds—is that presented in the umbelliferous order, which have Umbelli- somewhat feathery compound leaves, composed of small ®™® leaflets, and small flowers clustered together into those beautiful regular “umbels,” from which the order has its name. This resemblance between the manner in which the leaves and the flowers are each combined is a corre- lation which it seems quite impossible to refer to any adaptation of structure to function. The function of the leaves is to decompose carbonic acid and water, and to form organic compounds ; the function of the flowers is to mature the seed; and there is no such connexion between those two functions as to make it necessary or useful that 1 This is proved, not only by their morphology, but by the fact that the detached generative organs, or Meduse, after maturing and parting with the generative products, have been observed to root themselves and develop into common polypites. (See the Rev. Thomas Hincks on the Development of Zoophytes : Quarterly Journal of Science, July 1865, p. 416, note.) 2 This is Goethe’s well-known theory, awkwardly called the theory of the metamorphosis of plants. 234 HABIT AND INTELLIGENCE. [cmap. the leaves and flowers should be combined in the same Theircor- Way. The resemblance between the form of the com- eon pound leaf and of the umbel has, I believe, nothing to do to adapta- with their functions, and is comparable rather to those ay laws of symmetry that govern crystalline form. The resemblance, or difference, between flowers of op- posite sexes is, of course, of exactly the same kind as the resemblance or difference between flowers and leaf-bearing axes. Both the sets of flowers consist of modified leaves ; so that, however different they may be, the difference cannot be Differences fundamental. The difference between the sexes among all Se organisms that have the sexes on distinct individuals funda- = appears to be of this kind. It is often very considerable mental. : and most conspicuous, but never really fundamental. In- deed, it is a remarkable fact that the secondary sexual characters of any species are generally very variable as Secondary between individuals of the species.1 By secondary sexual Sexual ors chatacters are meant those sexual characters which do not are va- belong to the sexual organs. I believe it may be added mable. that such characters usually appear comparatively late in life. Thus, among the true insects, important as are the wings for the purposes of their life, their presence or absence is not a fundamental character, as is shown by these three criteria, that they are never developed till long after the insect has left the egg; that they are sometimes — present and sometimes absent in the same species ;? and that they are frequently a sexual character, being found on the males only. This general law, that the differences between the two sexes of the same species are not such as to affect the fundamental characters, is what might have been expected, Metamor- and it needs no special explanation. The same may be phosis, said of the general law of metamorphosis, which is, that the larva is formed on the same general type as the perfect form, but is of a lower grade of organization. The in insects, true insects are the highest of all articulated animals, and the larvee of those insects which undergo much metamor- 1 Darwin’s Origin of Species, p. 184. ? This is the case among some beetles. ‘dM Ge ae ea XIX. ] MORPHOLOGY. 235 phosis resemble articulated animals of a lower organization : caterpillars resemble centipedes, and maggots resemble worms. The Batrachians (frogs, toads, and newts), which Batra- are the lowest air-breathing vertebrates, in their larva or 2s tadpole state resemble some of the most lowly organized fishes, which are below the Batrachians in grade of organization. The crabs are the highest of the Crustacea, and their larvee are unmistakeably crustacean, but crusta- cean of a lower grade. But the law that metamorphosis is accompanied by advance in grade of organization is only a | Gms. general law, not a universal one. Among the Crustacea tacea. there are some remarkable exceptions to it; the largest of which, though not the only one, is that of the great order «4; of Cirrhipedes, or Barnacles, which are decidedly crusta- pedes. cean, but in their mature state are metamorphosed into fixed animals covered with shells, and having so great a resemblance to the Mollusca that they were classed as such until their larval forms became known. This must be regarded as a case of retrograde metamorphosis, or Retrograde metamorphosis to a lower type; for the larva, which has aed powers of sight and motion, is certainly a higher being than the mature animal, which has neither. Among the Echinoderms (star-fishes and sea-urchins) there are some Echino- : : derms. very extraordinary larval forms, which have no resem- blance whatever to the perfect form; but perhaps this exception to the usual law of metamorphosis is rather apparent than real, I shall have to speak of them in the chapter on Embryology. We have seen that there are three cases of unlikeness of form within the limits of the same species. These are— 1. Between the larval and the mature forms ; 2. Between the sexes; and 3. Between the forms that alternately produce each other by metagenesis. Under this last head, from a physiological point of view, comes, as we have seen, the relation between the leaf-bearing and the flower-bearing axes of plants. In all these three cases, as a general rule, the differences Rese™ blance ~ are not fundamental: what may be called the ground-plan betweer. 236 the forms vf the same species. We might expect this. Nipples in man. Relations of parts of the indi- vidual. Homology of hands and feet. Their tendency to vary together, HABIT AND INTELLIGENCE. [cHAP. is the same, although there may be conspicuous external unlikeness, as there is between the maggot and the fly, between the winged male and the wingless female of the glow-worm, and between a flower and a leaf-bearing axis.1 This is what might be expected. We need not seek for any special adaptation to purpose in the fundamental resemblance between forms that belong to the same spe- cies ; it would rather be necessary to seek for some special purpose, if we found a fundamental difference. There is, however, a well-known and remarkable in- stance, and there are probably many others of the kind, where the resemblance in external characters between the sexes is carried further than is in any way needed by the purposes of life. I speak of the existence of nipples, which are, in fact, rudimentary organs of lactation, in the male of the human species. These have no function, and con- sequently their existence cannot be referred to the law of the adaptation of structure to function: it must be due to a quite independent formative law.? The same is true of many of those relations of which I have now to speak, between homologous parts of the same individual, and even between parts which are not homo- logous. Thus, the hands and the feet in man are evidently homologous, not only in their relation to the rest of the skeleton, but in the number and position of the bones. The resemblance is carried much further than the law of adaptation to purpose will account for. That law will not account for the fact that there are both five fingers and five toes. If the hand needs five fingers for its uses, and no more, the foot might surely have done perfectly well with fewer than five toes. As I have mentioned in the chapter on Variation, the hands and the feet tend to vary together: thus, persons with large hands usually have 1 I say a leaf-bearing axis, not a leaf, because the flower consists of a number of modified leaves, and consequently is homologous with an axis bearing a number of leaves, not with a single leaf. 2 A perfectly parallel case is that of the Hibernia leucophearia, an insect of which the male is winged, while the female is “nearly, though not entirely apterous, the rudimentary wings being distinctly visible with a good magnifier.” (Mr. Inchbald, in the Zoologist, 1848, p. 2151.) er ae eh oe x1x.] MORPHOLOGY. bo oo x large feet, and the reverse; and they frequently present corresponding monstrosities. It seems impossible that this tendency of homologous parts to vary together can be in any way referable to the law of adaptation: I believe it has much more resemblance to the law, that the similar edges and angles of a crystal are similarly modified by the like simi- formation of secondary planes on them. In this case, as ce in that of the nipples, I do not say, nor do I believe, that there is anything inconsistent with the law of adaptation to purpose. I only say that there are relations which are not cases of that law, and are not to be explained by it. But still more remarkable are the parallelisms between Only un- parts which are not homologous. Thus, none but ungulate eee or hoofed animals have horns, though horns are not found havehorns, on all the Ungulata. It cannot be that no other animals have any need for them. Horns are weapons, to be used in fighting, and would have been in the highest degree useful to carnivorous animals, had they been endowed with them. The only reason, I think, that can be SU- Reason gested why ungulates alone should have horns, is a reason SUgs*sted. that does not in any way come under the law of adapta- tion ; it is, that they have a tendency to the production of horny matter, which appears in both the horns and the hoofs. I do not say that this conjecture is proved, or capable of proof; but it is supported by the fact that, when a tissue normally exists in one part of an organism, it is sometimes abnormally produced in another part, as for instance when the muscular walls of the heart are converted into a substance resembling bone. This, of course, occurs only in disease; but the tendency of the Ungulata to produce horny matter in their hoofs may in a somewhat similar manner determine its production in ‘their horns. But, though the law of adaptation wiil not, I believe, account for the existence of horns on Ungulates only, it does account for their growing on the head, and not on any other part; for it is only there they could be useful. But there is, again, a remarkable fact concerning the Position position of horns on the head, which cannot, I think, be ag 238 Rhino- ceros. Resem- blances HABIT AND INTELLIGENCE. [CHAP, brought under the law of adaptation. It is only Ungulates with an even number of toes on the foot—that is to say, cloven-footed animals—which have horns on each side of the head. The only Ungulate with an odd number of toes that has a horn is the rhinoceros ; and, instead of a horn on each side of the head, he has one on its middle line, on the snout. This coincidence is certainly remarkable. I do not, however, lay much stress on it, because the peculiar position of the horn of the rhinoceros may be an adapta- tion to his muscular structure, enabling him to use his weapon to the best advantage. But it is at least equally probable that his muscular structure is adapted to the position of the horn, and that the position of the horn is a case of correlation between the head and the feet, and is no more due to any adaptation of structure to function than are the formative laws of crystallization. It is very generally the case that different parts of the of different Same organism, even when not homologous, are formed in parts in the same organism. Verte- brata. Articulata. Mollusca. much the same way. In other words, both the tissues and the manner in which the tissues are combined into organs, are alike in the different parts of the same organism. Thus, bone is peculiar to the Vertebrata, and is found both in the spine and in the limbs; and the bones of both the spine and the limbs are covered by the muscles. In the Articu- lata, which in many respects form a contrast and a kind of inverted resemblance to the Vertebrata, the body is divided into segments, and the limbs also are jointed ; and in both the body and the limbs the hard parts cover the muscles, instead of being covered by them: so that the type of their construction is opposite to that of similar parts in the Vertebrata. In the Mollusca there is no skeleton at all, with the single exception of the cartilaginous skull of the cuttle-fish tribe (for the shell cannot be regarded as a skeleton); the body is not segmented, and there are no jointed limbs, only soft tentacles. Yet this is not con- nected with any inferiority of organization, for the organs of vegetative life among the Mollusca are very highly organized ; and the Cephalopoda (cuttle-fish and nautilus), which are the highest Molluscan class, stand as high in a i ll er ee xIx.] MORPHOLOGY. 239 the animal scale as any of the Invertebrata. It belongs to the same class of facts that the stem and the arms of the Crinoids. Crinoids are jointed in a nearly similar way. The conclusions arrived at in this chapter may be thus summed up :— All the facts of organic morphology are consistent with Summary the adaptation of structure to function; but there are ee many of the facts which are not cases of the law of adaptation, and cannot be referred to it: such as (to mention only a few examples) the resemblance, in a greater degree than is required by any purpose of adaptation, between the sexes of the human species, between homo- logous parts like the hands and feet, and between the leaf- bearing and the flower-bearing axes of the Umbellifere. These are what Darwin calls correlations of form. It Correla- ought not to surprise us to find this principle of correlation Pea pervading organic morphology. The wonder would be distinct. rather if we did not find laws of the kind. But what I wish to lay emphasis on is, that correlation in organic morphology is totally distinct from adaptation to function, and is much more nearly akin to the correlations of erystalline morphology. No doubt a correlation of form may serve a purpose. For instance, the correlation, amounting to identity of form, between the external organs on both sides of the body among nearly all the Vertebrata and Articulata is much more convenient than any unsymmetrical arrangement could be. But this will not apply to the correlation between the hands and the feet. The fact that the toes are the same in number as the fingers, is, as I have endeavoured to show, a case of . pure correlation, which has nothing to do with adaptation, If this appears doubtful, a proof of it, which may be almost called an experimental proof, is afforded by the fact that the hands and the feet habitually vary together, and sometimes present similar monstrosities. These facts ean, I think, be only compared to the laws of crystalline Crystalline morphology, that edges and angles which are similarly ee related to similar axes are themselves similar; and that phology, when the normal form of the crystalline species is modi- 240 ~ HABIT AND INTELLIGENCE. [cHap. xIx. fied, similar or homologous edges and angles are modified alike. This last, as has been stated in the chapter on Crystallization, is true not only of modifications that arise spontaneously in the process of crystallization, but of such modifications as are artificially produced by breaking off an edge or an angle. In this chapter we have considered only morphological relations within the limits of the species. In the next we are to extend our gaze over a much wider horizon, and to consider the morphological relations between widely separated groups. NOTE. Approach Ir has been stated (see p. 231) that metagenesis, like that of the a em Hydrozoa, is rare among flowering plants. Vallisneria spiralis, Vallisneria however, a plant which inhabits running streams, presents a case spiralis. somewhat similar to that described among the Hydrozoa. The female flowers float on the water, like those of a water-lily : the male flowers are matured below the surface, and when the pollen is ripe they become detached, and float on the surface, where they are carried by the current among the female ones. CHAPTER XX. COMPARATIVE MORPHOLOGY. Lee se ee ee . [* the last chapter we have come to a conclusion which may be thus briefly recapitulated. Although the law of the adaptation of every part of an organism to the rest, and of the whole organism to its mode of life, is true; yet there are many facts of the morphology of single species which this law will not account for, and which point rather to a principle of correlation of form, analogous to the formative law of crystallization. As yet I have spoken only of the morphology of Specific single species. It is obviously the logical order thus eeeeee to speak of the morphology of single species before the Ear eee comparative morphology of eiean species and classes ; compara- for, were there only one organic species in existence— #”* whether man or the oak-tree, or any other species that has a definite form—its morphology would be an object of science; but it would be impossible to study the com- _ parative morphology of different species and classes, without basing the study on our knowledge of the morphology of single species. Having arrived at the above-stated conclusion with respect to the principles of what may be called specific morphology, we shall find the facts of comparative mor- phology support that conclusion, and lead to other kindred results. It is necessary at the commencement to have a clear conception of the difference between analogical relations Analogy and homological relations. Two organs are analogous which hae perform the same function: for instance, the wing of the R 242 HABIT AND INTELLIGENCE. [cHar. Wing of bird and the wing of the insect are analogous, because they ee aoe are both organs of flight; but they are not in any way homologous, because they have no resemblance either in anatomical structure, or in position relatively to the other organs of the body, or in the mode of their development. This is perhaps the best example that could be mentioned of two organs which are analogous without being homologous. Conversely, it is possible for two organs to be homo- logous without being analogous: perhaps the best instance Lungsand of this is that of the lungs of the air-breathing Vertebrata, Pads which are now universally believed to be homologous with the swim-bladder of the fish. This homology is made out, partly from the fact that the lungs and the swim-bladder are similarly placed with respect to the other organs of the body; and partly from the existence of a tolerably perfect transitional series from fishes that have a swim- bladder to Batrachians (newts, frogs, &c.) that breathe by means of lungs, through the remarkable group of the Perennibranchiate Batrachians. Epes ; This instance of the homology between swim-bladder seaeatari and lungs introduces us to the fact, that the homologies of ors the respiratory organs of animals are peculiarly variable. This is not the case among the Vertebrata, except that the lungs of the air-breathing Vertebrata are not homologous ~ with the branchie of fishes. And among air-breathing tribes of animals generally, the homologies of the respi- ratory organs are tolerably constant for each tribe. But among the water-breathing Invertebrata, the branchiz are singularly inconstant, both as to their existence, and, where they exist, as to their position and homologies. I do not think this fact has received the attention it deserves. I shall have to recur to it as a most important one, and of great significance in accounting for that process of modi- fication which, as I agree with Darwin in believing, has given origin to all organic forms. Analogies Jt is to be observed that what has been said about and homo- - : ae 4 : logies homologies and analogies between similar organs in dif- aia ferent species is equally true of similar organs in the same individual. For instance, the fore-legs and hind-legs of a xx.] COMPARATIVE MORPHOLOGY. 243 quadruped are plainly not only analogous but homologous ; they are analogous as to function, being all of them organs of support and motion; and they are homologous in their relation to the spine, and in the number and relative posi- tion of their constituent bones. In man, the arms are as plainly homologous in the same sense with the legs, both in their relation to the spine, and in the number and relative position of their constituent bones; indeed, as remarked in the last chapter, the resemblance of the hands and feet is carried much further than is demanded by any mechanical necessity. But the arms and the legs are not analogous with respect to function—in commoner language, they do dif- ferent work. This, however, is not a very good example of the kind ; for every man now and then uses his arms as legs, A much better example is that of the nippers of crabs and Legs and lobsters, which, like the arms of man, are evidently homo- (73.01. logous with the legs, and yet are not capable of being used as legs. And the jaws of the Crustacea, and of all other Jaws of Articulata, are homologous with their legs. This is made A™Hculata. tolerably evident by the fact that, unlike the jaws of the Vertebrata, they open horizontally instead of vertically. Thus, organs belonging to the same individual may be homologous in position and structure without being ana- logous in function. The converse case is that of organs which are analogous in function without being homo- _logous. The best instance I can remember of this is that Fins of of the fins of fishes. The pectoral pair and the ventral #shes pair are homologous with each other, and homologous with the limbs of the higher Vertebrata. The caudal fin is, of course, not homologous with the pectoral and ventral fins ; _ but it is homologous with the tail of the higher Vertebrata. _ And the dorsal fin is not homologous either with any _ other of the fish’s fins, or with any organ whatever in _ any other vertebrate class," resembles the dorsal fin of the fish in position, is the so-called “foot” of the gasteropodous Mollusca, which in the Heteropoda is flattened into a vertical fin. This fin in the Heteropoda is ventral, not dorsal ; but the ventral side in the Mollusca and Articulata corresponds to the dorsal side in the i Eo a i 1 I believe the only organ in any other part of the animal kingdom that; : } 4 R 2 244 HABIT AND INTELLIGENCE. [cHAP. It cannot in the least surprise us that organs, or sets of organs, like the pectoral and ventral fins, the caudal fin, and the dorsal fin, which are in no degree homologous as to their relation to the rest of the body, should be adapted for the same work of swimming, and for that purpose have received the same structure; being all formed of skin, supported and extended on a framework of bony rays. This is only a case of that adaptation which is the purpose of organization, and which we expect to find everywhere. Homolo- But what really needs explanation is, that homological gical re- yesemblances are in many cases carried much further than semblances j ued is needed for any adaptation to purpose. We have seen in ur E . . . . . . ‘Shem nate: the last chapter that this is the case within the lmits of nays the same species; as, for instance, between the hands and the feet of the human species, and between the two sexes Homolo- of the same. It is equally true, that between different a species and groups there are homological resemblances blances of a degree and a kind for which no law of adaptation noe pe will account, though they are wonderfully harmonized ferences. with the law of adaptation. This is most evident among the Vertebrata, the general plan of which is much more constant than that of any of the lower groups, while it is equally susceptible of adaptive modifications for all pos- sible kinds of life. This wonderful combination and harmony between homological resemblances and adaptive differences is best seen in tracing the homologies of the skeleton. What I mean by the combination of homological resem- blances with adaptive differences may, if the expression is not of itself intelligible, be understood by comparing the hands and the feet of man, which have the strongest homological resemblance, and yet are adapted for different functions—the feet for walking, and the hands for grasping. Vertebrata, being the side where the nervous centres are. Huxley calls the side of the body which contains the nervous centres the neural side ; that which contains the circulatory centre, or heart, the hemal side. In the Vertebrata the neural side is the back, and the hemal side the belly: in the Mollusca and Articulata, their relative positions are reversed. (Huxley on the Morphology of the Cephalous Mollusca, Philosophical Transactions, 1853.) ee tee ae ae ae, Pe i Saas rr ney = xx. ] COMPARATIVE MORPHOLOGY. bo a Or In a precisely similar manner, organs in different species, which are plainly homologous as to their position with respect to the rest of the body, and the number and re- lative position of their parts, are modified to serve totally unlike purposes in life. Thus, within the boundaries of the single class of the Mammalia, the hand of the man, Hand, foot, the fore-foot of the dog, the wing of the bat, and the paddle adie all of the whale, are all homologous; and not only homo- homolo- logous in position and in general relation to the spine and ey the rest of the skeleton, but down to the minutest detail: every bone in one has its homologous or corresponding bone in the others, though they are all variously modified in shape and size, to suit the purposes of the various modes of life of the animals to which they belong. Thus, if we would know all that is to be known about the various forms of vertebrate skeleton, we must understand, not only the manner in which the skeleton of each species is modified in order to adapt it to its mode of life, but also the homological resemblances that underlie the adaptive modifications. To express the same by an example: if we would know all that is to be known about the man’s hand and the bat’s wing, we must understand the resem- blance, amounting to identity, between the number and relative position of the bones of both, as well as the struc- tural differences which adapt the hand for grasping and the wing for flight. The law of the adaptation of structure Adapta- to function will not account for this homological resem- us ve blance—not only general, but in detail—between organs Fagin that are unlike in function. logy. We see the same combination of two mutually modify- ing principles — morphological resemblance and adaptive difference—in comparing the vertebre of different animals. Among fishes and serpents these are distinct, and jointed Vertebre the one to the other, so as to be moveable. Among Mam- #P4ate 7 the lower malia and birds, some of the vertebre continue to be Vertebrata, moveable, but the form of the body, and the relation of the aa limbs to it, make it necessary for other vertebre to be ihe immoveably fastened together. Even when this is the case, however, they retain their visible distinctness as vertebree ; 246 HABIT AND INTELLIGENCE. [cHar. although the structure formed by their coalescence would be certainly as strong, and perhaps stronger, were it of a single bone, without any vestige of division into parts: at least, we know that in works of engineering it is always best, when it is practicable, to make any part where strength in a small compass is needed, out of a single piece. But even if there is no loss of strength, there cannot be any gain of it by retaining the visible distinctness of the vertebree. This fact is consequently not explicable as a case of the adaptation of structure to function; and it is a very re- markable case of homological resemblance continued along with, and notwithstanding, adaptive difference. es “3 How are we to explain these homological relations ? tobe. Shall we be satisfied with the answer, that it has pleased explained? the Creator to lay down a plan and keep to it? This is true: generally true, though not, like the law of gravita- Unity of tion, an invariable truth: but it is no explanation of the eee facts ; it is only a generalized statement of them. Perhaps nation. they are inexplicable, though I do not think so. I should not write as I do, were I not convinced that they are capable of explanation to a very great extent: but at least let us not advance that as an explanation which is no ex- planation at all, To say that the wonderful homologies of the vertebrate skeleton, to which I have little nfore than alluded, exist because the Creator has laid down a plan and adhered to it, is like saying that water rises in a pump because nature abhors a vacuum ;—perfectly true as a statement of facts, utterly unmeaning if offered as an explanation of them. Further, it is, I think, a true canon in the logic of Nolaw science, that no law can be an ultimate one which is sub- Eas ject to limitations or exceptions. By an ultimate law, I canbe = mean one which is not resolvable into any other. Gravi- ultimate. Hit . tation is such a law, and so, as I believe, are the elementary — laws of Habit. But so is not the law that nature abhors a vacuum; and when it was experimentally ascertained that nature abhorred a vacuum to no greater height than about thirty-three feet, it became evident, at least to such sound thinkers as Galileo, that the abhorrence of a vacuum pox. COMPARATIVE MORPHOLOGY. 247 could not be an ultimate law, but was one of which an explanation ought to be sought. Just so, the law of pre- Exceptions determined plan in morphology proves to be subject to ex- “° pian: ceptions. I will mention one very remarkable instance. Nearly all the Mammalia have seven vertebree in the neck, Cervical whether it is as long as in the giraffe, or as short as in the venice oe whale. The only known exceptions to this are among the various species of sloth and of manati; and these two families, it is to be observed, belong to orders that have little else in common. In the manati genus, the usual number of cervical vertebre is six: in the Brachypus genus of sloths, it varies from eight to ten: but in the Cholepus Hofmanni, a species of sloth lately described by Professor Peters, it is only six! Such an exception as this, in my opinion, goes very far to prove that the law of homology, or the law of adherence to types, is not an ulti- mate law, but one which is in its own nature capable of being explained by resolving it into simpler laws; even though the facts should prove to be too inaccessible, or too complex, to admit of its beimg so resolved by any sciénce possible to man. Such is the relation between the law of Homology and the law of Adaptation ; so wonderful is the adherence to the minutest details of a type, while at the same time it is _ modified so as to make it perfectly adapted to the most different purposes (as in the case of the man’s hand, the dog’s foot, the bat’s wing, and the whale’s paddle) ; that it seems as if an intelligent power were adapting materials Intelligent given to it by an unintelligent one. And this, I believe, is aieeae é no mere illustration, but the actual fact. The Intelligent powers. Power is that creative intelligence which, as I have already _ stated, I believe to direct the process of organic formation. The Unintelligent Power is the power of hereditary habit. I believe in the descent of all organisms from a few germs ; I am still more strongly convinced of the descent _ of all organisms of the same type from the same ancestor ; and as the vertebrate type is a very definitely marked one, IT am fully convinced of the origin of all Vertebrates from a 1 Quarterly Journal of Science, April 1865. we o> 248 HABIT AND INTELLIGENCE. [cHap. Homology single ancestor. All those homological resemblances which ere are Somceutly not due to adaptation are in my belief due to descent. community of descent. And those changes which are not pee enone due to adaptation, such as the abnormal number of cervical sponta- vertebra in the sloth and the manati, are, I believe, cases of Variation, Spontaneous variation. What is more than any other set of facts impossible to reconcile with the independent creation of every species, or indeed with any theory of the origin of species, except Rudi- that of descent with modification, is the existence of rudi- oe mentary or aborted organs. To the fact of the existence of such organs I have alluded in a former chapter, as very remarkable exceptions to the general law that all structure is adapted to function. It is impossible to assign any ee function for such parts as the rudimentary toes of some Ungulata. Ungulata (hoofed animals), which appear as if they were made for no purpose except to complete the number to five, being the usual number of toes among quadrupeds ; Leg-bones or for the rudimentary leg-bones of some serpents ; or for okie the rudimentary wing-bones of the apteryx, a bird of New Tene Zealand, which has no external wings. The discovery of apteryx. Tudimentary organs occupies a similar place in the history ; and in the philosophy of biology that the discovery of the Compa- general prevalence of fossils occupies in the history and these on philosophy of geology. The old notion that the earth was fossils. at once created as we see it would be difficult enough to reconcile with the evident marks of aqueous and igneous action during past time; but it became obviously un- tenable and absurd when it was discovered that the crust of the earth is full of the buried shells and bones of extinct races ; for it is impossible to believe that an Intel- ligent Creator would create dead shells and bones ; and, in my opinion, it is not less absurd to think that an Intel- ligent Creator would create animals with useless organs.1 1 It has been suggested that the purpose of apparently useless parts may be similar to that of excretory organs, and may consist in disposing of useless matter. This, however, is totally inconsistent with the facts of the case. Phosphate of lime, which is the mineral constituent of bone, is not a material of which growing animals are likely to have too much. (See Darwin’s Origin of Species, p. 538.) xx.] COMPARATIVE MORPHOLOGY. 249 The only real explanation ever given of their existence is, that they have been inherited from ancestors which had the same or homologous organs in a functionally active condition ; that the leg-bones of serpents, for instance, are Origin of proof of their descent from animals that had legs, and the His: By wing-bones of the apteryx are proof of its descent from a bird that had wings. The legs and wings of the ancestral forms have been lost as a result of that remarkable law in virtue of which a disused organ diminishes not only in strength but also in size. Thus, though the law of adaptation is generally true, Exceptions yet the existence of organs which are aborted, rudimen- eet tary, and useless, shows that it is subject to exceptions, and consequently is not a universal, ultimate, and all- explaining law. And though the law of homological parallelism—or, to use the common and very appropriate and of expression, the law of unity of type between organisms Oe that are externally unlike—is generally true ; as in the case already mentioned of the hand of the man, the fore-foot of the quadruped, the wing of the bat, and the paddle of the whale; yet the existence of such apparently capricious deviations from the type as that of the number of the cervical vertebre in the sloth and the manati, shows that unity of type is not an ultimate and universal law. On the view of all organisms whatever probably, and certainly all organisms between which any unity of type Unity of is discernible, being descended from the same ancestor, the Pe “of law of unity of type is fully explained. It is simply a commu- case of hereditary habit. The fact of externally unlike ay Oe organs, as alluded to in the last paragraph, being formed on what is fundamentally the same plan, is due to their being inherited from the same ancestor. But what needs further explanation, is the question how types have arisen Problem and how they have been modified ; how, for instance, and os by what transitional stages, an original vitalized but fication unorganized germ has been modified into the fish, and ORME oe the fish (which, as I shall show, is the original form of 1 Their disappearance is, however, in great part due to natural selection, of which I shall have to speak in a future chapter. 250 How do we know that rudi- mentary organs are aborted, and not nascent ? Classifica- tion, HABIT AND INTELLIGENCE. [CHAP. Vertebrates) into the reptile, the bird, and the mammal ? Of course, most of these questions cannot be answered in detail. As already remarked, most of the transitional forms have perished without leaving a ruin behind them. But the laws of Habit are perfectly well known, and we know something, though not much, of the laws of correla- tion; and with the aid of these laws it is, I believe, possible so to combine and co-ordinate the facts of em- bryology and comparative morphology, that in some cases, though not in all, we can arrive at definite, though general, conclusions as to the stages of modification through which organic forms have passed, and as to the causes and laws which have determined the modifications. If this chapter is read by any intelligent man un- acquainted with zoology, the question may probably occur to him, by what criterion are we able to assert that the leg-bones of serpents and the wing-bones of the apteryx are proofs of descent from animals that had legs and wings? Is it not as probable a conjecture that they are remnants of the races from which the animals that have legs and wings are descended? May not all birds be descended from the apteryx tribe, and all air-breathing Vertebrates from the serpents ? There are two answers to this question. The one is based on the facts of classification. Serpents are not on the line of ascent from fishes to the air-breathing Verte- brata. The transition is not through the serpents, but through the Batrachians (newts, &c.). And there is no reason for thinking that the apteryx is on or near the line of ascent from reptiles to birds. The ascent was, in all probability, through the reptilian birds, the former existence of which has been lately shown by geological evidence. But there is another answer, even more satisfactory than this, inasmuch as it is independent of comparative mor- phology, and depends only on the relation of structure to function in the species. A serpent can never have given origin to an animal with legs, or an apteryx to a bird with r = ox | COMPARATIVE MORPHOLOGY. 251 wings, because the rudimentary bones in question are Organs if useless : in other words, they are not at work, and therefore “Ss. are incapable of improving. Such organs as these, which aborted. have become rudimentary through disuse, are said to be aborted. But instances may be pointed out, of organs that are in the act of acquiring a new function; these may be called nascent organs for their new purpose. The best instance of this that I know of, is that of the lepidosiren, Nascent an animal concerning which it is a debated point whether Tes it ought to be classed as a fish or a Batrachian, and which sive. has its lungs in a state that presents an evident transition from the swim-bladder of a fish to the lungs of an air- breathing Vertebrate. In this chapter I have endeavoured to show, that homo- logical resemblances between organs which are unlike in form and function are explicable only on the supposition that they are proofs of a common descent. In the next, I intend to commence the attempt to explain, by means of the laws of habit and correlation, what the course of modification in certain cases has been. A * = re a 1) ee ee CHAPTER XXI. EMBRYOLOGY. Develop- he the chapter on Organic Development it has been stated ae that all development is from the simple to the complex. simple Every organism is developed from a simple structureless sem germ, and the germs and germinal matter of all organisms are in appearance exactly alike ; there is no test, chemical or microscopic, by which the germ, or small mass of germinal matter, that is capable of developing into the highest Vertebrate can be distinguished from that which is Species capable of developing into a worm. Consequently, if it pecene were possible to watch the development of a Vertebrate theirgerms and of a worm, or of any other widely separated species develop. of organisms, we should first see them perfectly similar, and afterwards see them becoming gradually more and more unlike, until their development was completed. Develop- It has also been stated that development essentially mentis ¢onsists in differentiation; that is to say, the process of differen- ; 2 2 : : : tiation. | development consists in the increasing unlikeness of tissues from each other, and the increasing separation of organs. When development is watched under the microscope (which can be done with many of those comparatively low animals that bave bodies composed of transparent tissues, and with the eggs of fishes and frogs), the original structureless and homogeneous germ is seen to transform itself into different organs and tissues, each occupying its own part of the body of the developing organism. From the truth that development consists in differentia- tion, it follows that the greatest differentiation is the highest development. Those organic species are the most ee Oe ee Ae CHAP. XX1.] EMBRYOLOGY. bo OU Oo highly developed in which differentiation, that is to say the distinctness of the different organs and tissues, has been carried furthest ; and those species are the lowest, or least developed, in which the original homogeneous germ has undergone the least differentiation, and remains most nearly in the original state. Consequently the undeveloped Embryos embryos of the higher forms bear some degree of resem- gitegiee blance to the mature states of the lower ones. That is to resemble say, an organism which has just begun a course of what is forms, destined to be very high development, has some resemblance to one which has completed a much lower course of de- velopment. This is a very general law, and it is, in a great variety of cases, carried out into a degree of detail that we could not have expected to find. The germs of those organisms which have any decided Develop- structure do not at once begin to develop into the structure 7475 which they are ultimately to attain. The germ at first most cases. acquires a simple cellular structure, and grows by the multiplication of its cells In this condition it resembles the mature states of the Protozoa and Protophyta (the simplest animals and plants, without any distinction of tissues except that of inside and outside of cell); and this cellular mass is afterwards transformed into the organs of the developing organism. Thus the process of develop- ment is seldom or never perfectly direct ; that is to say, the germ does not at once begin to transform itself into the organism that is to be. In some groups it is much more direct than in others; and, on the whole, the lower the Generally organization the more nearly direct is the development : Peis but this rule is subject to so many modifications and ex- direct in ceptions (due, as I believe, to causes that I shall endeavour ee to set forth at the end of this chapter), that it cannot be stated as even an average or general truth. Among the Process lower aquatic Invertebrata generally, the germinal mass tower ae lower In- early loses its cellular structure (if indeed it ever possessed Vertebrata. 1 Carpenter’s Human Physiology, p. 4, and Comparative Physiology, p. 176. All vegetable organisms, and the higher animal ones, appear to pass through this cellular stage of development, but it is certainly not proved of the lowest animals. They be- gin in the form of Protozoa. Change of plan in develop- ment. HABIT AND INTELLIGENCE. + . CHAR. it) by the fusion of the cells into a mass of homogeneous and structureless but living sarcode, which is in fact germinal matter. This sarcode mass swims about by means of cilia on its surface, and appears to perceive light and to avoid obstructions ; it almost exactly resembles one of the Protozoa, or animalcules. Within the sarcode mass the organs belonging to the organism at its next stage of development are gradually formed; and when they are complete and ready for action, the sarcode is either gradually absorbed, which is the most usual case, or else cast off in mass.?_ This mode of development is indirect ; the Echinoderms, and the marine Mollusca and lower Annulosa, begin their life in the likeness of Protozoa, before they begin to transform themselves into their mature forms.? The fact stated above, that the germs of the higher forms resemble the mature states of the lower ones, necessarily follows from the fact that the higher and the lower forms alike are differentiated out of perfectly simple germs, the higher ones being the most differentiated; and these truths hold good in direct and indirect development alike. But more than this is true in indirect ‘develop- ment. In the case of the aquatic Invertebrata just described, development begins and makes some progress on a plan of low organic type; this plan is not con- tinued but changed, and development begins anew on a higher type. Such change of plan during the course of development is very common among animals. In the cases just mentioned, the first plan of development is on the type of the Protozoa, which stands, as it were, at the base of the whole animal kingdom, and from which, on the theory of the origin of species by development, all 1 Dr. Wyville Thomson says of the embryos of the Echinoderms (star- fish, sea-urchins, &c.): “After impregnation of the egg, and complete segmentation of the yelk, the whole germ-mass is resolved into an oval ciliated animalcule, composed throughout, and consisting entirely, of homogeneous structureless sarcode.” (Natural History Review, July 1863.) I presume the “segmentation of the yelk” is an incipient formation of cells. 2 See note at end of chapter. 3 Dr. Wyville Thomson, in Natural History Review, October 1864. bo ot OU Xx1] EMBRYOLOGY. the rest have originated ; but where the higher forms of animals are developed by a similarly indirect process, their development, when the germinal mass first begins to differentiate into tissues, begins not on the plan of a Protozoon, but on that of a lower form of the same fundamental type to which the perfect form belongs. This remarkable fact of a change in the plan of develop- ment is well known in particular cases, under the name of metamorphosis ; the most familiar instance, and one of Insect the best, is that of the transformation of the worm into the ee fly, or of the caterpillar into the butterfly. But meta- morphosis is not confined to such obvious cases as these ; it is universal among the air-breathing Vertebrata: in most of which, however, the metamorphoses are finished before Metamor- the animal leaves the egg or the womb. All winged PRes's insects undergo metamorphosis, though some undergo birth. much greater metamorphoses than others. Some leave the egg in the six-legged form of a mature insect, only without wings; others in a many-legged form like centipedes: these are the caterpillars. Others, again, leave the egg in the form of worms, without any legs, in which state they are called maggots. Now, insects are the Insect highest class of the Articulata, centipedes are a lower ae class of that great group, and worms are a still lower lower : forms of class of the same than centipedes; so that the larval Articulata. forms of insects resemble the mature forms of those lower classes to which they are allied. The same is true ofSoof | the metamorphoses of the Batrachians (frogs, newts, &c.). ea These are air-breathing Vertebrates, and air-breathing organisms are, in general, higher than the water-breathing forms to which they are most nearly allied; but the larvee, or tadpoles, of the Batrachians are water-breathers, and have branchize which are altogether homologous with those of fishes. The resemblance of tadpoles to fishes, Larve however, is not to mature fishes, but to young or embryonic 7°" Pe ones ; and the same is true of insect larvee: they do not low forms. resemble mature worms or centipedes, so much as imma- ture ones.! To express this law in the most general terms: 1 Carpenter’s Comparative Physiology, p. 581. Transition between water- and air- breathing Verte- brates. HABIT AND INTELLIGENCE. [CHAP. when the original shapeless germinal mass begins to assume the form of the species, it first assumes an em- bryonic form, which is common to all organisms that are constructed on the same general plan, whether that be vertebrate, or articulate, or any other: it afterwards assumes the definite form of its species; and the high- est species, being the most differentiated, depart most widely from the common embryonic form. Thus in- sects, for instance, being higher than centipedes or worms, depart more widely than do centipedes or worms from the embryonic or larval form which is common to all. If the development theory of the origin of species is true, there ought to be a perfect series of species present- ing transitional forms between those lowest ones which depart least in the course of their individual development from the common embryonic form of the group, to those highest ones which depart from it most widely. Between worms and insects, most of the transitional forms appear to be lost; but between fishes and the highest Batrachians there is an almost unbroken series of intermediate forms. This series begins with the lepidosiren, an animal which is classed by Owen as a Batrachian, but by some naturalists as a fish. The series is continued through the Perenni- branchiate Batrachians, a remarkable transitional order which have two sets of respiratory organs: branchiz like those of the fish for breathing water, and lungs like those of the higher Vertebrata for breathing air. Next in the series come the newts, toads, and frogs, which differ from the Perennibranchiates in losing their branchie when their development is completed, so as to become air-breathers exclusively. And lastly comes the Salamandra atra, or land salamander, which, like the higher Vertebrata, passes through its metamorphoses in the egg, and leaves the egg in the air-breathing form. It is most important to observe, that there is a close parallelism between the series of specific forms in the Perennibranchiate order, and the series of forms through which each individual among the higher or exclusively XXI.] EMBRYOLOGY. 257 air-breathing Batrachians passes in the course of its ascending development.t Another instance of the higher forms of a group passing Branchie through stages of development similar to the mature AS forms of the lower species of the same group, is afforded by the branchiz of the higher Crustacea, which in the course of their development successively present the likeness of the branchize of different inferior orders of the same class.2 But probably the most instructive, if not the most obvi- ously interesting, of all cases of development, are those of the Vertebrata. I have spoken of the developmental changes of the respiratory system in the Batrachians; and there are facts in the development of the circulatory system Circula- of the air-breathing Vertebrata which, we can scarcely gon tke doubt, are profoundly connected with these. It is obvious, embryo. from the relation of respiration to the blood, that the posi- tion of the respiratory organs, and of at least a portion of the blood-vessels, must be determined the one by the other. The blood must flow where it will be aérated : consequently, the plan of so much of the circulation as ministers to respiration must of necessity be quite different in the air-breathing Vertebrata from what it is in fishes. _ But in the embryos of the air-breathing Vertebrata, the blood-vessels are at first formed on the same plan as in fishes: the blood flows towards that part of the body where the fish’s branchiz are, and flows through arteries which divide and reunite as the fish’s branchial arteries _ do; while slits are formed between the arterial branches, like those which in many fishes admit the water into the gills, although no branchie are ever formed. At a later period of development the plan of the circulation, so far as it is connected with the respiratory organs, is totally changed, and the “branchial slits” on each side of the _neck close up and disappear.? Another closely-related fact _ is, that bodies (the “ Corpora Wolffiana”) are formed in the 1 Carpenter’s Comparative Physiology, p. 706. * Milne-Edwards, quoted in Carpenter’s Comparative Physiology, p. 745. ; ® Carpenter's Human Physiology, p. 799. ) 258 HABIT AND INTELLIGENCE. [CHAP. Kidneys of embryos of mammalia and of birds, which are homo- same, Brain of human embryo. Develop- ment of spinal column. logous with the kidneys of fishes. Unlike the branchial arteries, these perform their function at first: they act as kidneys until, before birth, they are superseded by the permanent kidneys.? Tn the organs belonging to the nervous system, there is no case whatever, I believe, of any change in the plan of development: but, notwithstanding, the first embryonic brain of the highest animals is not a miniature likeness of its mature state; on the contrary, the human brain, when it can first be distinguished, is very like that of a fish; but, in conformity with the law already mentioned, it is more like that of the embryonic fish than that of the mature one. And, as the successive stages of development in the higher Batrachians correspond to the ascending series of specific forms among the lower Batrachians, and as the successive stages of development in the branchiz of the higher Crustacea correspond to the ascending series of forms among the lower Crustacea, so do the stages of the development of the human brain correspond to the brains of the ascending series of mammalian forms.?, This kind of correspondence, indeed, is a general law. One of the best instances of it is the development of the spimal column of Vertebrates, which begins in all as a mere tube of gelatinous substance, and afterwards becomes seg- mented: presenting successively, in the course of its — development among the higher Vertebrata, the likeness of different forms which are those of mature forms among the inferior orders.* Another instance may be mentioned, of what we may call physiological development, as distinguished from morpho- logical. Red blood-corpuscles are found in the Vertebrata only, and are all but universal in their mature forms :+ but 1 Carpenter’s Human Physiology, p. 809. 2 For these facts about the development of the brain, see ibid. p. 823. 3 Carpenter’s Comparative Physiology, p. 178. 4 The only known exception is the amphioxus, which is the lowest of all — fishes. It ought to be mentioned that some worms have red blood, but not red blood-corpuscles: the red colour is due to some diffused colouring — matter. xx] EMBRYOLOGY. 259 besides the red corpuscles there are white ones, which are Blood- similar to those found in Invertebrates. In the develop- Cnuscles ment of the vertebrate embryo the white corpuscles appear red : earlier than the red; and it is now believed that through- out life the red corpuscles continue to be formed by the metamorphosis of the white. So that we have this re- markable threefold relation between the two kinds: that their the white are found in lower organic forms than the red; ee the white appear in the embryo before the red; and the white are transformed into the red throughout life.! What inference are we to draw from these facts of in- direct development? and especially from the resemblance which we find to be so general, in various ways, through- out the animal kingdom, between the embryo or larva of the higher form, and the mature state of the kindred lower form? How are we to interpret the facts, that the lower aquatic Invertebrata usually begin their life in the likeness of Protozoa ; that the branchiz of the higher Crustacea are at first similar to those of the lower Crustacea; that many insects leave the egg as worms; that the frog and newt have the respiratory system of fishes, and consequently their circulatory system also, before they acquire those of air-breathers ; and, most surprising of all, that the embryos of the higher air-breathing Vertebrata at first develop “branchial slits ” and a branchial circulation like those of fishes, with the arteries dividing as the arteries of fishes divide ? It is a good rule, in all questions of this kind, to begin Can these __ by trying whether the facts are capable of being all referred atte fe _ to Cuvier’s principle of the adaptation of every organ and ca every function in an organism to the rest, and of all to its mate mode of life. But if it is difficult to apply this principle #°"? with any precision to the morphology of organisms, it is still more difficult to apply it to their development ; if it _is difficult to be sure that we thoroughly understand the purpose of any peculiarity of form or structure in the organism considered simply as a living mechanism, it is quite impossible to be sure that we understand all the 1 Carpenter’s Human Physiology, p. 163. 8 2 260 HABIT AND INTELLIGENCE. [cHar. purposes for which one mode of development is better than We do not another. We can see no purpose in indirect development ; Know the for anything the wisest of us can see, the original struc- answer. tureless germ of every organic species might as well have developed into its mature form by the most direct process ; but we are so utterly ignorant of the conditions of the problem, and any experimental test is so completely out of the question, that it would be unwarrantable pre- sumption in us to deny that there may possibly be as real a purpose for the process of indirect development, as there is for the process of circulation or of respiration. This, I say, may be true, but all that can be said in its favour is, that there is no evidence to disprove it: and it does not appear probable. It appears very improbable, that a change of plan while development is going on should be a necessary law of development ; it appears very much more They are, likely, that the first transitory stage of an animal’s develop- arobably, ment is a record, or, as it-were, a picture, of what the mature records of form of its remote ancestor was. I believe that the like- edge ness of the earliest forms of the lower aquatic Invertebrata to Protozoa is a record of their descent from Protozoa ; that the resemblance of the larve of many insects to worms is a record of the descent of insects from worms ; that the resemblance of the branchiz in the larval state of the higher Crustacea to the branchia of the lower Crustacea in their mature state is a record of the descent of the higher Crustacea from the lower; and that the water- breathing branchize of the tadpole are a record of the descent of the frog from water-breathers. As I have just stated it, I do not say that the argument is conclusive. I think it is comparable to the argument for the descent of all the Vertebrata from a common ancestor, grounded on the homolegical correspondences between their skeletons being so much closer than any community of function requires. But the argument from comparative morphology, which would have been otherwise only a strong, though unverified, presumption, is raised, as Be I think, into a certainty by the discovery of rudimentary and and useless members; which we cannot believe to have Ce XT. | EMBRYOLOGY. 261 been independently created, and which are explicable only as records of the descent of the organisms that possess them from ancestors to which they were of use. And here, in embryology, exactly the same confirmation comes in. There are not only useless organs, but useless modes of useless development, such as the formation in the embryos of air- feet breathing Vertebrates, of “branchial slits” and branchial ment. circulation, which afterwards disappear and never perform any function. This is an exactly parallel case to that of rudimentary organs; and some of what are strictly rudi- mentary organs are found in embryos, and disappear before Rudi- the organism comes to its mature state. Thus the embryo eae of the Greenland whale has teeth, which disappear by the the em- time it is fully grown; the calf has certain teeth before pry auly: birth which are afterwards absorbed ;! and the young of some of the naked “nudibranchiate” Mollusca have dis- cernible shells.? These, like the wing-bones of the wing- less apteryx, are only intelligible as records of organs which were developed in the ancestors of those species, and were of use to them. It is also a significant fact, that Rudi- rudimentary organs, even when they remain through life, Annee are relatively smaller in the mature form than in the largest embryo ;* this is no doubt a case of the law that unused anes organs diminish in size. As I have stated at the beginning of this chapter, all Differen- organisms are developed from germs which are perfectly Bee alike; and they grow more and more unlike each other as their development proceeds. This may be called the differentiation of embryos one from the other. It is now time to state the law according to which that differentiation takes place. It is a familiar truth, that organic species are naturally Groups of arranged in groups, and these groups, again, in wider 8°"?* groups. Thus, for instance, the domestic fowl is one of _ the group of birds, and birds are a part of the wider group of Vertebrates. The law in question is, that as the deve- 1 Darwin’s Origin of Species, p. 534. 2 Carpenter’s Comparative Physiology, p. 318. 3 Darwin’s Origin of Species, p. 537. 262 HABIT AND INTELLIGENCE. (CHAP. Characters lopment of any organism goes on, it acquires first the of the widest group appear first. Von Biir’s law. characters belonging to the widest group in which it can be classed, and afterwards acquires the characters of suc- cessively narrower and narrower groups. This is known as Von Biir’s law. Stated in these general terms, it will perhaps be scarcely intelligible, but an example will make it easily understood. Were it possible to watch the de- velopment of a chicken as it actually goes on in the egg, we should see it first acquire those characters which it has in common with all other Vertebrata; and it would be capable of being identified as a vertebrate embryo, before it could be known to which class of oviparous Verte- brata it belonged. It would next be seen to acquire the characters of a bird, without anything to enable us to say what kind of bird; afterwards it would acquire the characters of the gallinaceous family of birds, and after that, those of the domestic fowl; iast of all, the characters that distinguish the variety or breed. And this differen- tiating process does not stop at birth, for young animals belonging to different breeds of the same species, or to different species of the same genus, are generally—indeed almost invariably—more nearly alike than are the mature animals. The characters of the widest group appear first—or, what means the same, the characters that appear first are those which the species has in common with the greatest number of others; and those which appear last are the peculiar characters of the species—or, in variable species, the peculiar characters of the variety or of the individual. This is the statement of Von Biir’s great and simple law. 1 “Tn my possession are two little embryos in spirit, whose names I have omitted to attach, and at present I am quite unable to say to what class they belong. They may be lizards, or small birds, or very young mammalia, so complete is the similarity in the mode of formation of the head and trunk in these animals. The extremities, however, are still absent in these embryos. But even if they had existed in the earliest stage of their development, we should learn nothing; for the feet of lizards and mammals, the wings and feet of birds, no less than the hands and feet of man, all arise from the same fundamental form.” (Von Bar, quoted in Darwin’s Origin of Species, p. 519.) —- a ae xx1.] EMBRYOLOGY. 263 This law stands in the closest connexion with that which we have seen to be a general law of comparative morphology: I mean that those characters which distin- Characters guish the widest groups are also the least liable to variation ae as between orders, genera, species, or individuals within the Teast group. The characters which appear first in the embryo anaes are those of the widest group, and the characters of the of this widest group are the least subject to variation. From ane aes these two laws it follows, by syllogistic inference, that the characters which appear the earliest during development are the least subject to variation. The reason of this is tolerably evident. “If certain organs are formed early, Reason. those which come later must obviously accommodate them- selves to their predecessors; and any variations which have taken place in the latter will perturb the normal disposition of the former.”! Thus, the first-formed parts will vary only from such causes of variation as may arise in themselves; but the later-formed ones will vary, not ‘only from such causes as may arise within themselves, but from any cause that may produce variation in their pre- decessors: so that the later any part is formed, the more chances it will have of varying. Whether this explanation is satisfactory or not, a good instance of the fact is pre- sented by the contrast between the development of the Mollusca on the one hand, and that of the Articulata and __-Vertebrata on the other. In the Articulata and Vertebrata, earn the neural side of the body, or that containing the nervous and Ver- ‘ centres, is developed before the hemal side, or that con- ee Deaies taining the circulatory centre; and throughout those two great groups the general form of the nervous system is remarkably constant. In the Mollusca, on the contrary, the heemal side is developed first, and the neural side after it; and among them the plan of the nervous system is very much less constant.? The Mollusca present another remarkable instance of the same law. One of the most conspicuous and best-known molluscan characters is the very unsymmetrical form of their digestive organs, the 1 Huxley on the Morphology of the Cephalous Mollusca, Philosophical Transactions, 1853. 2 Thid. 264 Characters not em- bryonic HABIT AND INTELLIGENCE, [cHAP, alimentary canal being doubled back so as to bring its two extremities near each other. But this character is not are subject constant among the Mollusca: it is not found, for instance, to ex- ception. Unsymn- metrical mollusean develop- ment. Import- ance of embryonic characters in classi- fication. Cirrhi- pedes : their crustacean larve, Dorsibran- chiata, and twbi- cole. among the chitons; and it is also not an embryonic cha- racter—on the contrary, the molluscan embryo is sym- metrical, and the unsymmetrical form is subsequently produced by the right or left side, according to the species, growing more than the other.! The parts that appear the earliest are the most constant throughout wide groups. It is a result of this fact that the most important characters for classification are frequently those of the embryo or of the larva. It is usually said, de- velopment is the criterion of morphology ; but it would, I think, be more intelligible to say that development is the criterion of classification : in other words, the history of the development of any organism is the test of its true affinities. The earliest developed characters are the fundamental ones, and true classification is classification by funda- mental characters. The most remarkable instance of this is that of the Cirrhipedes, or barnacles; the position of which among animals was totally misunderstood so long as they were known in the mature state only. They were classed by Cuvier as Mollusca, which they resemble only in the possession of shells, and in other superficial characters ; but now that their larval forms are found to be unmistake- ably crustacean, they are classed among, or near, the Crus- tacea. Thus, of two crustaceans, which are at first almost alike,” one may end its life as a crustacean, while the other undergoes metamorphosis into the very different form of a cirrhipede. But this, though I believe it is by far the most remarkable instance of the kind, is not by any means the only one. A similar case exists among the marine worms. The dorsibranchiata and the tubicole are very unlike in external appearance: the former are free animals, with branchiz in rows along the back; the latter are fixed animals, inhabiting tubular shells, with branchie round 1 Huxley on the Morphology of the Cephalous Mollusca, Philosophical Transactions, 1853. 2 Dr. Knox’s translation of Milne-Edwards's Manual of Zoology, p. 448. “rs ee ee bo for] or xxi. | EMBRYOLOGY. the head. In some species these are magnificently deve- loped, and, being filled with the creature’s red blood, are described as presenting the appearance of a carnation flower. Yet the young of the tubicole are almost exactly like those of the dorsibranchiata: their subsequent unlike- ness is due to their afterwards fixing themselves, forming a shell, and developing branchiz on the head.! In a former chapter I have stated the fact of reversion, Reversion that individuals are sometimes found which present what ag appears to be the character of the ancestral form from which all the species of a genus, or, it may be, all the genera of a class, are descended. In this chapter we have seen reasons for believing that the image of that ancestral form is in some degree preserved in the embryo or larva. Consequently, if development is arrested, if the embryonic the reten- characters are in some degree retained, while the organism ears in other respects becomes fit for mature life, there will be characters. a reversion to ancestral characters. In fewer words, re- tention of embryonic characters is reversion to ancestral characters. This is beyond doubt the explanation of some eases of reversion, though I do not say that it will apply to all. The best instance I know of is that of the flat- fish or flounders, which differ from nearly all other verte- Flounders. brates in their unsymmetrical form. They habitually swim with one side uppermost, and both their eyes are on that side. But this is true of their mature forms only. Like the Mollusca, they are symmetrically formed when in their earliest state, and assume an unsymmetrical form in the course of their subsequent development. Young flounders swim vertically, and have both sides alike, and one eye on each side of the head, like fishes of the usual type. And fully-grown individuals are sometimes found among the various species of the tribe which continue to swim ver- tically, and have their two sides less unequally developed than is usual among flounders.” Considered in reference to 1 Carpenter's Comparative Physiology, p. 369. 2 See Dr. Wyville Thomson on the Obliquity of Flounders, Annals of Natural History, May 1865. He says of the wonderful change from the young form, in which one eye is on each side, to the mature form, in which 266 HABIT AND INTELLIGENCE. [cHAP. the species only, these are monstrosities ; but considered with reference to the type of the class, I have no doubt they are cases of reversion to it. Funda- The case of the flounders is a good instance of the ae difference between fundamental and adaptive characters. characters. Embryonic characters, as we have seen, are fundamental. On the development theory of the origin of species, they are inherited from a very remote ancestry; and to the ‘class of fundamental characters belong those homological resemblances, mentioned in the last chapter, between organs that are unlike in external form and in function, but present a minute resemblance in structure which cannot be accounted for by the law of adaptation of structure to function : such as the parallelism of formation between the hand of the man, the fore-foot of the quad- ruped, the wing of the bat, and the paddle of the whale. Adaptive characters, on the contrary, are acquired later in the course of development, and consist in modifications of the fundamental ones, so as to adapt them to the peculiar mode of life of the organism: such as the peculiar modifications of the common type, which fit the hand, the foot, the wing, and the paddle, each to its Homology special function. Homologous parts are those which are es developed in the same way, and their resemblance is consequently fundamental. When organs are analogous without being homologous, on the contrary, such as the wing of the bat and the wing of the insect, they are developed in different ways, and have no resem- blance in their embryonic states. Their resemblance is_ acquired later, and is not fundamental, but only adaptive. Homo- To put the contrast in its sharpest form—when organs logical are homologous without being analogous, the resemblance a ean is fundamental, and the difference is adaptive; and the mental, resemblance is greatest at first. Such are the man’s hand and the bat’s wing. When organs, on the contrary, are analogous without being homologous, the resemblance is both eyes are on the same side: “ The eye changes little in actual position : with the growth of the fish the associated parts are, as it were, developed past it, producing this singular obliquity.” we ~~ 2 el) le eee 22ers xx1.] EMBRYOLOGY. 267 adaptive and the difference fundamental ; and there is no Analo- resemblance at first. Such are the bat’s wing and the Si¢*! ms insect’s wing. And similarly, though the wing of the bat adaptive. is precisely analogous with that of the bird, its homologies are much nearer to those of the hand of man. To return to the subject of the flounders. We have Flounders. seen that their peculiar unsymmetrical form is not an embryonic character. But it is plainly an adaptive one; it is to be referred to their peculiar mode of life, requiring them to be able to swim as close to the bottom as possible ; for which purpose they have acquired the habit of swim- ming on one side instead of vertically, and have got their forms modified to suit that habit. Of course, the distinction between fundamental and adaptive characters is not an absolute distinction, but admits of gradations. It is, however, generally true that the fundamental characters are the embryonic ones; and that those characters which appear first in the course of development are the least variable as between individuals, species, genera, and classes. And if it is true, as is implied in the development theory, that the most fundamental characters are those which have been inherited from the Constancy remotest ancestry, and through the greatest number of oh generations, it follows that the comparative invariability characters, of fundamental characters is a case of that law of habit, {4° in virtue of which the habits of the longest standing are habit. the most tenacious and the least variable. We have seen that nearly allied organisms undergo a similar development; and that the characteristics of the genus appear before those of the species, and the charac- teristics of the wider group, generally, before those of the narrower group. These laws are subject to exceptions, Excep- some of which are very difficult to explain; as when, in ons some cases, characters that belong to the species appear earlier than others that belong to the genus." But there is a large class of exceptions of a different kind, which, 1 Stated on the authority of Agassiz, in Spencer’s Principles of Biology, vol. ii, p. 378. 268 Likeness of larval form proves affinity : but not the converse. Insects. Beetles. Land sala- mander. Crustacea, HABIT AND INTELLIGENCE. [CHAP. when properly understood, will, I believe, be found not to disprove but to confirm the principles by which I have endeavoured to explain the laws; and at the same time to throw an important light on the origin of species. When two organisms are developed from similar em- bryonic or larval forms, we infer that they are fundamentally alike ; as in the case of the-Cirrhipedes, which, as mentioned above, are now regarded as closely allied to the Crustacea, not from any likeness in their mature forms, but because they are developed from larve which are crustacean. It is indeed an axiom, that forms which are alike in their earliest stage of development are fundamentally alike, and are to be classed together. But the converse does not hold: organisms may be really allied, and yet may be developed out of very different larval forms. No one doubts, for instance, that the true or hexapod insects constitute a perfectly natural class; that is to say, a class whereof all the members have real and decided affinities to each other. Most of them leave the egg in a worm-like form; but “in some few cases, as that of Aphis, if we look to the ad- mirable drawings by Professor Huxley of the development of this insect, we see hardly any trace of the vermiform stage.”? And beetles constitute a perfectly natural order, yet some beetles are much more directly developed, and undergo much less metamorphosis, than others. Quite as remarkable is the case of the land salamander, which, unlike most of the higher Batrachians, does not pass through the tadpole stage, but leaves the egg as an air-breathing animal; yet we cannot doubt its affinity with the other salamanders, which begin their life as water-breathing tadpoles. Similar facts are observed among the Crustacea. “Fritz Miiller has lately made the remarkable discovery that certain shrimp-like Crus- taceans, allied to Penceus, first appear under the simple nauplius form, and passing through two or more zoea stages, and through the mysis stage, finally acquire their mature structure ; now in the whole enormous malacostra- can class, to which these crustaceans belong, no other 1 Darwin’s Origin of Species, p. 523. SOR ARETE ee eee XX1. | EMBRYOLOGY. 269 member is yet known to be developed under the nauplius form, though very many appear as zoeas.”! The Nauplius resembles the larva of the Cirrhipedes ; and Darwin agrees with Fritz Miller that a similar form was the ancestor of the whole crustacean class.” It is another most remarkable Fresh- fact, that the fresh-water Crustacea pass through no meta- (27 Crustacea morphosis at all; their mature forms are developed directly undergo from the egg.? I shall have to speak of the probable Se aetna significance of this fact further on. Supposing it to be true that the larva or embryo is a picture of what the ancestral form of the species was, these facts must be accounted for by supposing that there has been a substitution of direct development for indirect ; Direct de- or, in other words, that one or more stages of the process ee of development have been left out. I mean that, when a tuted for : : indirect. particular course of metamorphosis is characteristic of a class, but is not found in all the members of the class, it is most probable that those species which are developed directly without metamorphosis are descended from others which passed through the metamorphosis.- Such a change Laws of is consistent with what we know of the laws of Habit. b> In the chapter on that subject we have seen that it isa common case for an inherited variation to appear in the offspring, not at birth, but at the same age at which it first appeared in the parent; but that it sometimes appears earlier in the offspring. Now, if it is true that the embryo is a picture of the ancestral form, and that the mature form is descended from an ancestor resembling the em- bryonic or larval one, it follows, to use Darwin’s words, explaining that “the adult differs from the embryo [or larva], owing ve ae to variations supervening at a not early age, and being inherited at a corresponding age.”* But if, from some spontaneous variation, the offspring inherits and manifests and the the variation in question, not at the corresponding age ss of f : : : metamors but at birth, this will amount to leaving out the first phosis. 1 Darwin’s Origin of Species, p. 523. 2 Ibid. p. 531. 3 Ibid. p. 522. 4 Tbid. p. 406. I make this quotation, not as a testimony to a fact, for this can be only matter of inference, but as showing that I agree with Darwin’s opinion on this part of the question. 270 Batra- chians. Land sala- mander. Young pigeons of various breeds. Series. HABIT AND INTELLIGENCE. [CHAP, larval stage of development. For instance, the Batra- chians are, I believe, descended from an ancestor resem- bling a tadpole, which, from causes that I intend to speak of in a future chapter, was transformed into an air-breathing animal;' and in every successive generation the same change takes place, and at about the same age at which it took place in the original ancestor. But in the land salamander the change into an air-breather is in- herited, not at the corresponding age, but at birth; and thus it undergoes no metamorphosis. This account of the matter is, I think, in accordance with what we know of the laws of habit and variation, and of the facts of embryology. It is desirable, whenever it is possible, to adopt Darwin’s plan of reasoning from relations between varieties, the com- mon origin of which is known, to relations between species, the common origin of which can only be inferred ; and Darwin has stated an instance in which the variations that mark the several varieties of one species mostly appear some time after birth, but in one variety are very conspicuous at birth. “The young of the short-faced tumbler differs from the young of the wild rock-pigeon and of the other breeds in all its proportions, almost exactly as much as in the adult state ;” while the young of the other breeds of the pigeon, even of those which are most unlike in the adult state, are very nearly alike”? The short-faced tumbler, in presenting from the first the peculiar cha- racters in which it differs from the original race of pigeons, may be compared to the land salamander, which breathes air from the first; while the other varieties of pigeon, in presenting at first the character of the original race, resem- ble those Batrachians which commence their lives breathing water, like the fishes from which they are descended. To return to the subject of metamorphoses. We find in nature the following series :— 1 T do not mean that this transformation took place in one generation. On the contrary, I believe it must have occupied countless generations, and must have passed through many specific forms, like those preserved among the Perennibranchiates. 2 Darwin’s Origin of Species, p. 526. et mets i ax, | EMBRYOLOGY. 271 1. Water-breathing Vertebrates (fishes) producing their Fishes. like. 2. Water-breathing Vertebrates (tadpoles) developing Batra- into air-breathers, which again produce water-breathing “"™* tadpoles from their eggs. 3. Air-breathing Vertebrates (the land salamander Air- | among Batrachians and all the higher vertebrate classes) pee producing their like. brates. I believe these three form a succession by descent in the order enumerated; and I believe all the classes of air-breathing Vertebrates have been in this way descended Descent of from fishes, of which descent their embryos still bear the Ra marks in the “branchial slits” of their arteries. fishes. The existence of similar series in other parts of the Series. animal kingdom is to be explained in the same way. Thus we find this series :— 1. Worms producing worms. Worms. 2. Worm-like larve developing into hexapod insects, Insects which again produce worm-like larvee from their eggs. dove 3. Hexapod insects (as Aphis) producing hexapod insects, Vea which after they leave the egg undergo little or no meta- haa: morphosis except the acquisition of wings. No insect, ee however, acquires wings until some time after it leaves developed the egg. Among the higher, or malacostracan Crustacea, we find Crustacean quite as remarkable a series, as follows :— aan 1. Nauplius producing its like. This, I believe, is Nauplius. not known by direct evidence to exist or to have ever existed ; but we may infer its former existence from the fact that “forms wonderfully distinct from each other, as the suctorial parasites, Cirrhipedes, Entomostraca, and even [a few of] the Malacostraca, appear in their first larval state under a similar nauplius form.”! From sucha Nauplius all those forms have probably been descended. 2. Nauplius developing into zoea, thence into mysis, and thence into the mature malacostracan form of Penceus, Peneus. or a genus allied thereto, which again produces Naupli 1 Darwin’s Origin of Species, p. 531. Darwin agrees with Fritz Miller in drawing these inferences. 272 Other Malaco- straca. Fresh- water Malaco- straca have lost their meta- morphoses by varia- tion. Series in Hydrozoa. Hydra. Hydrozoa with flower-like generative organs. Generative organs be- coming detached as Meduse. HABIT AND INTELLIGENCE. [CHAP. from the egg. Of course it is not probable that the de- velopment of the Nauplius into any malacostracan form was effected in less than thousands, perhaps millions, of generations. 3. Malacostracans pursuing the same course of deve- lopment as the Penceus, except that they leave out the nauplius stage and begin as zoeas. 4. Fresh-water malacostracans developing by a direct process from the egg into the mature form. The fact of the development of the fresh- water species being direct, is very significant. The fresh-waters have, it is tolerably certain, been colonized from the sea, and not the reverse: the change from salt to fresh water, like any other change, must have acted as a stimulus to varia- tion ; and variation, among other modes of action, must have, in some cases at least, the effect of causing the young to acquire the characters of the parent at birth instead of by a subsequent metamorphosis. I have kept the case of the Hydrozoa for the last. In it we have to speak, not of metamorphosis, but of meta- genesis. In that class we find the following series :— 1. In the common fresh-water Hydra, the generative products are matured in organs which are mere swellings on the surface of the body. — 2. In other members of the class the generative organs are distinct flower-like expansions. From the mere swellings of the Hydra to the flower-like organs of the campanularian and sertularian Hydrozoa, there is a regular gradation ; and the same anatomical elements are discernible through all the various forms of that gradation, much in the same way that the same anatomical elements are traceable through the various forms of the vertebrate skeleton. 3. In the forms just mentioned, the flower-like genera- tive organs mature their products while in connexion with their parent stem, as do the flowers of plants. But in other nearly allied forms, the flower-like organs are de- tached before they arrive at maturity, and swim away. They grow to an enormous size in comparison with the ae) a EMBRYOLOGY. yay stock from which they have been detached, and are known as Medusz, or jelly-fish. From the ova which they pro- ‘duce, animals are developed, like the parent stem, but unlike the Medusa: and these again produce Meduse. So that we have this metagenesis, which is perhaps the Meta- most beautiful instance of the kind in the whole animal 8°! kingdom :— A. Parent stem, comparable to the leaf-bearing trunk of a plant, producing Medusz by a non-sexual process similar to the formation of buds by a plant. B. Medusa, comparable to the flower of a plant, except that it is detached; producing, by a sexual process, ova that develop into the likeness of the parent stem. There is no fundamental distinction between the species in which the flower-like organs mature their products while still in connexion with the parent stem, and those in which they become detached in the form of Meduse. The two ways are observed in allied species ; indeed they sometimes occur in the same species. 4, An instance has been observed in the genus Lizzia, Medusa and in all probability many more such instances yet ppeduewe remain to be discovered, of Medusze being directly pro- directly. duced by a Medusa, without any metagenesis or meta- morphosis.! This series is evidently a similar one to those which I Parallel have traced through the Batrachians, the Insects, and the °° Crustaceans ; and it is, to my mind at least, impossible to doubt that the members of this series, as of the others, have been descended each from the member enumerated before it.? The conclusions arrived at in this chapter may be thus Summary. summed up :— The embryonic or larval form of a species is most pro- _ bably a record of what the ancestral form was from which _ the species has been derived. | 1 See Dr. Allman’s Report on the Reproductive System in the Hydroida _ (Hydrozoa), British Association Reports, Newcastle, 1863. 2 Itis perhaps unlikely that the exact species from which any decidedly unlike species is descended can be still in existence; but very similar ones may be. . T 274 Nature of metamor- phosis. How lost. HABIT AND INTELLIGENCE. [ CHAP. Metamorphosis is due to variations taking place not at an early age, and being inherited at a corresponding age in the offspring.’ Metamorphosis, or metagenesis, may be lost by the cha- racters of the mature form appearing at birth instead of some time after it. This is a very probable result of spontaneous variation ; but the opposite change appears impossible. That is to say, it is a very probable variation for a species that has habitually undergone a metamor- phosis, to acquire its mature form by direct development, and so to lose its metamorphosis ; but it appears impossible that any species which has habitually acquired its mature form by direct development, should, as a result of any variation, begin to appear first in a larval form. I deduce this from the law of habit, that an inherited character sometimes appears in the offspring at the same age as that at which it appeared in the parent; sometimes earlier, but seldom or never later. If any one, previously unacquainted with the subject, considers and compares the facts that have been brought together in this chapter, he may very probably make some comment like this: “A plausible explanation is here 1 This does not apply to metagenesis. But metagenesis, though a less familiar fact than metamorphosis, is not nearly so difficult to account for. Such metagenesis as that of Aphis or of Cecidomyia (a dipterons insect) con- sists in the generation of larve by the larya, and needs no special explana- tion: what does need explanation is the metamorphosis of the larva into the perfect form, which, in Aphis at least, always occurs at intervals of a few generations. Such metagenesis as that of the Hydrozoa consists in the reproductive organs becoming detached, which may take place as the result of a very slight variation. In metamorphosis, form A is transformed into form B (as the caterpillar into the butterfly), and form B produces form A again. In metagenesis, form A produces form B, and form B produces form A again, as in the case of the hydra-like Hydrozoon and its Medusa. In Aphis, the two pro- cesses are complicated together. Larv produce larve for several genera- tions, and perish without undergoing metamorphosis; but at intervals they are metamorphosed into the winged state, and a new generation of larve is produced from their eggs. This is classed as a case of metagenesis, because there is one form in the cycle that remains permanently unlike the other: the larva that dies without undergoing metamorphosis never becomes like the winged form. — eS ney ee oF bo ~I On XXxI. | EMBRYOLOGY. offered of the manner in which stages of development have been dropped out, and the process of development thereby shortened. But this is no explanation of the Objection. Origin of Species. The difficulty is not to know how metamorphosis and metagenesis have been suppressed, but how they began. You tell us how the land salamander! has ceased to begin life as a tadpole: but how did the first salamander make the transformation from a water- breathing tadpole into an air-breather? You tell us how the Aphis has ceased to appear as a worm; but how was the first worm transformed into a hexapod insect? You tell us how the fresh-water Crustacea have ceased to leave the egg as zoeas ; but what determined the first transfor- mation of a zoea into a crayfish? You tell us how the Lizzia form of Medusa has come to produce its like with- out alternating with a hydra-like form; but how did the first hydra-like form begin to throw off Meduse? Until these questions, and such as these, can be answered, the mystery is unsolved.” T admit that what I have said leaves altogether unsolved Reply. the mystery of the origin of organic forms. I intend, in a future chapter, to show how far I think that mystery is capable of solution. But I cannot admit that we know nothing in any case where we are unable to ascend to a knowledge of causes. A true and correctly generalized view of facts not only is valuable in itself, but is often the means by which it becomes possible to ascend to the knowledge of causes. The discovery of Kepler’s laws is justly regarded as one of the most important contributions ever made to science, though Kepler died in ignorance of the reasons of those laws; but Kepler’s discoveries pre- pared the way for Newton’s discovery of the law of gravi- tation, which has explained the reason, not only of Kepler’s laws, but of the apparent exceptions to them. But before going on to the subject of the causes of these transformations, I shall endeavour to show how the deve- lopment theory of the origin of species agrees with the facts of classification. 1 Salamandra atra, or land-newt. 2 276 Metamor- phosis of Sitaris. Develop- ment of Echino- derms. Pseud- embryo. HABIT AND INTELLIGENCE. [CHAP. NOTE. ANOMALIES OF DEVELOPMENT. AurHoucH I am convinced of the truth of the principles set forth in the foregoing chapter, I admit that there are facts of development and metamorphosis for which I am utterly unable to suggest any way of accounting. One of these is that of the Sitaris beetle, which, when hatched, has six legs like a mature insect: these afterwards become rudimentary, and it assumes the ordinary form of a worm-like larva, and is transformed into a beetle in the usual way.? The development of some of the Echinodermata cannot be called an exception to the ordinary laws of metamorphosis only because it lies altogether outside of them.? As already stated, the aquatic Invertebrata are usually developed out of a germ which is nothing but a minute mass of structureless sarcode, bearing cilia on its surface, by means-of which it swims about. Dr. Thomson calls this the pseudembryo: perhaps pre-embryo would be a better word. Within it the true embryo is afterwards developed ; and in most cases the external ciliated “ sarecode layer” is absorbed by the developing embryo, and disappears. The peculiarity of some Echinoderms is, that the pseudembryo undergoes a development of its own, which has no morphological relation to that of the future animal. Its form varies greatly in different species. In some it is described as “ vermiform ;” in the sea-urchin and star-fish it appears more like the Ciliograda than any other form.* The difference between a larva and a “1 Stated on the authority of M. Fabre, in Darwin’s Origin of Species, p. 530. , 2 The facts in this paragraph are taken from Dr. Wyville Thomson’s papers in the Natural History Review for July 1863 and October 1864. 3 Professor Sars, who discovered the Bipinnaria, or pseudembryo of the star-fish, before he knew what it was, thought it might be allied to the Ciliograda. The resemblance in external form of Dr. Thomson’s figures of the various Echinus or Sea-urchin pseudembryos to the Ciliograda is, 1 think, obvious enough. — eo xx1.] ANOMALIES OF DEVELOPMENT. 277 pseudembryo is, that the larva is transformed into the mature form, but the pseudembryo is not so transformed ; its substance is in most cases absorbed by the growing embryo: there is thus a transformation of the substance, but this is not as the sub- stance of a larva is transformed into that of the perfect form ; it is rather as food is transformed into the substance of the organism. But the pseudembryo of the star-fish, instead of Pecu- being absorbed, is cast off, and continues to live for some days. eiy of Dr. Thomson quotes Dr. Carpenter’s remark, that the structures the star- first developed in the egg of the bird hold nearly the same fis relation to the rudimentary chick that the pluteus (pseudembryo) bears to the incipient Echinus or Ophiura, or the Bipinnaria to the incipient star-fish. There are also these distinctions between pseudembryos and Pseudem- ordinary larvee:—In the cases now under consideration, deve- beyosend lopment is not the criterion of morphology, nor of the true affinities of the species ; on the contrary, the pseudembryonic forms are much less constant throughout the class than the mature ones ; and, unlike what we find in metamorphosis and metagenesis, there is no morphological resemblance whatever between the pseudembryonic and the mature forms of the Echi- nodermata. A tadpole and a frog are both Vertebrates ; a maggot and a fly are both annulose animals ; a zoea and a crab are both Crustaceans; and a Medusa is morphologically a hydrozoon, though externally much modified ; but a pluteus or a bipinnaria is not in the least like an echinoderm. For these reasons I do not think it likely, or even possible, that the ancestors of the Echinodermata are in any way represented by their present pseudembryos. I am at a loss even to guess how this extra- ordinary mode of development can have originated. _ CHAPTER XXII CLASSIFICATION. Classifica- ie subject of classification is much more familiar than aantis that of embryology, and, apparently, has to do with on Embry- much more obvious facts. Nevertheless, classification as osy- now understood depends on embryology ; for which reason I have treated of embryology first. What is At the end of the last chapter I used an expression reant PY -which to many may appear strange. I spoke of the ala facts of classification. It may be asked: “What does this expression mean? What is classification but an affair of words and names? What other merit can the best classification have, than that of being the most convenient ?” Questions I have in a former chapter! stated my conviction, which eee Po is that of most if not all men who have given any attention real. and thought to the classificatory sciences, that there is not only a distinction in classification of convenient or incon- venient, but also a distinction of true or false. It is perhaps difficult to prove this to the satisfaction of any one who has not a general familiarity with the outlines of those sciences ; for such a conclusion is not like a mathe- matically demonstrated proposition, which is necessarily assented to as soon as the reasoning is understood on which it rests ; on the contrary, the general conclusions of biological science mostly depend on cumulative evidence derived from a variety of facts of various kinds, and of very different degrees of importance. 1 See p. 117. — »- =” ae a bo ~J te) CHAP. XXII. ] CLASSIFICATION, Before going on with the subject of this chapter, I wish to remove a possible source of confusion. I have stated that questions of classification are not mere questions of words and names, but have to do with realities. Thus - the assertions that the whale is not a fish, but a mammal having the external form of a fish, and that a cirrhipede is not a molluscan, but a crustacean which has put on the form of a molluscan, are assertions not merely concerning the names which naturalists have agreed to use, but con- cerning the real nature and affinities of those animals. These are cases of members of one group being disguised in the likeness of another, and in such cases we know nothing unless we know the real affinities of the species under review, and consequently its true classification. But Questions there are other questions of classification which are little a more than questions of words. It is, for instance, very beter difficult to say where the series of fishes ends, and that of verbai. Perennibranchiate Batrachians begins; it is very difficult to say whether the lepidosiren is a fish or a Batrachian. Position of But it is as needless as it is difficult to decide such a ite question. The two groups run into each other, and there is so complete a gradation of intermediate forms, that the line can scarcely be drawn between them except arbitrarily. It is scarcely a metaphor to say that the lepidosiren is a fish which we have caught in the act of acquiring lungs and transforming itself into a Batrachian. But Mammalia and fishes, or Crustacea and Mollusca, are not groups that run into each other; the whale has no tendency to become a fish, nor has the cirrhipede any tendency to become a molluscan. - It needs no proof that the value in classification of any The value character depends, not on the importance of that character 9 2", to the life of the organism, but altogether on the extent ap eation to which it is so correlated with other characters as to be depends an index to the general nature and affinities of the ™ being : an index organism. Thus, on the one hand, as I have mentioned to others. already, the presence or absence of wings in some groups of beetles is very inconstant, both as between otherwise similar species and between individuals of the same species ; 280 HABIT AND INTELLIGENCE. [cHAP. Value in while, on the other hand, organs which have become rudi- ee hee mentary, and therefore useless, like the nails under the rudiment- skin of the manati (an aquatic mammal), are regarded by aryors"™ the best authorities as of importance at least equal to that of organs homologous with them, but in a state of func- . andof _—_ tional perfection and activity. “It may even be given as organs not a general rule, that the less any part of the organization with is connected with special habits, the more important it eee becomes for classification.”! “With plants, how remark- able it is that the organs of vegetation, on which their whole life depends, are of little signification excepting in [separating the whole vegetable kingdom into] the first main divisions ; whereas the organs of reproduction, with their product the seed, are of paramount importance.” ? If we believe in the theory of the origin of species by development—or, to use more accurate language, if we believe that different species have been derived from a common ancestor by descent with different modifications— all this, and much more, at once becomes intelligible. The deve- Before the “ development theory ” became familiar to sci- ag entific men, and while the origin of species was regarded explains as something inscrutable, like the origin of matter or of all this. : ; eer : ; life, though naturalists were instinctively certain that their words had a true and important meaning when they spoke of ¢rue classification and of real affinities, yet they found it impossible to state what that meaning was, in a way that was satisfactory even to themselves. But if the development theory is true, real affinity simply means affinity of kindred by descent, and true classification is by Affinity genealogy. When we say, for instance, that the Cirrhipedes, indre a, ot Barnacles, though totally unlike in form and appearance, - of _ are proved by the character of their larve to be closely a Ns connected with the Crustacea, what we mean is, that they poe are literally akin to the Crustacea, being descended from Groups Crustacean ancestors. Not only every species but every within genus is descended from a single ancestor ; all the genera oevP® of an order are descended from a single ancestor further back ; so of all the orders of the same class, and all the 1 Darwin’s Origin of Species, p. 489. 2 Thid. p. 490. =~ ree OV j xx] CLASSIFICATION. 281 classes of the same great division, such as the Vertebrata. Thus far, I believe the development theory is as completely How far proved as any truth can be, of which we have neither cee direct evidence nor mathematical demonstration. More ry is than this I do not think we are yet in a position to assert eae with the same confidence ; but I believe that all organisms whatever are descended from a common origin. It will be Reason of seen why, on this view, so much importance in classifica- WP" tion should be attached to organs that have become rudi- rudiment- mentary and aborted; for they show the descent of the “7° organism as clearly as if they were conspicuous and at work. It will also be seen why the highest importance in classification is attached to those organs which are earliest formed in the embryo, and to embryonic characters gene- of embry- rally ; for the first developed characters, as shown in the me ch last chapter, are the least variable, and have probably varied the least throughout an indefinite number of generations. And it will be seen that although the of the flower and the seed-vessel in plants are not developed #°”™ early, yet they are of high classificatory- importance for the same reason that embryonic characters are; namely, that they are unlikely to be greatly changed by any change of an adaptive nature. Adaptation to a new habitat, for instance, or change of climate, will be much more likely to change its habit and mode of growth—to make the difference, for instance, between a tree and a herb—than to make any great change in the character of the flower. The foregoing remarks will all be familiar to the student of Darwin. What follows is not absolutely original; but it has not yet, I think, been stated with the emphasis it deserves. We suppose the following to be the rationale of the facts of classification :— All living beings have a capacity for variation, which is very limited in a single generation, but (like geological change) quite unlimited if sufficient time is allowed, so that the variations of successive generations may be added 282 Origin of organic forms by the accu- mulation of varia- tions. Divergent lines of variation. Diver- gence and re-diver- gence. Classifica- tion is genealogy. No re- union after diver- gence. Metamor- phosis generally is pro- gress. Exception in Cirrhi- pedes. Specific change also is generally progress. HABIT AND INTELLIGENCE. [ CHAP. together. All organic forms have come into existence by the accumulation of such variations, ever since they began in those first vitalized but unorganized germs from which IT believe all organisms to be descend Different forms have come into existence by the accumulation of different sets of variations along different and diverging lines of descent; and these lines ever diverge and re-diverge—that is to say, forms ever become differentiated and re-differ- entiated from each other—eiving origin to subordinate croups. Thus the true classification, could we find it, would be a genealogical table; and the best attainable classification is that which most nearly approximates to genealogical affinities. In the genealogical table of a human family, if it repre- sents the whole of the family, and not the leading branch only, there are such diverging and re-diverging lines of descent. In the human genealogy, however, lines that have diverged may reunite by the intermarriage of cousins; and thus a new line may arise, mixing the blood, and probably combining the characters, of the two parent lines. But in the organic genealogy such unions are impossible; for organisms cannot produce offspring together after they have diverged into decidedly different forms. In our organic genealogical tree, consequently, the branches, which are classes, should never reunite after diverging. Further, the facts of metamorphosis, as stated in the last chapter, go to prove that when any important change takes place it is generally towards a higher form. The frog is higher than the tadpole, the butterfly is higher than the caterpillar, and the crab is higher than the zoea; and though the cirrhipede is not higher than its nauplius-like larva, this case is exceptional. And if it is true, as I have shown reason for believing, that the ancestral forms from which the species of these groups have been developed resembled the larvee from which the individuals are still developed, then the direction of change of species, as well as of metamorphosis of the individual, is in general, though not invariably, towards higher forms. xxu.] CLASSIFICATION. 283 Thus the branches of our symbolical organic tree should never reunite after dividing, and should generally ascend. Do the facts support these @ priori conclusions ? So far as the facts are known they do. It is a familiar Groups remark to naturalists that groups are united by their lower erally rather than by their higher members. Were this univer- united by sally true, it is obvious that the organic tree would have Hie lover its branches always ascending and never reuniting. This is a subject of great importance, and I must illustrate it further. The organic tree consists of two main trunks, the vegetable and the animal. It is by the lowest mem- bers of both groups that they are brought into contact. There are many forms concerning which it is uncertain whether they are animal or vegetable; and perhaps, in- Animal deed, we make ourselves the slaves of our own words one when we assume that every organism must be definitely kingdoms. either the one or the other. The sarcode of which the bodies of the Protozoa are composed does not appear essentially to differ from the protoplasm contained in vegetable cells; and if it is true, as I think most pro- bable, that this community of properties indicates com- munity of origin, the germ, or germs, from which both Their the vegetable and animal kingdoms have been descended, age must have been neither decidedly vegetable nor de- ovgin. cidedly animal, but capable of acquiring the properties of either. . This is perhaps the best of all the many instances which might be enumerated of the general law that groups are united by their lowest members, and separated in their higher forms. We have seen that it is nearly impossible No abso- —I am inclined to think quite impossible—to make any BE lal absolute distinction between the lowest plants and the between lowest animals. But between their highest forms the ‘°™ distinction is not only fundamental, but is also so obvious that it cannot be mistaken. The warm-blooded Verte- Their brata are the highest of all animals; and, though there p'sres' is no vegetable class that stands so decidedly at the head totally of that kingdom, we cannot be far wrong if we regard a the Rosaceze and their allies as the most highly organized 284 HABIT AND INTELLIGENCE. [crap. of all vegetables. Now, there is no possibility of making any confusion between an animal and a rose-bush, or an apple-tree. Science is needed, not to inform us of the difference, but to tell us of the fundamental resemblance that all plants have to all animals. The same is true of the lowest of the unmistakeably vegetable classes, Thallogens. This class consists of three Affinities orders—Algee, Lichens, and Fungi. These in their highest ee genera are distinct enough, but of some of their lower and Fungi: genera, as Protococcus, it is nearly impossible to say of which of the three orders they are members ; and, indeed, on my view, they do not necessarily belong to one order more than to another. They may belong to that simple original type out of which Algee, Lichens, and Fungi have all been developed by differentiation. of fishes The next instance I shall mention is the connexion ae between the fishes and the air-breathing Vertebrata. The eciere air-breathing Vertebrata are on the whole much higher in the scale of organization than the fishes ; but the Perenni- branchiate: Batrachians, which are the lowest of the air- breathing Vertebrata, approximate, not to the highest, but to the lowest of the fishes. There are, however, some exceptions to this law, which may probably be regarded as parallel cases to that of the metamorphosis of Cirrhipedes, in which, as we have seen, Retrograde the change is retrograde—that is to say, from a higher to change. 4 lower form. One of the most remarkable instances Acari. of this kind that I know of is that of the Acari, or mites, which are the lowest members of a class which I believe to be descended from worms, and yet do not, at least normally,! present the slightest approximation to the worms. This subject must be explained in some detail. 1 Tsay “at least normally,” because there are instances of Acari pre- ~ senting a form which may be due to reversion to their ancestral worm- character. “Mr. Charles Robertson, Demonstrator of Anatomy in the University of Oxford, has lately described a form of Acarus found inside pigeons, chiefly among the connective tissue of the skin, the large veins near the heart, and on the surface of the pericardium. In some respects vr xxu.] CLASSIFICATION. 28 Or Those classes of animals whereof the body is composed of a succession of rings from head to tail are called the Annulosa. The annulose type is a very well-marked Annulosa. natural type. As in other groups, the lower limit of the Their _ Annulosa is somewhat difficult to trace, in consequence of eae the affinities of its lower classes being less definite than nite, their those of the higher ones ; but in its higher classes, the pace annulose type is as well-marked a one as the vertebrate. ines The annulose structure is best seen in the Annelids, or character. true worms, in which the division into ring-like segments Annelids. is distinctly visible to the eye. Above the Annelids, and, Arthro- as I believe, descended from them, are the four classes of eee the Arthropoda, which are distinguished from the Annelids from by the possession of well-developed jointed legs. The es classes of Arthropods are as follow :-— 1. Myriapods (millepedes and centipedes); 2. Crus- Number of taceans; 3. True or hexapod Insects; 4. Arachnids see (spiders and mites). In these four classes, according to and body Professor Huxley, the head generally consists of six seg- aaa ments consolidated together. In the Myriapods the number of the segments of the body differs greatly as between species, and, in the lower genera of the class, probably even as between individuals of the same species; but in the Crustacea, Insects, and Arachnids, most species, according to Professor Huxley, have the whole body, in- cluding the head, consisting of twenty segments. Such characters as these cannot be adaptive; they must be hereditary, and due to community of origin. These homo- logies are similar to the fact mentioned in a former chapter, that nearly all the Mammalia, whether their necks are long or short, have seven neck-vertebre. But this number is subject to variations ;' and if there are similar variations in the number of segments in the Arthropoda, this in no the Acarus described agrees with Sarcoptes, but has an extraordinary maggot-like appearance. The discovery of an external parasite inside an animal, in such numbers as Mr. Robertson records, is very remarkable. Colonel Montagu found such Acari in the Gannet, and Mr. Robertson has since found them in the Pelican.” (Quarterly Journal of Science, January 1867.) 1 Pp. 247, 286 Acari, . though the lowest Arthro- pods, do not revert to the worm- type. Pygno- gonidee. Classifica- tion in a single series is impos- sible. “ Natura’ non facit saltum.” HABIT AND INTELLIGENCE. [CHAP. degree invalidates the evidence of community of descent ; it is only another instance of the truth, that no morpho- logical character is quite invariable. It is to be observed that the determination of the number of segments in an Arthropod is generally difficult, and sometimes impossible, in consequence of the almost perfect obliteration of the joints between some of the consolidated segments, analo- gous to the consolidation together of some of the vertebree in all the higher vertebrate forms. But to return to the Acari: these are a family of Arachnids, and, though they are the lowest of all Arthropods, they have not — reverted to the worm-like structure ; on the contrary, like the rest of the Arachnid class, they have their segments more completely consolidated and less distinguishable from each other than is common among the rest of the Arthropod classes. The same remarks apply to the Pyg- nogonidee, a crustacean family of organization so low that they have neither a circulatory nor a distinct respiratory system. Their respiration is through the general surface of the body, as it is in all the lower Invertebrates ;+ and yet their forms are not worm-like, but crab-like. These facts concerning the Acari and the Pygnogonidze may be, I think, most easily interpreted by supposing that they are tribes which, instead of advancing in organization, have fallen below the general level of the classes to which they belong, and yet have not in any way reverted to the ancestral worm-structure. The first attempts that were made at zoological classifi- cation, generally aimed at the arrangement of all species of animals in a single series, according to their affinities, from the lowest to the highest; or rather, as the early schools would have expressed it, from the highest to the lowest. “ Naturalists,” says Agassiz,” “were bent upon establishing one continuous 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 1 Carpenter's Comparative Physiology, p. 403. 2 Spencer’s Principles of Biology, vol. ii. p. 299. Sn xxi] CLASSIFICATION, 287 chaine @étres.” The doctrine that natura non facit saltum is now almost become an axiom, and is, I think, suffi- ciently proved by the fact that modern researches among extinct as well as among living organisms have not made known a single type of form fundamentally different from those which have been familiar since the dawn of the science ; while they have made known a vast number of intermediate forms, and in some cases (as, notably, in that of the Cirrhipedes) they have filled up a gap by the dis- covery of a larval form. As it has been truly expressed, “all newly discovered forms can be arranged either in known groups or between them.” ! But such expressions as “la chaine d étres,” or “ the organic scale,’ are inaccurate. A little familiarity with organic _ classification is enough to show that no single series is possible which shall represent organic affinities. The possibility of such a series is excluded by the fact that different groups generally approximate by their lower, and not by their higher members. Thus, for. instance, among the Vertebrata, if we were to write the names of all the orders and genera of fishes in a series from the lowest to the highest (though not even a single class can be truly arranged in such a series), we could not go on straight to the air-breathing classes; on the contrary, it would be necessary to go back far down in the series of fishes, in order to begin that remarkable series of Perennibranchiate Batrachians which constitutes the transition from the fishes to the air-breathers. It is the same generally. When The whole groups are capable of being compared, though one may be tee ‘ higher than the other on the whole, it is seldom or never seldom __ that all the members of one group are higher than all the Tee members of another. ‘Thus, though animals are on the Mee whole very much higher than vegetables, the higher group. vegetables are very much more highly organized than the Protozoa, or lowest animals. When the student has become convinced of this fact, Organic that the order of organic affinities is not a single series, his as seem like a Unless the pseudembryonic forms of the Echinodermata are an excep- EWE tion to this. See Note to last chapter. 288 HABIT AND INTELLIGENCE. [cHAP, first impulse will probably be to conclude that it is an inextricably complex network. Such a conclusion would no doubt be incomparably nearer the truth than the notion which it has superseded, of a chain; and I think it likely that the majority of naturalists are at present rest- ing in that conclusion. Nevertheless, I believe it will be found, that as the notion of a single chain of affinities rested on a total misconception of the facts of comparative morphology, so that of a network, with affinities in all directions, rests on an incomplete knowledge of them. As I believe already stated, I believe the true form of classification is they are that of a tree, the branches of which, after diverging, never like a tree. reunite. The analogy of a tree may help us further. Were it possible for an intelligent man to see for the first time a large and many-branching tree without knowing anything of its mode of growth, he would at first fancy that its branches formed a network ; and it would require a good deal of careful examination before he could be quite con- vinced that such an inference was a purely visual illusion, and that in not a single case do they reunite after diverg- ing. Just so, I believe the natural and plausible notion of a network of affinities is due to our imperfect knowledge. Groups The organic tree has two main trunks, the vegetable and nnite after 2€ animal : these, at least, do not reunite, but diverge and diverging. re-diverge into classes, orders, genera, and species, though not with the regularity that appears to be implied in those somewhat technical expressions. I do not say it is proved that groups never reunite after diverging; I only say that I believe every advance that is made in systematic biology and in true classification tends to prove it. Affinities It is, however, scarcely to be hoped that the outline of will peataily the organic genealogical tree will ever be traced in all its never be parts with even approximate fulness and accuracy: Great perfectly : traced, progress, no doubt, has been made, and more remains to make. Many connecting links of affinity have been found: some, by the more careful anatomical or micro- scopic study of organisms already known; others, by the discovery of new forms, fossil as well as living; and 2259 eae ae aes ee ee ~ Fig pa Rte Xx11.] CLASSIFICATION. 289 others, again, by the discovery of larval forms. And, with Many the clearer understanding of affinities, we have come to see j2X* fave that some apparent affinities are not real affinities at all, covered, but merely adaptive modifications. Thus the whale tribe aor are not, as they appear on a superficial view to be, a group a ae connecting the Mammalia with the fishes, but a group of not to be Mammalia modified for an aquatic life, and having no re- oe poe semblance to fishes except a merely external one. Such a Nes conclusion as this, though merely negative, is as important as any positive conclusion: perhaps more so, for the pur- pose of clearing away difficulties and anomalies in classi- fication. But it is likely that our organic classifications will ever remain incomplete and in some degree provi- sional, because the materials of our knowledge are incom- plete. Much has no doubt been lost that cannot be re- covered, It is probable that whole classes once connected Lost links. the lower animal forms with the Vertebrata, which have died out during geological time, and, being without shells or skeletons, have left no fossil remains. And, what is quite as important in accounting for apparent gaps in the series, it is likely that forms that once underwent a meta- morphosis have ceased to undergo it, and have come to be developed by a direct process. Had the Cirrhipedes lost their metamorphoses and come to be developed directly from the egg, we should probably never have guessed how closely they were connected with the Crustacea ; and had they been known only by fossil shells, their resemblance to the Mollusca, which is only external and adaptive, would certainly have been misunderstood, and would have caused them to be mistaken for molluscans. But, though I do not believe that all the gaps in our classifications will ever be filled up, I believe it will before long be generally recognised that affinities are divergent and re-divergent without ever reuniting; as they ought to be on the hypothesis that true classification is genealogical, and that the true bond of affinity is community of descent. It is to be observed, that on the genealogical theory of Affinity : : F : -___ is distinet classification, affinity by descent does not necessarily in- from re- volve resemblance; and the groups which are most nearly semblance. 16 290 Analogy of human kindreds, Why is there organic progress ? HABIT AND INTELLIGENCE. [oHar. akin by descent do not necessarily resemble each other the most. Thus the Cirrhipedes are nearly akin to the Crus- tacea, though in their mature state they do not resemble them; and if they had lost their metamorphoses, their kindred to the Crustacea would never have been suspected. There may be, and probably are, many such cases in the organic kingdom of real affinity without visible resem- blance; and this possibility almost indefinitely increases the difficulty of ascertaining the true classification by descent : just as in human kindreds there is such a thing as family likeness, but the degree of likeness is no measure of the nearness of kindred ; brothers are sometimes met with who have no family lkeness to each other, and cousins sometimes resemble more than brothers. But in human genealogy we have records or tradition, while we have to make out the facts of organic genealogy as we best can, from the resemblances between the various groups. There is this further parallelism between human and organic genealogy, that in men there are some kinds of characteristics, such as the form of the features, which are original and not acquired, and are consequently in some degree an index to the man’s kindred; while there are others, such as peculiarities of voice and of manner, and to a certain extent complexion, which are much more capable of alteration by the action of circumstances, and conse- quently are no index of kindred. Just so, in organic genealogy there are some kinds of characteristics which are subject to alteration to suit special habits of life, and, as has been already stated, are thus merely adaptive characters, and are of much less value as indications of the real affinities of the organism than those why are not so alterable. I have shown in this chapter why, as I believe, the branches of the organic tree should diverge and re-diverge. But why should they ascend? Why is there organic pro- gress? The laws of habit and variation may account for variety, for divergence and re-divergence ; but why is it that groups, after being formed, generally produce members XX11.] CLASSIFICATION. 291 that are superior to themselves in organization, rather than members that are inferior ?—for this is implied in the fact that groups generally approximate by their lowest members. In other words, the earliest formed species or classes of a group are generally the lowest; thus, according to the theory detailed in the last chapter, the Batrachia were the first-formed group of air-breathing Vertebrata, and have given origin by descent to the groups above them. The question now before us is, why progress is upward, and not downward ; why the later-formed groups are in most cases of higher organization than their ancestors. I shall have to consider this question in the next few chapters. T have said that the leading branches of the organic tree An ex- diverge and re-diverge into classes, orders, genera, and ee species. To make the enumeration complete, however, J tion would - should have added races, families, and individuals. If ae it is true that community of descent is the real bond of acer organic affinity, a really exhaustive classification would viduals. include not only every species, but every individual that has ever existed; just as a really exhaustive cosmology would include not only every nebula, every star and planet, and every continent and ocean, but the position of every pebble and the fall of every rain-drop. NOTE. Ir has been urged against the validity of the argument from classification in favour of the origin of species by descent with modification, that crystalline species also are naturally classified Crystal- into groups, and into groups of groups, and yet there is no bond sa ane of descent between crystalline species. I think this is suffi- species. ciently answered by the fact that the unformed but formative material of organisms can only, so far as we know, be produced by organisms ; but this is not true of crystals, as their formative material has a merely chemical source: so that the cases are not parallel. U 2 How has the trans- mutation of species been caused ? How has organic structure been pro- duced ? We must begin by trying CHAPTER XXIII. THE CAUSES OF DEVELOPMENT. ye the foregoing chapters I have stated a mass of cumu- lative evidence which, when taken together, amounts, as I believe, to a conclusive proof that species have not been independently created ; but that all the speeies of the same fundamental type certainly, and all organic species whatever probably, have been descended from the same original stock. I regard it as certain, for instance, that all the Vertebrata are descended from one ancestor, and all the Annulosa from another; and so among plants with Endogens and Exogens. And I regard it as highly pro- bable that those ancestors, in their turn, were descended from a still older stock, which was the original germ of all organisms whatever. I believe that in the chapter on Embryology, especially, I have pointed out the course that has been followed by the successive changes which have developed some of the lower forms into higher forms of the same fundamental type. But I have as yet scarcely approached the question, by what agency the development of one species out of another by descent has been brought about. And if it is true that all organisms are descended from a few original vitalized though unorganized germs, the further question arises, how the matter of those original germs was caused to assume an organic structure ; or, in other words, by what causes, acting on vitalized matter, organic structure is produced ? It is a cardinal principle in scientific reasoning to begin by trying whether known causes are adequate to account for the phenomena under investigation; and it is only when Tee ee er ee, CH. XXITI. | THE CAUSES OF DEVELOPMENT. 293 the causes already and independently known to exist are known demonstrably inadequate to produce the effects, that we “"*** can with any certainty infer from the effects the existence of distinct and separate causes. From this point of view, the questions of the origin of species, and of organization, “may be thus stated :— Given the laws of the relation of life to matter and energy—or, in other words, given the chemical and dyna- mical properties of vitalized matter ;—given also the laws of habit and variation :—are these, In combination with the action of external causes, sufficient to account for the facts of organization and morphology? Will vitalized matter assume organization under the action of external forces, and in obedience to no other laws than chemical . and dynamical ones, and the vital laws of habit and variation ? . Before we endeavour to answer these questions, let us go back on the subject, and recount what organization is. Organization, as defined in a former chapter, is the adap- tation of structure to function; or, in other words, the Organiza- adaptation of every part of an organism to every other Lianee part, and of all to the mode of life of the organism. It is tion, obvious that if any theory of the origin of organization is morpho- to be complete, it must account for histological as well as een nd morphological adaptations ; that is to say, it must not only sical. account for the adaptation of the foot for walking, of the hand for grasping, of the wing for flying, of the jaws for masticating, and of other organs each for its own special. work; it must also explain how each different kind of tissue has been formed and fitted for its function—bone for support, muscle for transforming vital into motor energy, and the other tissues of which organisms are built up, each for its own peculiar function. These two problems, it is The evident, run into each other; nevertheless they are dis- Proper tinct: and it may be thought that the problem of the origin of morphological adaptations—that is to say, the adaptation of every organ to its particular work—is a partly soluble problem; while the origin of histological adaptations—that is to say, the adaptation of every kind of The problem stated. Two pos- sible processes : self-adap- tation, and natural selection. HABIT AND INTELLIGENCE. [cmar. tissue to its function—is totally insoluble. And certainly, at a first glance, the question how germinal matter has acquired the power of transforming itself into cellular tissue, bone, muscle, and nerve, appears at least as far beyond the possibility of solution as the question why oxygen and hydrogen combine into water, or nitrogen and carbon into cyanogen. Nevertheless it is worth while to inquire whether it is possible to suggest any physical causes for these transformations: if we come to the conclusion that none such are possible to assign, even such a negative conclusion will be valuable; and if we are compelled to keep our judgment in suspense, it will be well to have stated the question. With respect to morphological adap- tations—-as, for instance, the question how, and by what physical process or physical cause, a imb which is homo- logically identical with the pectoral fin of the fish has been transformed into the leg of the quadruped, into the wing of the pterodactyle, the bird, and the bat, and into the hand of man ;—with respect to this class of questions, I say, it does not appear so hopeless to seek for a solution. In speaking of the laws of habit and variation in a former chapter, it has been explained at some length how organ- isms have a very considerable power of adapting them- selves to changing circumstances, necessitating a change in the mode of life. The problem before us may be thus stated, in words somewhat different from those employed above :—Is it possible that all of the almost infinitely com- plex adaptations of the organic creation can be produced by any action, direct or indirect, of inorganic forces on the organism, and the actions of the organism in response thereto; the effects of all such actions being, of course, accumulated by hereditary habit, and complicated with the laws of the correlation of variations ? When I speak of a direct process, I mean, of course, the process of spontaneous self-adaptation to circumstances, of which all organisms are in some degree capable. By an indirect process, I mean that of “natural selection among spontaneous variations,” according to Darwin’s law. These two causes—self-adaptation and natural selection—are the ee or Xxut.] THE CAUSES OF DEVELOPMENT. 29 only purely physical causes that have been assigned or that appear assignable, for the origin of organic structure and form. But I believe they will account for only part of the I believe facts. It is my belief that all such solutions of the problem eee are inadequate, and that no solution of the questions of the Eee origin of organization and the origin of organic species can above be adequate, which does not recognise an Organizing Intel- "es ligence over and above the common laws of matter. Or, in other words, I do not believe that the relation of means to distinct purpose in organization is a mere case of the law of cause FEareal and effect. This is what I aim at showing in the present causation. and the following chapters. But we must begin our inquiry by considering how much of the facts of organic structure and vital function may be accounted for by the two laws of self-adaptation and natural selection, before we assert that any of those facts can only be accounted for by supposing an Organizing Intelligence. Here I must guard myself beforehand against a miscon- ception. I shall be compelled to say that some organs and structures may have been formed by the forces of ordinary matter acting on and through the laws of Habit, while other organs and structures can only be the work of Intelligence. But this is not an accurate way of speaking, It is only a concession to that narrowness of our under- standing which compels us to think and speak of things apart which in nature are always and necessarily united. Life does not suspend the action of the ordinary forces of matter, but works through them. I believe that wherever Where life there is life there is intelligence, and that intelligence is ee i at work in-every vital process whatever, but most dis- gence: cernibly in the highest. In every vital action, whether one eee formative, motor, or sensory, whether conscious or un- mathe, conscious, the ordinary forces of matter are at work, functions. directed and controlled by life and intelligence. But, though I believe that these two sets of causes act in every manifestation of life-—the forces of inorganic matter on the one hand, and life with intelligence on the other,— it is a mere statement of obvious fact to say that the inorganic causes are most discernible in the lower vital The laws of habit are not in- telligent. Formation of cellular tissue, HABIT AND INTELLIGENCE. [cHar. functions, as in nutrition, circulation, and respiration ; and life and intelligence are most discernible in the formation and in the action of the organs of sense and of thought. Nutrition, circulation, and respiration are in a great degree to be explained as results of physical and chemical laws ; and I believe that, as I shall endeavour to show, the origin of the organs in which those functions are carried on may be in some degree explained by the operation of those same laws. But sensation, perception, and thought cannot be so explained. They belong exclusively to life; and, similarly, the organs of those functions—the nerves, the brain, the eye, and the ear—can have originated, I believe, only by the action of an Organizing Intel- ligence.. . ‘ I believe intelligence to be a concomitant of all life, but I do not think that all the laws of life are to be referred to vital intelligence. The laws of Habit are purely vital laws, but I do not see any element of intelligence in them. I have now to explain in what way I think that some of the simpler of the formative processes of organization may be accounted for by the action of inorganic forces on vitalized matter. What follows on the origin of cellular tissue, of circulatory vessels, and of respiratory organs, is all suggested by the second volume of Herbert Spencer’s “Principles of Biology,” and is mostly taken from it. We have seen that in most cases, though not in all, the first product of organization is the cell. Many of the lowest Algze consist of but a single cell, and the embryo of the higher plants and animals before they begin to assume any distinctive form consist of cellular tissue. A very pro- bable cause of the first formation of cells is that various influences from without acting on a minute mass of vital- ized but unorganized matter; determine a slight hardening of the surface. The exact nature of the action is probably undiscoverable ; but all I wish to insist on is, that it must be due to some agency that acts differently on the inside and on the outside of the mass of germinal matter. By = d s ¢ 7 . é ; Xx11.] THE CAUSES OF DEVELOPMENT. bo © 4] repetition of this process, a habit is produced of forming cells; and this becomes hereditary, so that cells continue to be produced in the midst of the germinating seed or of the growing embryo, and other situations where the chemi- eal actions are alike in the inside and on the outside of the cell, and where consequently cells could not continue to be formed in the same way that I suppose them to have been formed at first. This is a mere speculation, and, so far as I am aware, has no evidence except what it derives from internal probability. But there is more direct evidence as to the origin of circulatory vessels. We know that streams have Circu- excavated their own channels, and there appears to be a eee very strong probability that the circulatory channels in how organisms have been formed by the flow of the nutritive ers fluids. It is stated on the high authority of Von Bar, that in the animal embryo blood appears before blood-vessels are formed, and circulation begins before there is a heart to propel it ;1 and we know that the embryonic states of the higher forms generally represent the permanent states of the lower ones. We may consequently infer that the earliest circulation was not due to the action of a heart, or to muscular action at all. The circulation in air-breathing Cause of plants is principally due to evaporation from the leaves, Serpe: but this will not apply to aquatic or embryonic organisms, breathing A physical cause for their circulation, however, may be panes assigned. It has been ascertained? that if two liquids have access to the same capillary tube, and one of these has a stronger affinity than the other for the substance of the tube, the one that has the strongest affinity will drive the other before it. Arterial blood has a stronger affinity ’ 1 Carpenter’s Comparative Physiology, p. 717. 2 Carpenter’s Human Physiology, p. 255. The discovery of this law, and its application to the facts of circulation, are ascribed to Professor Draper. Of course it is true of porous substances, as well as of capillary tubes. This law will account for the capillary circulation in the respiratory organs, as well as throughout the tissues. Throughout the tissues, the arterial or oxygenated blood is attracted by the carbon of the body. In the respiratory organs, the venous or carbonated blood is attracted by the oxygen of the lungs. 298 HABIT AND INTELLIGENCE. [cHAr. and in the than venous blood for the substance of the tissues; and lowest animals. Tendency of circu- jJation to form channels for itself. Dr. Carpenter regards this action as being probably at least one cause of the capillary circulation. Some such action as this must probably be the cause of the motion of the nutritive fluid, not amounting to circulation, which has been observed in the Sertularian Hydrozoa. These animals have nothing of the nature of a heart, and yet the nutritive fluid, which corresponds to the blood of the higher animals, flows towards growing parts, and away from dying ones.! Suppose now that a movement of the nutritive fluid— of sap or blood—by permeation through the cellular tissue, has been commenced, whether by this means, or, as in plants, by evaporation from the surfaces exposed to air and sunbeams ;—and if proof is needed that the circulation of air-breathing plants is so caused, it is proved by the fact that there is hardly any circulation in the water- breathing kinds ;—suppose, I say, that the circulation has begun from either of these two causes, or from any other ; the currents will tend to excavate channels for themselves through the cellular tissue, exactly as rivers tend to ex- cavate their own channels; and the formation of such channels will become hereditary. This is no mere hypo- thesis ; it has been as directly verified as the nature of the case admits of. A perfect gradation may be seen in air- breathing plants, from cellular tissue to perfectly-formed vessels. First, ordinary cellular tissue; next, cells elongated in the direction in which the sap flows, as determined by the place of greatest evaporation ; next, cells having their separating walls broken or dissolved away, so as to unite them into tubes; finally, tubular vessels showing no longer any vestige of cell-structure. I do not mean that this whole course of development takes place in the lifetime of any one plant ; but the comparison of these transitional stages makes it highly probable that such has been the course of development through successive generations ; and there are many cases where a part of the transition may be witnessed in the individual.” 1 Carpenter’s Comparative Physiology, p. 633. For details see Spencer’s Principles of Biology, Part V. chap. iv xxut] THE CAUSES OF DEVELOPMENT. 299 I now come to the organs of respiration; and I have to Respira- begin this subject with the remark, that among the lower ae a tribes of animals generally, those organs are surprisingly aeealite inconstant, not only as to their position, but as to their existence. This, I think, is more remarkably the case among the water-breathing Gasteropodous Mollusca than in any other class, but it is generally true of the aquatic Invertebrata ; and in close connexion with this is the fact, that the most remarkable cases in the animal kingdom of organs that assume new functions totally unlike their original ones are among respiratory organs. This extra- ordinary plasticity of the respiratory organs is evidently connected with, and I believe is a result of, the fact that among the lowest animals, and in some degree among the highest, the function of respiration belongs to the whole surface. “The alimentary canal respires, digests, and excretes in the larva of the dragon-fly and in the fish Cobites,” * neither of which is a very low organism. Among the Batrachia a great proportion of the respiration takes place through the skin;? in the axolotl; which is one of that class, the importance of the branchie is so slight that their removal does not appear to injure the animal, or even to increase the necessity for coming to the surface of the water in order to breathe air;? and even in man there is a slight amount of respiration through the skin.t I believe it may be safely asserted that all land animals have distinct respiratory organs.® Respiration, though it takes place in living beings, is Respira- essentially a merely chemical and physical process; it 173° physical consists in absorbing oxygen, and giving out the carbonic Process. . ; : 4 , ‘ ; 1 Darwin’s Origin of Species, p. 220. 2 Carpenter’s Human Physiology, p. 293. ‘3 Memoir by Auguste Dumeril, translated in the Annals of Natural History, December 1867. 4 Carpenter’s Human Physiology, p. 293. 5 Those of the earthworm have remained unknoywn till lately. But it is now stated that its “respiratory apparatus is an aqueous sac, lined with vibratile cilia, within the abdominal cavity, on either side of the body.” (Dr. Coote on Nerve Structure and Force, Quarterly Journal of Science, April 1867.) The respiratory organs of the land mollusca are well known. 300 HABIT AND INTELLIGENCE. [cHAP. acid which is formed by slow combustion throughout the body. The means by which the organism effects this change is to bring the blood into virtual contact with the air or water, according as the animal is an air- or a water- breather, over an extensive surface of very thin membrane, through which the exchange takes place spontaneously by that physical process known as the “ diffusion of gases.” } If now in one of those animals, such as many of the naked marine Mollusca, in which. the function of respira- tion is discharged by the whole surface of the body, and peste there is no distinct respiratory organ ;—if in one of these, pee pirabons I say, a part of the skin, from any spontaneous variation, organs. hecomes thinner than the rest, or more abundantly sup- plied with blood-vessels ; or if the blood under one part of the skin contains a somewhat larger proportion of car- bonic acid, from the waste of the body, than in other places; any one of these causes will produce a more rapid exchange through that part of the skin than through any ‘other, between the carbonic acid of the blood and the oxygen of the external medium, and that part will be a rudimentary, or nascent, respiratory organ; the flow of blood to and through that part will be increased by the chemical unlikeness between venous and arterial blood, in virtue of the law already stated ; and it appears tolerably well established, that an increase in the flow of blood through any part will cause it to grow and-develop. In order to form a complete theory of this subject, it would be necessary to explain, further, why the nascent respiratory organ not only develops, but develops into that structure which is specially needed for the purpose of respiration. This, however, I am not able to do, and I doubt whether it is possible. Unlike the origin-of cellular tissue, or the formation of sap-vessels by the union of cells, the present problem is an extremely complex one. Whatever may be thought of this speculation (and I only advance it as such), it is consistent with what we know of the development of the branchie of the Mollusca, 1 Carpenter’s Human Physiology, p. 264. Xxu11.] THE CAUSES OF DEVELOPMENT. which arise by budding from the skin of the embryo. For if embryology is an index of descent (and this, I think, cannot be reasonably doubted), we must suppose that the development of the individual repeats, in a short time, the development which occupied untold generations of the species. It can scarcely be necessary to repeat, that all actions whatever, tending to modify the nature of an organism, will accumulate their effects through successive generations. I will now go on to mention two remarkable instances, in which organs originally employed for another purpose have become converted into respiratory organs. These two instances include the principal, though not the only classes of air-breathing animals. The leech and the earthworm have two small sacs in each segment opening on the external surface of the body. The function of these appears to be to secrete mucus. But there is a gradation, through the Millepedes and Centipedes, between these sacs and the air-tubes or tra- chez by which insects respire.2 The trachez of the insect are consequently homologous with the mucus-sacs of the worm ; and, if the development theory is true, the sacs, being no longer needed for the purpose of secreting mucus, have, through successive generations, changed the character of their lining, and have extended through the body, so as to become transformed into respiratory organs, adequate to meet the wants of such active animals as insects. What made the transformation possible, was the fact that every surface exposed to the air or the water in which the animal lives is (if not too much indurated, like the shell of a tor- toise or of a crab) in some degree a respiratory surface.® The wings of insects are due to a transformation which is even more wonderful than this. Unlike the wings of Pterodactyles, birds, and bats, they are in no way homo- logous with legs; on the contrary, they are now generally believed to be appendages of the respiratory system; the 1 Spencer's Principles of Biology, vol. il, p. 293, 2 Carpenter’s Comparative Physiology, p. 748. 8 Carpenter’s Human Physiology, p. 293. Homo- logies of the respi- ratory organs of insects : 302 of air- breathing Vertebrata. Origin of the latter. The axolotl. HABIT AND INTELLIGENCE. [cHar. only organs with which they are homologous are the leaf- like expansions that arise from the openings of the trachez of some aquatic larve ; though it is very difficult to guess by what kind of a process leaf-like branchiz came to be used as wings. The difficulty is not so much to under- stand how the wing-form was produced (for branchiz may be produced in one form almost as easily as in any other), as to understand how the habit of flight could originate ; for, as a rule, there are no sudden transitions in nature. But: it is to be remembered that motor habits are more variable than formative ones. The next transformation that I have to mention is not only one of the most important, but one of the best under- stood, in the whole organic creation. I mean the transi- tion from the swim-bladder of fishes to the lungs of the air-breathing Vertebrata. I have spoken of this several times already; but I have now to notice the significant fact that all the Batrachia, through which class the transi- tion takes place, are inhabitants of fresh water Fresh water is liable to be dried up; and I believe the Batra- chians to be descended from fishes, which, when water was failing them, acquired the power of breathing air by means of the swim-bladder.2. This would be almost incredible, were it not for the general fact of the inconstancy of the form and position of the respiratory organs, and the special fact of the preservation of the Perennibranchiate order, which presents a series of connecting forms. And what still further strengthens the case, is the remarkable set of facts lately discovered concerning the axolotl. This animal is a Batrachian, and has till now been regarded as a Peren- 1 Except the land-newt, or land salamander, and perhaps one or two other species, which are not aquatic at all. 2 See Spencer’s Principles 8f Biology, vol. i. p. 395, and vol. ii. p. 325. I must say, however, that I think his attempt to account for the origin of the swim-bladder is quite unsuccessful. He thinks it arose from fishes acquiring the habit of swallowing bubbles of air, at a time when the supply of oxygen in the water was failing in consequence of the increasing heat of the weather. This appears to be possible only in shallow fresh waters, and [ do not think thére is the slightest reason for believing cod and other sea-fish that have swim-bladders to be descended from fresh-water species. j . . XXII. | THE CAUSES OF DEVELOPMENT. nibranchiate. But the axolotl as hitherto known, though it is a mature form by the usual test of being able to pro- pagate,’ proves to be a larva in the sense of having yet to undergo metamorphosis. The specimens observed in captivity in France, from which alone our knowledge of their metamorphosis is derived, sometimes undergo meta- morphosis and sometimes not, without any assignable reason. Like other Batrachians, they lose their branchize when they undergo metamorphosis, in addition to other changes ; and if the branchize are removed (under which operation the animal does not appear to suffer in health), the chance of the metamorphosis taking place is increased, though it is still very uncertain.” These facts are most interesting, partly as showing that an animal which has not undergone its final metamor- phosis may yet propagate; and partly as showing how variable is the fact of fianl metamorphosis, as between in- dividuals of the same species. This last is an instance of the general law, that those characters which are variable as between allied species are also variable as between indi- viduals of the same species: for no one doubts the real and near affinity between the Perennibranchiates, which never lose their branchiz and are in fact permanent tadpoles, and the Caducibranchiates, such as the newt and the frog, which lose their branchiz when they cease to be tadpoles.? The irregularity of the metamorphoses of the axolotl sug- gests also, that the first individual of a Perennibranchiate species that lost its branchiz, and gave origin to a race of Caducibranchiates, may have done so accidentally, in consequence probably of the branchie drying up and withering for want of water. 1 7.c., sexually. No vertebrate ever propagates in any other way. 2 Memoir by Auguste Dumeril, translated in the Annals of Natural History, December 1867. 3 T do not mean to imply that it would be a natural classification to divide the Batrachia into Perennibranchiates and Caducibranchiates. The right division of the class is into (1) the newt or salamander tribe, having four limbs and a tail; this includes the Perennibranchiates : (2) the frog tribe, having four limbs but no tail: (3) the Cecilia genus, which are like serpents in form, having no limbs, 30 9 v 304 HABIT AND INTELLIGENCE. [CHAP. A fact may be mentioned of the vegetable kingdom, which is strictly analogous with the power of various parts of the external surface of animals to assume the respiratory function. Leaves are the organs with which plants decompose the carbonic acid of the atmosphere ; Stems —_ but there are some tribes of plants, of which the Cacti are pains the the best known, that have no true leaves ; and in them the of leaves. stems are green like leaves, and discharge the function of Tater: leaves. And it belongs to the same order of facts, that one eet secreting organ frequently shows itself able, though im- between perfectly, to discharge the function of another. This has ee been observed in cases of disease in man, and in animals when one of the organs has been removed or otherwise experimentally interfered with. I have now brought forward a number of facts and arguments, which all tend to show that wp to a certain point tissues and organs are capable of being formed by the Will action of purely physical forces on vitalized matter. But Eee it is important to observe, that all the instances I have actions mentioned are taken either from the vegetable kingdom, or account for : HD : the origin from the vegetative, or nutritive, systems of animals ; and ee no way appears conceivable by which the actions of ex- ternal forces on the organism can account in any similar way for the origin of the peculiarly animal tissues—for the formation of muscle and nerve. If their origin is expli- cable at all, it must, I think, be due, not directly to the action of inorganic forces on the organism, but to the actions of the organism itself in response to impressions from without. But a satisfactory physical theory of the origin of muscle and nerve is out of the question in the present state of science, nor do I feel sanguine of ever attaining to it. I only do not say it is impossible.? 1 Carpenter’s Human Physiology, p. 374. 2 I say this after reading the speculations on this subject in the second volume of Spencer's Principles of Biology. He admits that his theory of the origin of nerve-fibres does not account for ganglia. Now nerve-fibres and ganglia are always found together, in all classes of animals, and they are developed together; so that no theory is good for anything that xxull. | THE CAUSES OF DEVELOPMENT. 305 One of the most important laws to be borne in mind in Organs all speculations of this class, is that law in virtue of which Vif“. every organ improves with use. This indeed is a parti- the diffi- cular case of the law of self-adaptation. Exercise, pro- fae : vided that it is not beyond the limit of what is good for °™sin. health, increases the strength of a muscle or the sensitive- ness of a nerve. The difficulty is not to account for almost indefinite improvement in an organ, but to account for its origin. I cannot make any suggestion whatever as to the possible origin of nerve, but perhaps it is not absurd Origin or to think that the fibrous structure of muscle was originated Wace fy by the sarcode substance of the earliest structureless though living beings acquiring the habit of contracting in one direction more readily than in any other. We know that the sarcode has contractile power in organisms which have no visible structure, and the simplest rudimentary muscular structure consists in a fibrillation of the sarcode.! I have now stated my belief that some of the simpler structures belonging to the vegetative system have pro- bably been produced by the action of inorganic forces on the organism ; and that muscular structure may possibly, though I do not say probably, have been produced by the action of the organism itself in response to impressions from without. Of course these two factors are always both present, though acting in very unequal proportions in different cases. But there are structures for the origin of which it is, I believe, utterly impossible to account by any such merely physical theory; and which can only be referred to an organizing intelligence. I refer to such organs as of the eye the eye and the ear. If it is certain, as I think it is, that pa a the flow of the nutritive fluids through cellular tissue, for successive generations, must have a tendency to form a rudimentary circulating apparatus, it is at least equally obvious that the action of light falling on the eye for any _ number of generations can have no similar tendency to produce the optical apparatus of the eye. Nor can the attempts to account for the one without the other. It would be different if the ganglia were an outgrowth of the fibres, as the brain is of the spinal cord. 1 Dr. Wyville Thomson on the Embryology of the Echinodermata, Natural History Review, October 1864. x 306 No physi- cal causes will account for the origin of the eye and the ear ; HABIT AND INTELLIGENCE, [cHAP. constant exercise of the eye in the act of seeing have any such effect. The exercise of the eye, within the limits of what is healthful, does no doubt tend to increase the sensitiveness of the retina; and I do not say it is im- possible, though I do not admit it as probable, that the muscular arrangements to which the mobility of the eye- balls and eyelids is due may have been produced by the effort to move them, continued through successive genera- tions; and that the expansion of nerves over the retina may have been produced by the constant stimulation of the nerves themselves. But no such merely physical theory will account for the origin of the special complexities of the visual apparatus. Neither the action of light on the eye, nor the actions of the eye itself, can have the slightest tendency to produce the wondrously complex histological structure of the retina; nor to form the transparent humours of the eye into lenses ; nor to produce the deposit of black pigment that absorbs the stray rays which would otherwise hinder clear vision; nor to produce the iris, and endow it with its power of partly closing under a strong light so as to protect the retina, and expanding again when the light is withdrawn ; nor to give the iris its two nervous connexions, of which one has its root in the sympathetic ganglia, and causes expansion, while the other has its root in the brain, and causes contraction.! I have spoken first of the organs of special sense, because it is in their case that the impossibility of formation by any physical action is most obviously evident. But there are other cases where the impossibility is equally demon- strable. When any structure is formed or modified either by the action of external forces, or by the action of the organism itself, this is a case of self-adaptation—not of adaptation only, but of self-adaptation. But, as we have seen, the complexities of the eye and of the ear cannot be so produced; and, quite independently of any special complexity, there are many structures in which the rela- tion of structure to function is such as to make any phy- sical explanation of their formation as totally impossible as in the case of the eye and the ear, though for an entirely 1 Carpenter’s Human Physiology, p. 639. xxm.] THE CAUSES OF DEVELOPMENT. 307 different reason. Take the case of protective structures ; the skin hardens and thickens in any place that is exposed to rough usage; this is a case of self-adaptation, and it may become hereditary, as in the instance of the knuckles _ of the gorilla.t But such cannot be the origin of the shell nor of the that protects the bird’s egg. Without this protection, no peernel egg could be hatched, and the whole tribe of birds would perish. But the contact of comparatively hard and rough substances, which makes the shell necessary, cannot have had the slightest tendency to produce the shell; for the shell is formed, and from the necessity of the case must be formed and completed, before any such contact can take place And the law of hereditary habit, universally true as it is, makes no difference here; for it appears utterly impossible that egg-shells can ever have been formed under different circumstances from the present. The same nor of the is true of the bony or cartilaginous skulls that protect ae the cephalic ganglia in the Cephalopoda (cuttlefish and nautilus), and in the Vertebrata, and of the hard woody nor of shells that protect the seed in nuts. The fact that there ee is something to protect, is not a physical cause of the production of a protective structure. In this chapter I have considered how far the facts of Summary. organization can be accounted for by the direct action of inorganic forces on the organism, and by the actions of the organism itself; and I have come to the conclusion, that such a purely physical account of the origin of organization is probably valid in the case of some of the simpler structures belonging to the vegetative system ; very doubt- ful in the case of the peculiarly animal ones, as muscle and nerve; and demonstrably, not merely inadequate, but totally inapplicable to the case of the complexities of the _ organs of special sense, and to many cases, not necessarily _ complex ones, in which the structure is produced under such circumstances that the peculiar work for which it is adapted can have no tendency to originate it. In the next chapter I shall have to consider how much may be explained by the law of Natural Selection. 1 Spencer’s Principles of Biology, vol. ii. p. 295. _ 2 Ibid. vol. i. p. 440. x 2 Natural selection defined to be among sponta- neous variations. CHAPTER XXIV. NATURAL SELECTION. i the chapter on Distribution, while stating my belief that the remarkable similarity, without identity, between the living species inhabiting the same region during successive geological periods is due to the bond of descent, I said also that the species inhabiting any region at present are not the direct lineal descendants of those which dwelt there in the last geological age ; but that the descent has been modified in accordance with a peculiar law. The law I mean is that of “natural selection, or the preservation of favoured races in the struggle for life.” (I quote these words from the title-page of Darwin’s work on “The Origin of Species.”) But as Darwin’s work, in which the law of natural selection was first stated and proved, is a well-known and very readable book, it is needless for me to go into much detail. I speak of it as a law that is proved, There is the greatest possible differ- ence of opinion as to how much the law will account for; but no one who has the slightest knowledge of the subject can doubt that it is a truly operative law. Before there can be selection, there must be something among which to select; and the full designation of the theory is “ natural selection among spontaneous variations.” In the chapters on Habit and Variation, we have seen that species are not absolutely constant in their characters, but are liable to slight, and sometimes considerable variations, which may become very great if they are added up and accumulated through successive generations. We have seen also that variation is stimulated by change of the nl ES a CHAP. XXIV. ] NATURAL SELECTION, 309 circumstances of life and by mixture of races, when these do not go further than what is good for the health and vigour of the species; and that some species are more variable than others. It has also been shown that when any particular character in a race once begins to vary, it is apt to acquire a habit of varying; and it is obvious how this fact must facilitate the accumulation of variations through descent in particular directions. I mean that when any one character becomes variable, such as the form of the beak in the domestic pigeon, it will be com- paratively easy to obtain by selection new breeds that differ from the parent ones in that point. Domestication is a great change in the circumstances of the life of both animals and vegetables, and cross-breeding, so as to produce slight mixtures of race, is systematically practised with many of the domestic races of animals; and these two causes are amply sufficient to account for the great variability which is common, though perhaps Causes of not universal, among domesticated races, vegetable as well 700". as animal. It is, I think, an unsettled question, whether tic races. a perceptible degree of variation ever takes place, at least among the higher animals and vegetables, without the stimulus either of change of circumstances or of mixture of race ; and it will not be easy to decide the question, for evidence is difficult to obtain concerning wild races, and _ the variability of the domestic ones is probably due to the change in the conditions of their life effected by their first domestication, and to subsequent crossings of the breed. But it is certain that changes in the circumstances of Changes life must, throughout geological time, be constantly oceur- oh eee ring, and stimulating variation. Geological, and I may geological add astronomical,! revolutions alter the climate and oe physical geography of whole continents ; and quite inde- pendently of this, geological revolutions effect vast changes in the circumstances of the life of species, by either forming 1 See Mr. Croll’s Papers on the Glacial Climate, and kindred subjects, in the Philosophical Magazine. I do not agree with all his results, but I think he has done great service to science by showing that the changes in the earth’s orbit must produce climatic changes. 310 Struggle for existence. Races probably mix little in nature. Rapid increase. HABIT AND INTELLIGENCE. [CHAP. or removing barriers to migration. Besides—as Darwin was, I believe, the first to state, with any emphasis proportioned to the importance of the fact—any change in the distribution of one species is almost certain to produce changes in the conditions of the life of other species, by altering the character and abundance of their food, the character and number of their enemies, and the character of the species with which they have to compete for a subsistence. The importance of this element of competition, or “struggle for existence” (to quote again from Darwin’s title-page), was also first in- sisted on by Darwin. I shall have to return to this subject shortly. It is very difficult to say how much effect is to be ascribed to mixture of races occurring in a state of nature. Among plants the pollen of one is carried to the stigma of another, by the wind or by insects, quite at random; and in this way races may be mixed. But, for reasons to be explained further on, it is not common for two varieties of the same species to be found in the same habitat, and this must tend to prevent mixture. The same is true among animals; and besides, among the higher animals, which have a mental nature and a power of choice, it seems certain that this will be exercised in a way that will keep races separate.1 For these reasons, I am inclined to think that there is very little mixture of races in a state of nature, and that the stimulus to varia- tion is chiefly given by changes in the circumstances of life. Having spoken of these preliminary questions, it is time to state the manner in which natural selection will take place. All organisms multiply so rapidly that when any geolo- gical or climatic change opens a new habitat to a species, a time which is geologically very short will suffice to stock the new habitat with as large a population of that species as it can support ; and when this has been done, many more 1 This is true of domestic races, When any degree of freedom is per- mitted, different breeds of the same species, as sheep or pigeons, prefer to herd and to breed each apart. (Darwin on Variation under Domestication, vol. ii. chapter xvi.) XXIV.] NATURAL SELECTION. dll individuals are born every year than can possibly come to ‘maturity. Consequently there will be a “struggle for existence.” If any spontaneous variation takes place Favour- which gives an advantage in that struggle to its pos- eos Uae sessor, the individual so favoured will have the best be pre- Set : a served and chance of surviving and leaving offspring, and the favour- inherited. able variation will probably be inherited. Thus, by pre- serving the best and weeding out the inferior ones, the race will be gradually improved. This process is exactly the same in principle as that by which the breeds of domestic plants and animals have been improved; the best are preserved, and the others are destroyed or not permitted to breed. This short explanation will make it obvious how selec- tion, whether by natural or by human agency, can improve and perfect a breed. But it must be further explained how selection can produce, from the same parent stock, two or more races differing not only from the parent stock, but from each other. For this purpose, however, it is only necessary that selection should act on two different and divergent lines of descent, and that the individuals selected in the different lines should be selected for different qualities. The various breeds of the dog, for instance, have chiefly Diver- arisen by selection under domestication; and they are Se distinct, because selection has been applied for different how pro- purposes in the different breeds : in some for fidelity and Sear sagacity ; in others, for power of hunting by scent; in in do- others, again, for fleetness. I believe this account of the ar origin of the breeds of dogs is historically true; it is certainly possible; and there cannot be the slightest doubt that such has been the origin of the domestic breeds of the pigeon) which vary from each other more than those of any other domestic animal. Selection, constantly applied for different qualities, must necessarily give origin to dis- tinct breeds, because the different qualities can be but seldom found together in any unusual degree. Thus, among dogs, if the qualities of keenness of scent and fleet- ness are not correlated, which they certainly are not; and 1 Parwin’s Origin of Species, p. 24. and in wild races. HABIT AND INTELLIGENCE. [cHAP. if one dog in every hundred excels in one of those qualities, and the same proportion in the other, to a sufficient degree to attract the attention of the selector ; the chance of any dog excelling in both will be only the second power of one in a hundred, or one in ten thousand, which chance is so small] that it may be left out of account. It is obvious that selection by natural agency may act in exactly the same way as selection by human agency. Wild beasts of prey that excel either in fleetness or in scent will have the best chance of surviving and leaving off- spring ; and these favourable variations will be added up and accumulated through an indefinite number of genera- tions. As I have just shown, it is not at all likely that two favourable variations will occur in the same individual ; and an unusual degree of fleetness in one of a race that hunts by scent, or an unusual power of scent in one of a swiftly running race that hunts by sight, would probably be comparatively useless, and consequently would not tend to the preservation of that individual. What will be most valuable, and consequently most conducive to the preser- vation of the individual, is probably a slightly increased degree of some favourable peculiarity that the race already has in a tolerably high degree ; especially as, by the laws of self-adaptation and hereditary habit, the race will have begun to be adapted in its whole structure to the mode of life to which its favourable peculiarity is suited. Thus we may expect sagacity and wariness to become characteristic of those animals which hunt by scent, especially if they are gregarious. Besides, a race produced by the selection of individuals for any one character will necessarily have that character, on the whole, more variable than the rest of the organism; and, as we have seen in speaking of the laws of variation, a character, or an organ, that begins to vary is apt to continue variable ; so that favourable varia- tions will be more likely to occur in the character that constitutes the principal differentia of a breed, than in any other of its characters. It is, as already stated, an observed fact that this is so. For these reasons, different breeds will continue distinct and divergent. XxIv.] NATURAL SELECTION. 313 Here occurs a difficulty which Darwin appears not to How will see, but which H. Spencer thinks fatal to the theory as Py" applied to the highest animals. Among animals that et unite for each birth and wander much, will not variations, ue no matter how favourable, be lost and merged in the second generation, by the union of the individual possessing the favourable variation with others that are without it?! It is quite obvious that the domestic breeds of animals could not be kept distinct, if free inter-breeding between different varieties were permitted.? I am inclined, however, to think that this objection is sufficiently answered by the fact, that most varieties in a state of nature are local varieties: so By being that while natural selection is forming a variety, suited to °° the locality, by preserving the individuals suited to it and destroying the rest, this action will not be much interfered with by the race getting crossed with other varieties. Dissimilar habits of life will also probably have con- How wild siderable effect in keeping the individuals of different joe °" races apart: and, as we have seen, races are kept apart by distinct. their instincts among the higher animals, and probably among all animals. I believe that natural selection will account for much which self-adaptation will not account for. To take one very curious instance: so far as I can see, natural selec- tion is capable of giving origin to the bat’s wing. Once Origin of the wing is formed, self-adaptation will no doubt give the se necessary power to the wing-muscles ; but self-adaptation cannot, I think, have any tendency to produce that ex- + Spencer’s Principles of Biology, vol. i. p. 454. 2 “The prevention of free crossing, and the intentional matching of indi- vidual animals, are the corner-stones of the breeder’s art. No man in his senses would expect to improve or modify a breed in any particular manner, or keep an old breed true and distinct, unless he separated his animals.” (Darwin’s Variation under Domestication, vol. ii. p. 85.) “Cats, which from their nocturnal habits cannot be selected for breeding, do not yield distinct races in the same country.” (Ibid. vol. ii. p. 236.) 3 J may here mention Darwin’s belief, that what is called the acclima- tization of a race of plants is really, in some cases, the production of a new and hardier race by selection. It is obvious that natural selection may operate among plants grown in a garden but not under glass, by killing the least hardy and preserving the most so. (Origin of Species, p. 169.) 314 Extract from Darwin, Flying squirrels, Flying Jemur. HABIT AND INTELLIGENCE. [CHAP. tension of the fingers, and of the skin over the fingers, which constitutes the wing. The bat’s wing, when in the nascent state, probably resembled the parachute of the flying squirrels. I quote the following from Darwin:* “Look at the family of squirrels : here we have the finest gradation from animals with their tails only slightly flattened, and from others, as Sir J. Richardson has remarked, with the posterior part of their bodies rather wide and the skin on their flanks rather full, to the so-called flying squirrels; and flying squirrels have their limbs and even the base of the tail united by a broad expanse of skin, which serves as a parachute, and allows them to glide through the air to an astonishing distance from tree to tree. ... Let the climate and vegetation change, let other competing rodents or new beasts of prey immigrate, or old ones become modi- fied, and all analogy would lead us to believe that some at least of the squirrels would decrease in numbers or become exterminated, unless they also became modified and im- proved in structure in a corresponding manner. Therefore I can see no difficulty, more especially under changing conditions of life, in the continued preservation of indi- ~ viduals having fuller and fuller flank-membranes, each modification being useful, and each being propagated, until, by the accumulated effects of this process of natural selection, a perfect so-called flying squirrel was formed. Now, look at the Galeopithecus, or flying lemur, which formerly was falsely reckoned among bats. It has an extremely wide flank-membrane, stretching from the corners of the jaw to the tail, and including the limbs and the elongated fingers: the flank-membrane is also furnished with an extensor muscle. Although no graduated links of structure fitted for gliding through the air now connect the Galeopithecus with the other Lemuride, yet I see no difficulty in supposing that such links formerly existed, and that each had been formed by the same steps as in the case of the less perfectly gliding squirrels; and that each grade of structure was useful to its possessor. Nor can I 1 Origin of Species, p. 208. Xx1v.] NATURAL SELECTION. 315 see any insuperable difficulty in further believing it pos- sible that the membrane-connected fingers and fore-arm of the Galeopithecus might be greatly lengthened by natural selection : and this, as far as the organs of flight are con- cerned, would convert it into a bat. In bats which have the wing-membrane extended from the top of the shoulder to the tail, including the hind-legs, we perhaps see traces of an apparatus originally constructed for gliding through the air rather than for flight.” It may be necessary to state distinctly, that no one Bat and supposes the bats or the flying lemur to be descended from fying a squirrel. The unlikeness of the other parts of the descended organism excludes this. But Darwin believes, and I agree heath with him, that their wings have been produced by the improvement of an organ which originally resembled the parachute of the flying squirrels. I mention this instance as the best I can find, of organs that may have been produced by natural selection, and cannot have been produced by any process of self-adapta- tion. The membranes of the flying squirrel, the flying lemur, and the bat, are simple organs ; and those of the flying squirrel, at least, are not closely connected with the other parts of the organism : that is to say, their presence does not cause any very great deviation from the usual structure of squirrels. These two facts, that the mem- branes in question do not consist of a number of corre- lated parts, but are simple organs, and that they have no close correlation with the other parts, make this a . peculiarly good case to isolate in thought, so as to make it the subject of reasoning. It is certain that the flying squirrels can take much longer leaps than they could if they had no membranes, and it needs no proof that this power must be useful to them. But it is, I think, equally Mem- obvious, that the habit of leaping cannot have any ten- ines ; dency to develop such membranes, or to increase their by self- . size after they begin to be developed. Self-adaptation Palisa thus fails to account for this very simple change. Natural by natural selection, however, here comes into play. I agree with poe Darwin in thinking that the preservation, for generation 316 Origin of the bat’s wing. Self-adap- tation and natural selection co-ope- rating. Extensor muscle of wing of flying lemur. HABIT AND INTELLIGENCE. [cuar. after generation, of squirrels that had the fullest skin on their flanks, and were thus best able to escape their enemies by taking long leaps, is quite an adequate cause for the origin of such a structure as the membrane of the flying squirrel. Further development into such a perfect flying apparatus as that of the bat is a much greater change than that from the common to the flying squirrel; but self-adaptation will account for this in a great degree; once the animal began to gain anything by flapping its membranes, the muscles used in doing so would strengthen and grow, in virtue of the law that every muscle strengthens and grows with use. There is no difficulty in believing that it would begin to flap its membranes as soon as it became useful to do so; if mere animal intelligence were not sufficient to originate such an instinct, it might begin in some spontaneous variation, much less strange than that in which the characteristic habit of the tumbler pigeon began. This is exactly the same as the question how animals with legs first learned to creep and to walk. I agree with Darwin in believing that natural selection can produce changes which self-adaptation has no ten- dency to produce. But the process of self-adaptation, in whatever direction it is going on, will be so furthered and assisted by natural selection that the effects of the two will be impossible to separate. Self-adaptation gradually adapts organisms to their mode of life ; natural selection destroys those which are least adapted; and it is obvious that when these factors are both in operation, they will work to the same result. ; But is this explanation of the probable origin of the bat’s or the flying lemur’s wing satisfactory? Does it explain all the facts? I greatly doubt whether it does. I do not see how it will account for the fact that in the flying lemur “the flank membrane is also furnished with an extensor muscle.” I do not think that any clear evidence has been brought, proving that either self-adapta- tion or spontaneous variation will suffice to produce a new muscle. But there are so many instances of organs 1 See quotation from Darwin above. XxIv. | NATURAL SELECTION. 317 which appeared to be quite peculiar, proving to be really homologous with organs in other species,! that I attach little weight to such an objection. I only mention it as an unsolved difficulty, without implying that it is an insoluble one. If the extensor muscle of the flying lemur’s wing-membrane can, for instance, be shown to be homologous with some muscle in any other lemur, similar to those muscles by means of which the horse shakes his skin when tickled, it will be safe to infer that the two muscles are the same in origin, but modified for different work by habitual self-adaptation. These wing-membranes—at least in what I believe to be their initial stage—are, as I have already remarked, them- selves very simple organs, and also not in very close correlation with any other organs. But will natural selec- win | tion account for changes in organs which are so closely petal correlated together that no variation in one of them can be account for : onc ¢ closely cor- of any use to the organism unless it is accompanied by yelateil, or corresponding variations in all the rest? And will it complex account for the complex perfection of an organ like the a 1 eye or the ear, consisting of many closely correlated parts ? ' On this subject H. Spencer says :—“ The co-operative parts Quotation must vary together, otherwise variation will be detrimental. Hoan 3 } A stronger muscle must have a stronger bone to resist its Spencer. contractions ; must have stronger correlated muscles 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 spontane- ously increase or decrease together? The assumption that they do seems to me untenable.”? He goes on to show that if, for instance, the horns of a deer grow larger through 1 See especially the facts stated in the preceding chapter about respira- tory organs, and those in the chapter on Comparative Morphology on the homological relations of the limbs of Vertebrates. 2 Principles of Biology, p. 453. 318 Com- plexities of the eye and the ear not due to natural selection. Darwin on the sim- plest eyes. HABIT AND INTELLIGENCE. [cHap. spontaneous variation, and this, being advantageous to the deer by increasing its fighting power, is perpetuated by natural selection, it will be necessary for the muscles and bones of the neck and fore-legs to acquire increased strength in order to carry the increased weight of the horns ; and it will be almost infinitely improbable that the necessary changes should all occur together from spontaneous variation only. But I agree with H. Spencer, that such cases present no difficulty whatever. Let natural selection increase the size of the horns, and self-adaptation will produce the corresponding increase of strength in the muscles and bones. But what are we to say of the complexities of the eye and the ear? We have seen that they cannot have been produced by self-adaptation. Neither the action of light on the eye, nor any action of the eye itself, can have any tendency whatever to produce the deposit of black pigment that absorbs the stray rays, nor to shape the transparent humours into lenses, nor to form the iris and its nervous connexions. And the hypothesis of natural selection appears equally inapplicable. In speaking of the flying squirrel, I have shown that I think natural selection adequate to account for any amount of im- provement in a simple organ; but it does not follow that it will account for any improvement in a complex organ. H. Spencer’s reasoning to show that the increase in the strength of an animal’s neck and fore-legs at the very time when this is needed in order to carry heavier horns cannot be a result of mere spontaneous variation, applies, I think, with much greater force to so complex an organ as the eye. “The simplest organ which can be called an eye consists of an optic nerve, surrounded by pigment cells, covered with translucent skin, but without any lens or other refractive body. We may, however, according to M. Jourdain, descend even a step lower, and find agerega- tions of pigment cells, apparently serving as an organ of vision, but which rest merely on sarcodic tissue not fur- nished with any nerve.”’ Darwin remarks “that, as some 1 Darwin’s Origin of Species, p. 216. : XxIv.] NATURAL SELECTION. 319 of the lowest organisms, in which nerves cannot be detected, are known to be sensitive to light, it does not appear im- probable that certain elements in their tissues or sarcode should have become aggregated and developed into nerves endowed with special sensibility to its action.”! This is in accordance with all that we know of the process of development; and I believe as firmly as Darwin can do that the eyes of the highest animals, like everything else in the organic creation, have been brought to their present perfection by adding together a number of small improve- ments, continued through an immense number of gene- rations. But it does not follow that these improvements, even though almost infinitely small, can be due to natural selection among spontaneous variations. The higher the organization, whether of an entire organism or of a single organ, the greater is the number of the parts that co-operate, and the more perfect is their co-operation ; and consequently, the more necessity there is for corre- sponding variations to take place in all the co-operating parts at once, and the more useless will be any variation whatever, unless it is accompanied by corresponding Natural variations in the co-operating parts; while it is obvious eae that the greater the number of variations which are needed able to the highest in order to effect an improvement, the less will be the organiza- probability of their all occurring at once. It is no reply %™ to this to say, what is no doubt abstractly true, that what- ever is possible becomes probable, if only time enough be allowed. There are improbabilities so great that the youproties common sense of mankind treats them as impossibilities. eal fa It is not, for instance, in the strictest sense of the word, impossi- bility. impossible that a poem or a mathematical proposition should be obtained by the process of shaking letters out of a box; but it is improbable to a degree that cannot be distinguished from impossibility ; and the improbability of obtaining an improvement in an organ by means of several spontaneous variations all occurring together is an impro- bability of the same kind.? It is, of course, out of the question to find numerical data on such subjects, but the 1 Origin of Species, p. 215. 2 See Note at end of chapter. 320 Algebraic statement. HABIT AND INTELLIGENCE. [CHAP. reasoning admits of algebraic statement. If we suppose that any single variation occurs on the average once in am times, the probability of that variation occurring in any individual will be 1 m’ and suppose that x variations must concur in order to make an improvement, then the probability of the neces- sary variations all occurring together will be 1 me Now suppose, what I think is a moderate supposition, that the value of m is 1000, and the value of x is 10: then ds fn devia m= 1000" — 10” To many readers these numbers will have no meaning. But it will speak to the understanding, though it will batfle the imagination, to say that if these suppositions are true, the probability against the concurrence of such vari- ations as would constitute an improvement is expressed by a number which is about ten thousand times as great as the number of waves of light that have fallen on the earth since historical time began.! And it is to be further observed, that no improvement will give its possessor a certainty of surviving and leaving offspring, but only an extra chance, the value of which it is quite impossible to estimate.” Of course this argument falls to the ground if it can be shown that it is not necessary for more than one variation to take place at the same time in order to constitute an improvement. But I think this is inconsistent with the unquestionable fact, on which I have already insisted, that in the highest organization there are both the greatest 1 In 6,000 years there are, making no correction for leap years, 189,216 x 10° seconds. In one second there are 535 x 10? undulations of yellow light. The product of these two numbers is 101,230,560 x 10%, or not much more than 106, which is the ten-thousandth of 10°. 2 See Note at end of chapter. XXxIv.] NATURAL SELECTION. 321 number of co-operating parts, and the closest co-operation between the parts.’ It is to be observed, that the fore- going argument is greatly understated. I have spoken only Co-opera- of the co-operation of parts in one organ. But in nature 8 of parts in an we have also to do with the co-operation of organs in one organ, and organism; and most organs are complex; so that there of aie must be co-operation of many organs, each of them con- °'ganism. sisting of many parts. In some cases, no doubt, the prin- ciple of self-adaptation will account for one part becoming adapted to the rest, as we have seen in speaking of the deer’s horns and the bat’s wing-muscles ; but this will not be always the case. What greatly increases the difficulty The eye of supposing that the eye can have been formed by natural fs Pe selection is, that it has been formed not on one, but on three : three distinct lines of descent. Well-developed eyes ATC lites of found in the higher orders of Annulosa, Mollusca, and descent. Vertebrata. We need not now discuss the question, whether these three groups are descended from a common ancestor ; if they are, that ancestor must have been, in all probability, of too low an organization to have eyes. We may infer this from the facts that the lowest known ver- tebrate, the amphioxus, has only rudimentary eyes,’ and the lower Mollusca and Annulosa have none. The eye must consequently have been separately perfected in those three groups. It is a fact of the same class, that the Skulls of Cephalopoda (cuttle-fish and nautilus), which are the Sea highest of the Mollusca, resemble the Vertebrata not only © of oe in the general higher development of the nervous system ”” and of the eyes, but in the special and very remarkable 1 H. Spencer has given the emphasis to this argument that it deserves (see. the extract above). Yet he says, though without offering any proof, that the complexities of the sensory organs must be the result of natural selection. I cannot avoid thinking that he has been biassed by a deter- mination to assign a physical cause, such as either self-adaptation or natural selection, for all biological facts, to the exclusion of intelligence. (See his Principles of Biology, vol. ii. p. 307.) 2 “ An eye, such as exists in the fish called the lancelet (amphioxus), which is so simple that it consists only of a fold-like sac of skin, lined with pigment and furnished with a nerve, but destitute of any other apparatus, being merely covered by transparent membrane.” (Darwin’s Origin of Species, p. 218.) Yi 322 Striated muscular fibre in Annulosa and in Verte- brata. Sponta- neous variation and natural selection is a pro- cess of blind trial, and in- applicable to complex conditions. HABIT AND INTELLIGENCE. [ CHAP. character of having a skull formed of cartilaginous plates, to protect the cephalic gangha; and yet there is no true affinity between the Cephalopoda and the Vertebrata, by the test either of morphological anatomy, or of deve- lopment, or of a gradation of intermediate forms: so that the skulls of the two groups, like their eyes, must have been formed separately. Exactly the same remarks will apply to the fact, that both the Vertebrata and the higher Annulosa have their voluntary muscles formed of striated fibre, while the involuntary muscles of those groups, and all the muscles of the Mollusca, are non-striated.1_ If the Vertebrata and the Annulosa have been descended from a common ancestor, that ancestor must have been far too low in the scale to possess striated muscle ; and consequently that tissue must have been formed in those two groups in- dependently. The origin of tissue, however, is so totally unknown, that no argument drawn from it can be of much weight. I have already admitted that I do not see any absurdity in the idea of the muscular, and perhaps even the nervous tissues being, in some way, originally produced by a process of self-adaptation. Natural selection among spontaneous variations may be described, without a metaphor, as a process of experiment by mere blind trial, and preserving the results of the suc- cessful experiments, while the failures are destroyed and forgotten. I do not deny that great results have been ob- tained in this way. Such has been the case in some chemical arts; above all, I believe, in photography ; and it is by such a process of blind experimenting, without any acoustic theory to guide the experiments, that musical in- struments have attained to their present high degree of perfection. But this method is applicable only to cases where the process of experimenting is comparatively simple. Were it necessary for ten different experiments at once to be successful, in order that any result at all might be obtained, such a method would be totally inapplicable ; the improbability of success would be an improbability of 1 Carpenter’s Comparative Physiology, p. 442. Xxtv.] NATURAL SELECTION. 323 the same kind (to recur to a former illustration) as that of obtaining a poem by shaking letters at random out of a box. I conclude from these facts and reasonings, that the facts Summary. of organization may be accounted for in part by the direct action of external inorganic forces on the organism; in part by the action of the organism itself, producing self- adaptation ; and in part by natural selection among spon- taneous variations: but that in addition to, and in co- operation with, all these, there must be a principle of Organizing Intelligence. I shall defer the further con- sideration of the bearings of this conclusion to a future chapter, and shall end this with some observations on that very remarkable class of adaptations known as Imitative Resemblances. Many birds, insects, and other animals have colours re- Imitative sembling those of the objects among which they live. °°!" The purpose of this, beyond doubt, is protection against enemies: but what is the cause? Natural selection is the most obvious cause to assign, and in many cases it is pro- bably quite adequate :—those individuals which were coloured like the objects among which they lived, escaped their enemies oftener than the ones not so favoured, and consequently survived and bequeathed their colouring. This view is supported by the fact that imitative colouring is worn by no birds except those which inhabit exposed in birds, places, where they would be in danger from birds of prey.! But this cannot be the purpose of the white colour of the Polar bear, which has no enemies, at least on land; its in the white clothing must have been given to it for warmth, as rite white substances are those which radiate heat the slowest. Natural selection, however, will apply equally well to this ease; and if all cases of adaptive colouring were single, I should think this explanation would account for them all. But they are not all single ; there is the remarkable double 4; 41. fact of some animals, of which the ermine is the best-known ermine. 1 This is stated by the Duke of Argyll in “The Reign of Law.” ¥2 324 The cha- meleon, Minicry, Quotation from Darwin. Mr. Bates on mimic- ry among butterflies, HABIT AND INTELLIGENCE. [CHAP. instance, having a white covering in the winter, and one of another colour in the summer. I cannot think that this is a result of natural selection among spontaneous variations, because I cannot think that it could ever arise as a spon- taneous variation; I think it must be in some way due, fanciful as the notion may appear, to an action of the light reflected on the animal from the surfaces among which it lives. The same is perhaps true (though this is a very large concession to those who refer all the facts of organi- zation to the action of inorganic forces)—the same, I say, is perhaps true of the wonderful power possessed by the chameleon and a few other animals, of: assuming a colour similar to that of the neighbouring surface, what- ever that may chance to be. These, in my view, can scarcely be regarded as cases of self-adaptation: they belong rather to the class of adaptations produced by the direct action of external inorganic forces. But the fact of mimicry, as distinct from mere imitative colouring, can, I think, be accounted for by natural selec- tion, and by natural selection only. Mr. Bates, the author of “The Naturalist on the Amazons,” has shown “that in a district where, for instance, an ithomia abounds in gaudy swarms, another butterfly, namely a leptalis, will often be found mingled in the same flock, so ike the ithomia in every shade and stripe of colour, and even in the shape of its wings, that Mr. Bates, with his eyes sharpened by col- lecting during eleven years, was, though always on his guard, continually deceived. When the mockers and the mocked are caught and compared, they are found to be totally different in essential structure, and to belong not only to distinct genera, but often to distinct families. If this mimicry had occurred in only one or two instances, it might have been passed over as a strange coincidence. But travel a hundred miles, more or less, from a district where one leptalis imitates one ithomia, and a distinct mocker and mocked, equally close in their resemblance, — will be found. Altogether no less than ten genera are enumerated, which include species that imitate other butterflies. The mockers and mocked always inhabit the XXxIv.] NATURAL SELECTION. 325 same region ; we never find an imitator living remote from the form which it counterfeits. The mockers are almost invariably rare insects; the mocked in almost every case abound in swarms. In the same district in which a species of leptalis closely imitates an ithomia, there are sometimes other lepidoptera mimicking the same ithomia; so that in the same place, species of three genera of butterflies and even moths may be found, all closely resembling a species of a fourth genus. It deserves especial notice, that many of the mimicking forms of the leptalis, as well as of the mimicked forms, can be shown, by a graduated series, to be merely varieties of the same species ; whilst others are undoubtedly distinct species. But why, it may be asked, are certain forms treated as the mimicked, and the others as the mimickers? Mr. Bates satisfactorily answers this question, by showing that the form which is imitated keeps the usual dress of the group to which it belongs, whilst the counterfeiters have changed their dress, and do not resemble their nearest allies.” 4 The purpose of mimicry is the protection of the Its pur- mimickers. The species which they mimic, probably in fyotection. consequence of some odour that they emit, are not preyed on by birds; and of course it is a protection to the butter- flies which do not emit any such odour, to be mistaken for those which do. The mimicking forms have no doubt Its cause been produced by the survival, through successive genera- panty tions, of those individuals belonging to defenceless species, which most nearly resembled the kinds that have natural means of protection. For the same reason, insects that sting are never known to mimic others, though others sometimes mimic them.” 1 Darwin’s Origin of Species, p. 503. 2 Thid. p. 506. 326 Grey- hound. HABIT AND INTELLIGENCE. [CHAP. NOTE. FORMATION OF COMPLEX ORGANS. In answer to the objection that selection is inadequate to produce a co-ordinated structure of many parts, Darwin?’ points to the instance of the greyhound, all the parts of which are together “adapted for extreme fleetness, and the running down of weak prey ;” yet the greyhound has been formed by selection ; and the most competent selectors do not attempt to obtain all kinds of excellence at once, but try for one point at a time.” This instance is conclusive as to the correlation of characters, each of which may vary a little without necessitating variation in the rest. But I do not think it will apply to such an organ as the eye. On this subject Darwin says :?— ‘‘A writer has recently maintained that ‘it is probably no exaggeration to suppose that, in order to improve such an organ as the eye at all, it must be improved in ten different ways at once. And the improbability of any complex organ being im- proved and brought to perfection in any such way, is an impro- bability of the same kind and degree as that of producing a poem or a mathematical demonstration by throwing letters at random on a table.’* If the eye were to be abruptly and 1 Variation under Domestication, vol. ii. p. 221. 2 “The excellence of one sub-variety (of pigeon), the Almond Tumbler, lies in the plumage, carriage, head, beak, and eye; but it is too presump- tuous in the beginner to try for all these points.” The great judge above quoted (J. M. Eaton) says :—‘“ There are some young fanciers who are over-covetous, and go for all the above five properties at once: they have their reward by getting nothing.” (Variation under Domestication, vol. ii. p- 198.) Though these remarks are quoted of pigeons, Darwin evidently means them to apply to the improvement of the breeds of all animals. Compare what he says in p. 221: “ From what we know of the method which different men follow in improving their stock—some chiefly attend- ing to one point, others to another point, others again correcting defects by crosses, and so forth.” 3 Variation under Domestication, vol. ii. p. 222. 4 Darwin here quotes from my opening address to the Belfast Natural History Society, reported in the Northern Whig, Nov. 19, 1866. a xxiv.] FORMATION OF COMPLEX ORGANS. 327 greatly modified, no doubt many parts would have to be simul- taneously altered, in order that the organ should remain service- able. But is this the case with smaller changes?” He then speaks of the changes that would be needed in order to adapt the eye of a diurnal species for a nocturnal life—a case which presents no great difficulty ; and afterwards says :—‘‘ Amphibious animals, which are enabled to see both in the water and in the air, require and possess, as M. Plateau has shown, eyes con- Eyes of structed on the following plan: ‘The cornea is always flat, or at aD least much flattened in front of the crystalline and over a space animals. equal to the diameter of that lens,! while the lateral portions may be much curved.’ The crystalline is very nearly a sphere, and the humours have nearly the same density as water. Now, as a terrestrial animal became more and more aquatic in its habits, very slight changes, first in the curvature of the cornea or crystalline, and then in the density of the humours, or con- versely, might successively occur, and would be advantageous to the animal whilst under water, without serious detriment to its power of vision in the air.” I confess I do not see much force in this argument. The subject is most intricate, but I know of no reason for thinking it possible that any apparatus consisting of lenses can be im- proved by any method whatever, unless the alterations in the density and the curvature are perfectly simultaneous ; and the probability against this taking place by mere spontaneous variation is practically infinite. 1 The purpose of the flat cornea is evidently to produce as little differ- ence as possible in the way in which the rays of light enter the eye from air and from water. Origin of life in Creative Power. Only two theories are pos- sible as to the origin of species: separate creations, or develop- ment. Question raised by geological discovery. Difficul- ties of de- velopment theory partly are probably insoluble, CHAPTER XXV. GENERAL REMARKS ON THE DEVELOPMENT OF SPECIES. ie giving a brief summary of the argument on the origin of species, it is best to begin by repeating, that the origin of life is a distinct question. We have every reason to believe that life, like matter, has had its origin in the direct action of Creative Power. Accepting, then, the facts of life, with the laws of habit and variation, as primary data, the question is, how particular organic structures, and particular organic species, have come into existence. Let us take the question of the origin of species first, as indeed it includes the other. Only two answers to this question appear to be possible. Either every species (with of course great indefiniteness as to the question what are in point of fact real and distinct species),—either every species, I say, has been separately created, or all species, both animal and vegetable, have been derived, by descent with modification, from one, or at most a few, germs that were originally vitalized by Creative Power. So long as the world was believed to be only a few days older than man, the theory of separate creations was necessarily accepted. But when geological time was found to be indefinitely long, and when geology further disclosed the fact of a succession of species, the one set displacing the other and succeeding to it throughout geological time, the question of the possibility of a change of species through descent was immediately raised. Such an hypothesis is no doubt surrounded with difficulties; or, it would be more correct to say, such an hypothesis raises a vast number of questions, most of which are not yet solved, and may pro- GENERAL REMARKS ON DEVELOPMENT OF SPECIES. 329 bably remain ever unsolved. But all that we have learned, ee they of the variability of form and function in organisms, tends Lape to diminish the difficulties of this hypothesis; while all advance of that we have learned concerning the facts of development, tedie, embryology, morphology, and classification, tends to in- crease its probability; and that probability is, as I think, raised to the rank of a certainty by the discovery of rudi- mentary and useless structures, both in mature and in embryonic forms. It appears to be a prevalent notion, that, whatever may be hereafter concluded on as true, the presumption is at present in favour of the theory of separate creations, and will continue to be so until the argument is closed. I cannot admit this ; I think that it has no logical foundation No pre- whatever, and that it owes the force it has in men’s minds aa merely to the theory of separate creations being familiar ; develop- while the theory of the origin of species by descent =: ee modification (or, briefly, the development theory) is still comparatively strange. It is not to be hoped that the difficulties of the development theory can be cleared up at once, or that they can be fully cleared up at all. The evidence is too scanty, and the experimental method, which has created the sciences of physics and chemistry, is scarcely applicable here. But the ordinary objections to - the development theory are utterly futile. It is said that argu. we have never seen a new species come into existence by Beats ar descent. This is scarcely true, for many races of plants perience and animals have come into existence under domestication, *8%™s* which it is nearly if not quite impossible to distinguish from true species.1 But it is quite true that we have never seen any such change as the descent of a bird from a reptile would be; and yet the birds and the reptiles are both vertebrates, and must, therefore, on the development theory, be descended either the one class from the other, or both from a common origin. It is true, I say, that such a change is quite out of our experience; but the separate ne worth- and special creation of any organic form out of dust, or out !** of nothing, is equally outside of our experience, and far 1 See Note at end of chapter. 330 The argu- ment from experience is in its favour. Difficulties aboutman. Develop- ment theory applicable to man. Man’s brain not essentially unlike the ape’s. HABIT AND INTELLIGENCE. [CHAP. more remote from it ; for we have experience of the origin of races by descent with modifications, though very small modi- fications, but we have no experience at all of the origin of a race by special creation. I believe this kind of argument from direct experience, in the narrowest sense of the word experience, is good for very little ; but, such as it is, it is in favour of the development theory, and not against it. This is the place for referring to those difficulties of the development theory which appear to be keenly felt by many, in connexion with the origin of man. Man is developed, like all other organisms, out of a minute structureless germ, which cannot by itself be dis- tinguished from that which will develop into the form of any other species. Man is an air-breathing Vertebrate; I have brought forward what I believe to be conclusive proofs that all air-breathing Vertebrates have been de- scended from fishes; and if these arguments are good for other species, they are good for man also. Man is closely allied in his physical structure to the apes; the best autho- rities appear to be agreed with Professor Huxley, that there is no difference, either in the brain or in any other part of the body, between man and the apes that can be regarded as in any way fundamental. The superiority of man’s mental powers to those of the highest animal is no doubt so great that it may be regarded as infinite; but we have seen that motor and mental characters are so variable in comparison with formative ones, that they are no index whatever to the affinities of a species. As a question of biological science, consequently, I see no reason to doubt that man, like other species, has been developed out of lower forms. The repugnance to this belief rests on the notion that where there is gradual change there can be no funda- mental change of nature; and, consequently, that if man has been developed out of an ape, he is still essentially an ape. Now, without entering on any logical or metaphy- sical discussion, I meet this argument by saying that such is not the fact: I say that a gradual change by develop- ment may be a fundamental change. The logical, or meta- xxy.] GENERAL REMARKS ON DEVELOPMENT OF SPECIES. dol physical, difficulty of conceiving how such a being as man The deve- can be descended from an ape, the ape from a fish, and yume the fish from a Protozoon, is paralleled in the life of every from the human being, by the facts that the child, before it learns peer to speak, appears to have no higher mental nature than paralleled that of a dog; that for some considerable time after birth at ae, it appears to ‘have no mental nature at all; and that at ™dvidual. the first moment of its conception it is not only without mental nature, but without any higher organic nature than that of a Protozoon. The development of the individual is in the highest degree mysterious; but the mystery is only repeated, and the difficulty is not increased, if it is true, as I believe it is, that the development of the indi- vidual, from the structureless germ up to the man, has had its parallel in the development of the race. The relation of man’s spiritual nature to his animal nature is, no doubt, one of the greatest of all mysteries ; but the relation of life to matter is equally mysterious, though it is a lower kind of mystery. No merely physical science can elucidate a spiritual mystery like this; but, in my opinion, the dis- covery that man’s brain has no anatomical superiority to that of the highest apes, from which his mental superiority could possibly be guessed, is so far from lending support to a materialistic view of our spiritual nature, that it tends to cut away the ground from under any materialistic argument. I do not see any improbability in the belief Man’s that the same Creative Power which at the beginning ee created matter, and afterwards gave it life, finally, when the may be a action of that life had developed the bodily frame and ae ai the instinctive mental powers of man, completed the work Creative by breathing into man a breath of higher and spiritual life. ce The theory, or rather the law, of organic types has been The law of sometimes set up in opposition to the theory of develop- ae ment. The notion of any opposition between them, how- ever, is a pure misconception. By the law of types is meant the fact, which has been stated at greater length in the chapter on Morphology, that homologous parts are found through vast numbers of species—as, for instance, 532 HABIT AND INTELLIGENCE. [CHAP. through the whole vertebrate group ; and that when a new organ is needed—as, for instance, a wing instead of a leg— the type is not radically altered, but modified in such a way as to leave the same anatomical elements still dis- cernible. There are some who regard it as a sufficient answer to the question of the origin of species, to say that the Creator has laid down plans, or types, for the Verte- brata, and for each of the other great groups, and then modified the vertebrate type to suit the special modes of life of the fish, the quadruped, and the bird; and the other types in the same way. Now, as a statement of fact, this is perfectly true; but it is only a generalized statement of explained fact—it is no explanation. The development theory takes Deyithe , up the question at this point, admits the law of types as develop- true, and shows why it is true; namely, that homological gal resemblances are due to permet of descent. The deve- lopment theory does for the multitudinous facts of mor- phology, embryology, and classification, what Kepler’s laws did for the mass of facts previously known concerning the planetary motions: it shows how they are to be referred to simple and intelligible principles. But Kepler’s laws left unanswered the question of the agency by which the planetary motions were determined ; and the development The deve- theory, considered merely as such, leaves unanswered the pat question by what agency the development of species has theor does not been produced. Any theory that will explain this will do answer the ; question for the development theory what the Newtonian theory vig? of gravitation has done for Kepler's laws. Darwin develop- believes that he has discovered such a theory. I regard Thee his theory as not’ false, but totally insufficient; partly because, as I have shown at some length in the last chapter, I do not believe it will in any degree explain the more complex facts of organization ; partly because, while it throws very great light on the variation of forms, it throws little or none on the origin and variation of © tissues. I have not yet considered the geological evidence on the subject of the origin and succession of organic forms, I oe are xXxv.] GENERAL REMARKS ON DEVELOPMENT OF SPECIES. 339 believe the geological evidence is too scanty and frag- mentary to be of much value;! but what we have is cer- Geological tainly favourable to the belief that there has been, on the ae whole, an advance in grade of organization during geolo- be in gical time. Thus, reptiles have been superseded, to a great ae extent, by warm-blooded animals; the great marine rep- have given tiles have given place to whales and other Cetaceans, the W*Y,*° great land reptiles to Mammalia, and the Pterodactyles, or blooded flying reptiles, to birds. This, however, says nothing either aay for or against the theory of natural selection among spon- taneous variations as the principal cause of improvement. But I think that, on the whole, geological evidence is Geological against that theory. Were it true—were natural selection eee not only one cause of change, which it certainly is, but the Darwin’s principal cause—improvement ought to take place most te": rapidly in the classes where there is most variability, and least fixity of form: in other words, selection ought to do most where there are the most numerous and the greatest variations to select from. Now, variability as between individuals is greatest in the lowest classes: but this does not cause the production of new species to go on among them with any corresponding rapidity. Geologists, Improve- on the contrary, appear to be agreed that old species ey oes disappear, and new ones come to take their places, most rapidly in rapidly in the highest classes. I think this is an impor- oe eee tant argument in favour of believing that advance in development is due to some vital power which is most energetic in the highest classes, and not to any mere inorganic agency like natural selection. We have to account not only for the fact of improve- ment, or, in other words, for advance in grade of organiza- tion, but for the fact that improvement goes on in divergent Diver- . E : é gence of lines. The organic tree not only grows higher, but branches ieee out into classes. This, however, presents no difficulty, if it is admitted, as the development theory requires, that if 1 See the chapter, in Darwin’s Origin of Species, on the Imperfection of the Geological Record ; also “ Illogical Geology,” in the second volume of Herbert Spencer’s Essays. 334 Effect of isolation in pro- ducing change. The largest areas pro- duce the most dominant species, General effect of geological conditions. HABIT AND INTELLIGENCE. [cHAP. time enough is allowed variation is perfectly indefinite in amount, at least within the limits of the same fundamental form. Isolation alone appears to produce change. If a small colony of any species whatever were to be isolated from its kind, it can scarcely be doubted that after some generations it would come to constitute a distinct variety. Such cases have occurred under domestication ; and they must have been of constant occurrence in nature whenever some of the individuals of a species have been isolated from the rest by the formation of mountain or sea barriers. But it is on the largest areas obviously that there will be the largest number of chances of favourable variations occurring ; and there consequently those species will be perfected which will be most successful in the “struggle for existence.”2 Thus European and Asiatic species of plants are now overspreading Australia and New Zealand, while the Australian and New Zealand species, which have been introduced as garden plants, have little tendency to become wild in Europe. No doubt the most favourable state of things for the promotion of variation and advance- ment is that which geological conditions have produced,— constant, but generally slow changes in the conditions of life ; general severe competition, with many isolated areas in which it is less intense; colonies frequently shut off from the parent stock, and left alone to vary or to remain unchanged ; and species which had been matured in sepa- rate regions suddenly brought into competition by the removal of barriers. Every geological change will tend to 1 “Youatt gives an excellent illustration of the effects of a course of selection, which may be considered as unconsciously followed, in so far that the breeders could never have expected, or even have wished, to produce the result which ensued—namely, the production of two distinct strains. The two flocks of Leicester sheep kept by Mr. Buckley and Mr. Burgess, as Mr. Youatt remarks, ‘have been purely bred from the original stock of Mr. Bakewell for upwards of fifty years. There is not a suspicion existing in the mind of any one at all acquainted with the subject, that the owner of either of them has deviated in any one instance from the pure blood of Mr. Bakewell’s flock, and yet the difference between the sheep possessed by these two gentlemen is so great, that they have the appearance of being quite different varieties.’” (Darwin’s Origin of Species, p. 37.) 2 Tbid. p. 119. 8 og od. pi il ieee ee xxy.] GENERAL REMARKS ON DEVELOPMENT OF SPECIES. 335 promote change of the living species in different ways : by causing self-adaptation to the new conditions of life; by directly stimulating variation, and by selecting those variations which are best adapted to the new conditions. What I wish now to insist on is, that all this will neces- sarily produce “divergence of character;” for the new influences will most probably act on only part of the indi- viduals of a species, and among those which do come under the new influences it is probable that different variations will be adapted to different habits of life, and will be formed by hereditary habit and natural selection into permanent varieties or new species. Thus the constant tendency of Tendency change will be to produce new varieties; and it appears poe impossible that any retrograde change can reverse this, poset! or that it can be undone by any cause except their creater extinction. variation. It is to be remembered also that with any particular Correla- variation other variations will probably be correlated ; but pies veer of the laws of such correlations we know nothing. It can scarcely be doubted that the lowest organisms are Lowest the most plastic, and consequently the most capable of ev, assuming organization of a new type. It is probably for plastic. this reason that allied groups, as already remarked, are generally united by their lowest, and not by their highest members. In more technical language, the least differen- asses are tiated species have the greatest capacity for further dif- ee te ferentiation, and are capable of giving origin to widely their distinct groups above them. Thus, for instance, the lowest sewed air-breathing Vertebrates—the Batrachians—are allied not with the highest, but with some of the lowest fishes. The lowest fishes, on the development theory, have given origin to the higher fishes on the one side, and to the air-breathing Vertebrates on the other. But we cannot assert that this There may is universally the case. The affinities of birds, I believe, Pers are rather with the higher than with the lower reptiles ; this. and the same is probably true of the mammalia. Any intelligent man who has taken the trouble of fol- APpatent inconsis- lowing my reasoning so far will very probably make these tencies of 336 HABIT AND INTELLIGENCE. [cHap. my argu. two remarks—that I have first rejected natural selection ment. among spontaneous variations as a complete theory of the origin of species, and afterwards gone on as if I thought it true; and that I treat intelligence as a Deus ex machind, explaining vital phenomena, so far as I can, by merely physical causation, referring them to the action of external agents, self-adaptation, and natural selection, and only calling intelligence in as a solution when the physical causes fail to account for the facts. I shall reply to these two objections together. I believe, as I have more than once stated already (and I think I here state the universal belief of scientific men)—I believe, Life does I say, that the action of life does not supersede the ordi- ae nary physical laws of causation—or, in other words, does works not supersede the ordinary properties of matter—but ees > that life produces its results of organization by guiding pane i the action of causation, and by working through the ordinary properties of matter. So it is with intelligence. sointeli- As life works through- the inorganic forces, so intelli- shan. gence, I believe, works through the unintelligent forces. intelligent All the inorganic forces are unintelligent, and so, I forces: believe, are the laws of habit and variation. The rela- These tion of life to the inorganic forces is totally inexplicable, Asien and the relation of intelligence to the unintelligent forces plicable. is equally so; but the latter, though inexplicable, forms Wehave part of our mental experience. Our mental life is partly eae of intelligent and partly merely habitual. The intelligent the action and voluntary powers of the mind, as I shall have to Gee show in a future chapter, do not supersede the action of the habitual powers, but work through them ; and it is, I believe, the same in our unconscious or bodily life. Self adaptation takes place according to the laws of habit, and according to that remarkable physical law in virtue of which every part that has increased work thrown on it within the lirnits of what is good for its health, increases in size, in strength, and in general efficiency; but it appears to be universally admitted that there are many adaptations for which no mere blind habitual process of self-adaptation will account. Natural selection also is a eo xxv.] GENERAL REMARKS ON DEVELOPMENT OF SPECIES. 337 purely physical process, a case of purely physical causa- tion; but I have given reasons, which to my mind are conclusive, against accepting natural selection as an explanation of the complexities of organic structure. But I believe that these physical agencies, self-adaptation and natural selection, are what produce organization, and are Self- _ the operative causes of progress in organization ; not, ea however, acting alone, but under the guidance of intelli- by intelli- gence. I believe that intelligence guides the process of self-adaptation, producing adaptations which no unintelli- gent process could produce. I have no doubt the law of natural selection is universally operative, in this sense, that natural selection by “survival of the fittest”! in the struggle for existence is the reason why new and improved races, when once formed, supersede the old and unimproved ones; but I have stated my reasons for believing that spontaneous variation alone can never produce any com- plex organization. I believe that intelligence determines Intelli- those variations to occur together which are needed together. 5°7°° 3 determines To return to a former instance :—If I am right, such an co-ope- organ as the eye has been formed by a series of gradual ee successive improvements, in each of which those different Gein variations in the different parts of its complex structure which are necessary to make one another useful have been determined to occur together by the mysterious power of unconscious organic intelligence. But every improvement, when once made, has been preserved, perpetuated, and multiplied by the action of natural selection. I believe that organizing intelligence co-exists and co- Intelli- operates with the unintelligent forces through all life; but 74 most domi- that intelligence is most completely dominant in the highest nant in the life. It is where intelligence is most completely dominant He ie that, as we have seen, organic progress is most rapid; though there is least spontaneous variation among the most highly organized forms, and consequently it is among them that the least could be done by mere natural selec- tion among unguided spontaneous variations.2 And it is 1 This expression is used in Spencer’s Principles of Biology. 2 See p. 333. 338 Why does natural selection preserve the highest? Because the Bei been said already. The most highly organized beings have an are most efficient. HABIT AND INTELLIGENCE. {CHAP. also where intelligence is most completely dominant that we find those organs, such as the eye and the ear, of which the purpose is the most evident and the most definite, and which appear most completely beyond the power of any merely physical causation to construct. Purpose, as I remarked early in this work, is most traceable where cause is least so. In the first chapter of the second volume I shall*have to consider what this organizing intelligence is; and this will further open the question of the relation between the unconscious life and the mind. NOTE A. THE OPERATION OF NATURAL SELEOTION. 7 Tue first question of an intelligent man when he first hears the theory of the Origin of Species by means of natural selection among spontaneous variations, will very probably be something like this :—Granting its postulates, your theory no doubt ac- counts for the origin of different species, one from the other, by descent. But the most important and conspicuous fact in com- paring species and classes is not mere variation, but advance. __ Granting that natural selection among spontaneous variations is adequate to effect the transition from the Protozoa up to the highest warm-blooded animals, the question still remains, why the changes it effects are in this direction? Why does natural selection, or “ survival of the fittest,” on the average and on the whole, preserve those variations that constitute advance in | organization, and destroy the retrograde ones ? The answer to this is obvious, and is contained in what has advantage over others in the struggle for existence. Strength © of muscle, perfection of the organs of motion and prehension, acuteness of the nerves of sense, perfection of the eye and ear, and increased development of the nervous centres producing mental intelligence,—all these constitute advance in grade of organization, and all at the same time give their possessor an — xxv.] THE OPERATION OF NATURAL SELECTION. 339 increase of power to fight the battle of life, and consequently an increased chance of surviving and leaving offspring. This, however, is only an average result, not a uniform or constant one. For instance, if an animal becomes internally or externally Excep- parasitic on other animals, it will have little or no occasion for tions. acute senses or great motor power; its organs of sense and Retro- motion will consequently degenerate, and the whole organism, see taken altogether, will undergo a retrograde change; while the great facility of obtaining abundant nourishment which its parasitic life affords, will cause the variation to be a favourable one, and it will be perpetuated. Such has no doubt been the Suctorial origin of the “suctorial parasites” on fishes and whales, into P@tsites. which many freely swimming crustacean larve are metamor- phosed. But such cases are distinctly exceptional. As a rule, natural selection will preserve only those variations which con- stitute an advance in organization, and the rest will perish. Here another question arises, which Darwin has not seen, but which H. Spencer has seen and satisfactorily answered! The ability of an individual, or of a species, to survive in the battle of life no doubt depends on its organization ; and the chances Chance of are, on the whole, in favour of the highest organization. But oe the probability of its not only surviving but leaving offspring, partly does not depend on this exclusively ; it depends partly on its ee os organization, but partly on its fecundity. Other things being fecundity. equal, the highest organization will have the best chance. But, other things being equal, the greatest fecundity will have the best chance. The chance that any variety will have of being preserved by natural selection will be in a ratio compounded of its organization and its fecundity. The advantage that fecundity gives to a species may be seen Rabbit by comparing the rabbit with the hare. The hare would pro- 4 bt. bably be by this time extinct in the cultivated parts of our country, were it not preserved, while the rabbit has no diffi- culty in maintaining its position; a difference which may be partly due to the burrowing habits of the latter, but much more, _ I think, to its great fecundity. To resume the argument. Fecundity and high organization High do not in general accompany each other, but the reverse. The °8*™4* 1 What follows is in substance taken from Spencer’s Principles of Biology, Part VI., especially chapters ix. and xi. The algebraic statement of the reasoning is my own. zZ2 340 tion and fecundity are opposed. HABIT AND INTELLIGENCE. [CHAP. highest organisms have on the whole the least fecundity. Com- pare the comparatively small number of young produced by birds and Mammalia with the thousands of eggs produced by some fishes and many insects, and the countless germs of the Bearing of lowest animals and plants. Might it not be expected, that this law on natural selection. Algebraic statement, whatever is gained in the chance of leaving offspring by ad- vanced organization will on the average be lost by diminished fecundity ; that the greater fecundity of the inferior types will give them as good a chance of leaving offspring as the superior ones ; and that the superior and the inferior species will thus be in the long run evenly matched in the battle of life, giving the superior ones no permanent advantage ? This would be so if all other things were equal. But all other things do not remain equal. The advantage of any favourable variation to a race consists generally either in greater facility of obtaining food, or in greater facility of escaping enemies ; and either abundant food or tranquillity of life will tend to increase fecundity. So that advance in grade of organization, though it will entail diminished fecundity as its direct effect, will tend to counteract this effect in an indirect way, namely by placing the organism in external circumstances favourable to its fecundity. In such a question, verification by observation or experiment is totally out of the question. But the above reasoning, con- sidered merely as reasoning from what data we have, is, 1think, sound and satisfactory. It may be well to put it into an algebraic form. Call grade of organization x, and fecundity y. Suppose, what is approximately true, that the chance of surviving and leaving offspring is proportionate to their product xy; and suppose, what is also approximately true, that, other things being equal, the values of # and y are inversely as each other; then, other things being equal, zy will be a constant quantity, and no increase in the value of « from any favourable variation will increase the chance of leaving offspring. Call now abundance of nutrition a. Within limits, the value of y, as already shown, will increase with that of a, and the value of a will increase with that of «; so that any increase of the value of x from spontaneous variation will tend to increase that of y, and con- sequently of the product xy; while, on the contrary, no increase of the value of y will have any tendency to increase that of either x or xy. 7 eA Pee ee xv.] THE OPERATION OF NATURAL SELECTION. 341 NOTE B. Ar the time when the foregoing chapter was written, I was The not aware of the fact mentioned by Mr. Lewes in an article on donee Darwin’s theory in the Yortnightly Review, that the domestic Guinea-pig and the wild Guinea pig are not fertile together. Mutual fer- oe tility is the usually accepted test of identity of species ; so that together. by this test these two are distinct species, Variation is slow: butI think races have been, and are, improved. I am inclined here to pp eer differ from Darwin. I think it most likely that, in many maintains. cases, species have been formed at once by considerable Possible sudden origin of new species. Such has occurred under domesti- cation. Poppy: Datura tatula. CHAPTER XXVI. THE RATE OF VARIATION. T is a necessary result of any form of the develop- ment theory, indeed I may say a part of it, that the process of variation by which any great change in organic forms has been effected has been extremely slow: so slow as to be measurable only by geological time. Darwin constantly insists on this. Of course it is impossible to doubt that such a change as that involved in the descent of warm-blooded animals from fishes, or of winged insects from worms, must have occupied immeasurably long geological ages. But Darwin goes much further than this ; he thinks that all species whatever have been formed by a process of variation not more rapid than that by which, in most cases, our domestic a oe wy ay , neat Eee Le eee Ee ate a ee variations ; variations not comparable to that which would be necessary to derive an air-breathing animal from a water-breathing one, but amounting to the sudden forma- tion of new species and new genera. In this belief there is nothing inconsistent with the laws of life. Variations, equal in magnitude to the pro- duction of new species, do occur under domestication. I have mentioned, in the chapter on the Laws of Variation, a case of a new variety of the poppy appearing suddenly, which had “a remarkable variation in its fruit, a crown of secondary capsules being added to the normal central capsule:” and a similar case of the datura tatula, present- — CHAP. XXVI.] THE RATE OF VARIATION, 343 ing a variety with smooth fruit instead of spinous! Here are two cases of variations amounting to specific differ- ences, which appeared suddenly, and were propagated by seed: they are in fact cases of the production of new species, not by gradual, but by sudden variation, and they would be recognised as such if they had appeared in the wild state. The otter, or Ancon, sheep of North America Ancon was also the result of a sudden variation ; and the differ- ree ence in the form of its skeleton from that of the common sheep amounted to a specific if not a generic difference. Thus we see that sudden as well as gradual variation may give rise to what are really new species, both of plants and animals. It is true, these instances, and others that could be mentioned,? have arisen under domestication ; but why may not the same have taken place in the wild state? Darwin thinks they do not: “ Various general reasons,” he says, “could be assigned against such a belief: for instance, without separation a single monstrous variation would almost certainly be soon obliterated by erossing.’* I think this difficulty is quite imaginary. We know that vegetable species and varieties usually propagate their kind truly ; and among animals, once a race has been formed, the instinct of every race to unite with its own Instinct kind will, in the wild state, prevent crossing. Besides, ee even if they unite, it is by no means certain that their crossing. union would produce a mixed race, having characters Crossing intermediate between the two parent stocks. When two aad slightly different individuals unite, the offspring, no dowbt, mixedrace. have on the average an intermediate character; and thus it is that where free intercrossing is permitted, separate breeds do not usually arise in the same country. But this is not always the case when the union is between indi- ; viduals of very unlike races. “When the Ancons (or ; 1 P. 197. a 2 “Tt is certain that the Ancon and Manchamp breeds of sheep, and 4 almost certain that the niata cattle, turnspit and pug dogs, jumper and “ frizzled fowls, short-faced tumbler pigeons, hook-billed ducks, &c. and 4 with plants a multitude of varieties, suddenly appeared in nearly the same state as we now see them.” (Darwin’s Variation under Domestication, vol. ii. p. 414.) * 3 Ibid. 344 HABIT AND INTELLIGENCE. [cHap. otter sheep) are crossed with other breeds, the offspring, with rare exceptions, perfectly resemble either parent.”} . For these reasons, I think there is no improbability in 4 sudden, as well as slow, variations occurring among | organisms in the wild state ; though, as wild organisms | are much less variable than domestic ones, they must occur much more rarely.2. It may be true that we have no evidence of the origin of wild species in this way. But this is not a case in which negative evidence proves any- Sudden thing. We have never witnessed the origin of a wild oe species by any process whatever; and if a species were to eee come suddenly into being in the wild state, as the Ancon covered, Sheep did under domestication, how could we ascertain the fact? If the first of a newly-begotten species were found, the fact of its discovery would tell nothing about its origin. Naturalists would register it as a very rare species, having been only once met with, but they would have no means of knowing whether it were the first or the last of ew ee oe ee possible, but probable. And I further doubt whether the ee ey development theory is tenable unless we admit it. I am variation. Inclined to think that geological time is too short for the evolution of the higher forms of life out of the lower by that accumulation of imperceptibly slow variations, to which alone Darwin ascribes the whole process. ~ This will surprise many readers. It is almost as start- ling now to be told that geological time is less than infinite, as it would have been a hundred years ago to be told that historical time was but a small fraction of the age of the: earth. Nevertheless, the reasoning which has been briefly stated in the chapter on “the Motive Powers of the Lae ae Universe,” proves that the order of things on the earth earth must have had a beginning; and Sir William Thomson according has roughly estimated the past duration of the solar to Sir W. Thomson. system in its present state at about five hundred millions | its race. Geological I consequently think that sudden variation is not only ; 1 Darwin’s Variation under Domestication, vol. i. p. 100. 2 “Sports” are extremely rare under nature, but far from rare under domestication. (Darwin’s Origin of Species, p. 10.) xxvi.] THE RATE OF VARIATION. 345 of years; previous to which time it was condensing out of its original nebula. Of course this calculation does not make the slightest pretension to precision, but all that is needed for the present argument is the order of the mag- nitude Five hundred millions of years is a period so overpowering to the imagination, that it is no wonder if most readers think it equivalent for all practical purposes to eternity. I think, however, that for the present purpose this is very doubtful. Darwin justly mentions the greyhound as being equal The grey- to any natural species in the perfect co-ordination of its ne a parts, “all adapted for extreme fleetness and for running species ; down weak prey.”” Yet it is really an artificial species,’ formed by long-continued selection under domestication ; and there is no reason to suppose that any of the varia- tions which have been selected to form it have been other than gradual and almost imperceptible. Suppose that it has taken five hundred years to form the greyhound produced out of his wild wolf-like ancestor: this is a mere guess, ain ee but it gives the order of the magnitude. Now, we have by slow seen that the past duration of the earth has been about beat five hundred millions of years, or only one million times greater than this ; yet during that period ali those changes have taken place by which the highest animal—let us say the elephant—has been developed out of a Protozoon ; or, if we admit the hypothesis of a separate ancestor for the vertebrate and for every other fundamentally distinct form, during that period the elephant has been developed out of the earliest and most lowly organized fish. Now, if it takes five hundred years to obtain the present race of greyhounds from a wolf-like ancestor by an accumulation of slow variations, how long would it take to obtain an Loe ae elephant from a Protozoon, or even from a tadpole-like production 1 See the article on Darwin’s theory, in the North British Review for June 1867. 2 Darwin on Variation under Domestication, vol. ii. p. 221. 3 This expression will be objected to. But, as I have already stated, I think Darwin, in his Origin of Species, has proved that the distinction between species and variety is only one of degree. 346 of the highest forms from the lowest require, by the same process ? Variation is slower among wild than tame races. How selec- tion will act in the wild state. HABIT AND INTELLIGENCE. [CHAP. fish? Ought it not to take much more than a million times as long? I ought perhaps to repeat, that if accuracy of numerical statement here is unattainable, it is also unnecessary ; what we have to compare is not the magni- tudes themselves, but only the orders of the magnitudes. But the argument, as stated here, is much too favourable to the theory of none but imperceptibly slow variations ; for it compares variation under domestication with varia- tion in a state of nature, as if it were equally rapid in both. And this is not the case: on the contrary, we may say, without much risk of exaggeration, that variability is the rule among domesticated races, but the exception among wild ones. Wild races remain unchanged through- out whole geological ages, though, I have no doubt, they become variable under the influence of changes of habitat, climate, and food; when these changes take place, new varieties and new species, more suitable than the old ones to the new conditions of life, will be formed by the conjoint action of self-adaptation and natural selection. But, so long as variation takes place so slowly as not to produce new races at once (which in Darwin’s opinion is always the case in the wild state), the formation of distinct races in the same habitat will be prevented by intercrossing, and it will be impossible for the processes of change to do more than to modify the whole race together, so as to suit the new conditions of life This will not prevent the action of “natural selection among spontaneous varia- tions,” in giving origin to new species, but it must tend greatly to restrict its operation so long as the variations are so small as not to give origin at once to new species, or strongly marked races. The absence of mixtures of race in the wild state will tend to prevent variation, for, as we have seen, pure races are less variable than mixed ones. 1 Tf some individuals of a race were so far to change their habits as not to be brought into contact with their kindred,—as, for instance, in the case of becoming nocturnal instead of diurnal,—they would, no doubt, give origin to a new race. But such a change of habit comes to the same thing as a change of country. XxXv1.] THE RATE OF VARIATION. 347 Thus there will be less variation in the wild state than in the domestic ; and such variations as occur will be much less likely to be kept separate so as to produce modified races. For, as we have seen, the improvement of domestic breeds depends altogether on separation, and would be impossible if the animals were permitted freely to obey their instincts as to breeding. It is true that instinct tends to keep well-established breeds distinct, but this is not the case when they are only beginning to diverge. To recapitulate the foregoing argument : Geological time is only about a million, or a few million, Summary. times as long as the period needed to form what is really a new species by the accumulation of small variations under domestication. Variation goes on much less rapidly in the wild than in the domestic state, and favourable variations, when they occur, are much less likely to be so preserved as to produce a modified race. We shall probably be greatly within the mark if we assume that variation is ten times less rapid in the wild state than in the domestic, and that the chance of any favourable variation being so preserved as permanently to modify the race is ten times less. If these numbers are correct, then the efficiency of selection among small spontaneous variations in producing new races 1s one hundred times as great under domestication as in the state of nature. If this is so, then five hundred millions of years of variation and selection under nature are equivalent to only five millions under domestication. And if it takes five hundred years, or anything approach- ing to that period, to form a race like the greyhound out of a wild dog, it can scarcely be maintained that five millions of years of change at the same rate would suffice to form an elephant out of a fish. Five millions are only ten thousand times five hundred, and the proportion between those two changes is certainly measurable by no such number as ten thousand. For these reasons, I believe there must have been sudden variations, amounting to the origin of new species all at once; not widely different in any case, perhaps, from the 348 How far I agree with Darwin. HABIT AND INTELLIGENCE. [cHaAP. old ones, but sufficiently different either not to mix with others, or to preserve their distinctness (like the Ancon sheep) in spite of intercrossing. It may, perhaps, be asked whether I am after all a follower of Darwin or not. This question cannot be answered by a mere yes or no. I agree with the theory of which Darwin is not the founder but the best known and most original exponent, that all organic species have been descended from one or a few germs. I believe that Darwin has done most important service to science by pointing out the importance of natural selection among spontaneous variations, as a cause of organic progress. But I do not agree with him that it is almost the only cause. I believe that geological time is too short to admit of the progress that has taken place, unless variation is a much more rapid process than Darwin admits. I believe (as stated in the preceding chapter) that the facts of variability being greatest in the lowest organisms, while progress has been most rapid among the higher ones, shows that there is something in organic progress which mere natural selection among spontaneous variations will not account for. Finally, I believe this something is that Organizing Intelligence which guides the action of the inorganic forces, and forms structures which neither natural selec- tion nor any other unintelligent agency could form. ee ee Ce aR TT ane tee Te XXVI.] THE RATE OF VARIATION. NOTE. I rivp that, taking Sir William Thomson as my authority, I have greatly understated the argument of the foregoing chapter. 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