marck has elaborated the same view in detail: using for the pm'pose, with great ingenuity, his extensive knowledge of the animal kingdom. From a passage in the Avertlssement, it would at first sight seem, that he looks upon direct adapt- ation to new conditions, as the chief cause of evolution. He says — "Je regardai comme certain que le mouvement des fluides dans I'interieur des animaux, mouvement qui c'est progressivement accelere avec la composition plus grande de Torganisation; et que Vinfluence des circonstances nouvelles, k mesure que les animaux s'y exposerent en se repandant dans tons les lieux habitables, furent les deux causes g^ne- rales qui ont amene les differens animaux a I'^tat ou nous les voyons actuellement." But elsewhere, the view he expresses appears decidedly different from this. He asserts that "dans sa marche, la nature a commence, et recoromence encore tons les jours, par former les corps organises les plus simples; " and that " les premieres ebauches de I'animal et du vegetal etant forme es dans les lieux et les circonstances convenables, les facultes d'une vie commencante et d'un mouvement or- ganique etabli, ont necessairement developpe pen a peu les organes, et qu'avec le temps elles les ont diversifies ainsi que les parties." And then, further on, he puts in italics this proposition: — " La progression dans la composition de Vor- 408 THE EVOLrTIOiN OF LIFE.
ganisation suhit, ^d et Id, dans la serie generate des animaux, des anomalies operees /par Vinfluence des cir Constances d'hdbi- tation, et par celle des habitudes contractees.'' These, and sundry other passages, joined with his general scheme of classification, make it clear that Lamarck conceived adaptive modification to be, not the cause of progression, but the cause of irregularities in progression. The inherent tend- ency which organisms have, to develop into more perfect forms, would, according to him, result in a uniform series of forms; but varieties in their conditions work divergences of structure, which break up the series into groups: groups which he nevertheless places in uni- serial order, and regards as still substantially composing an ascending succession.
§ 147. These speculations, crude as they may be considered, show much sagacity in their respective authors, and have done good service. Without embodying the truth in a de- finite shape, they contain adumbrations of it. Not directly, but by successive approximations, do mankind reach correct conclusions; and those who first think in the right direction, loose as may be their reasonings, and mde of the mark as their inferences may be, yield indispensable aid by framing provisional conceptions, and giving a bent to inquiry.
Contrasted with the dogmas of his age, the idea of De Maillet was a great advance. Before it can be ascertained how organized beings have been gradually evolved, there must be reached the conviction that they have been gradu- ally evolved; and this conviction he reached. His wild notions as to the way in which natural agencies acted in the production of plants and animals, must not make us forget the merit of his intuition that animals and plants wei^e pro- duced by natural causes. In Dr Darwin's brief ex- position, the belief in a progressive genesis of organisms, is joined with an interpretation having considerable definite- ness and coherence. In the space of ten pages he not only indicates several of the leading classes of facts which support HOW TS ORGANTC EVOTJTTTON CAUSED 409 the hypollicsis of evolution, but he does something towards elucidating tlie process of evolution. Ilis reasonings show us an unconscious mingling of the belief in a supernaturally- impressed tendency to develop, with the belief in a develop- ment arising from the changing incidence of conditions. Probably had he pursued the inquiry further, this last belief would have gro^\Ti at the expense of the first. Lamarck, in elaborating this general conception, has given greater precision to both its truth and its error. Asserting the same imaginary factors and the same real factors, he has traced out their supposed actions in detail; and has, in con- sequence, committed himself to a greater number of un- tenable positions. But while, in trying to reconcile the facts with a theory which is only an adumbration of the truth, he laid himself open to the criticisms of his con- temporaries; he proved himself profounder than his con- temporaries, by seeing that evolution, however caused, has been going on. If they were wise in not indorsing a theory which fails to account for a great part of the facts; they were unwise in ignoring that degree of congruity with the facts, which shows the theory to contain some fundamental verity.
Leaving out, however, the imaginary factors of evolution which these speculations allege, and looking only at the one actual factor which Dr Darwin and Lamarck assign as accounting for some of the phenomena; it is manifest from our present stand-point, that this, so far as it is a cause of evolution, is a proximate cause and not an ultimate cause. To say that functional adaptation to conditions, produces either evolution in general, or the irregularities of evolution, is to raise the further question — why is there a functional adaptation to conditions? — why do use and disuse generate appropriate changes of structure? ^N^either this nor any other interpretation of biologic evolution which rests simply on the basis of biologic induction, is an ultimate interpretation. The biologic induction must itself be interpreted. Only when 410 THE EVOLUTION OF LIFE.
the process of evolution of organisms, is affiliated on the process of evolution in general, can it be truly said to be explained. The thing required is to show that its various results are corollaries from first principles. We have to reconcile the facts with the universal laws of the re- distribu- tion of matter and motion.
CHAPTER IX.
EXTERNAL FACTORS.
§ 148. When illustrating the rhythm of motion {First PrincijjleSy § 94) it was pointed out that besides the daily and annual alternations in the quantities of light and heat which any portion of the Earth^s surface receives from the Sun, there are alternations which require immensely- greater periods to complete. Reference was made to the fact, that *' every planet, during a certain long period, presents more of its northern than of its southern hemisphere to the Sun at the time of its nearest approach to him; and then again, during a like period, presents more of its southern hemisphere than of its northern — a recurring co-incidence which, though causing in some planets no sensible alterations of climate, in- volves in the case of the Earth an epoch of 21,000 years, during which each hemisphere goes through a cycle of tem- perate seasons, and seasons that are extreme in their heat and cold." Further, it was pointed out that there is a varia- tion of this variation. The slow rhythm of temperate and in- temperate climates, which takes 21,000 years to complete, itself undergoes exaggeration and mitigation, during epochs that are far longer. The Earth's orbit slowly alters in form: now approximating to a circle; and now becoming more eccentric. During the period at which the Earth's orbit has least eccentricity, the temperate and intemperate climates which repeat their cj^cle in 21,000 years, are 412 THE EVOLUTION OF LIFE.
severally less temperate and less intemperate, than when, some one or two millions of years later, the Earth's orbit has reached its extreme of eccentricity.
Thus, besides those daily variations in the quantities of light and heat received by organisms, and responded to by varia- tions in their functions; and besides the annual variations in the quantities of light and heat which organisms receive, and similarly respond to by variations in their functions; there are variations that severally complete themselves in 21,000 years and in some millions of years — variations to which there must also be a response in the changed functions of organisms. The whole vegetal and animal kingdoms, are subject to a quadruply- compounded rhythm in the in- cidence of the forces on which life primarily depends — a rhythm so involved in its slow working round, that at no time during one of these vast epochs, can the in- cidence of these forces be exactly the same as at any other time. To the direct effects so produced on organisms, have to be added much more important indirect effects. Changes of distribution must result. Certain redistributions are occasioned even by the annual variations in the quantities of the solar rays received by each part of the Earth's surface. The migrations of birds thus caused, are familiar. So too are the migrations of certain fishes: in some cases from one part of the sea to another; and in some cases from salt water to fresh water. Now just as the yearly changes in the amounts of light and heat falling on each locality, yearly extend and restrict the habitats of many organisms that are able to move about with some rapidity; so must these alternations of temperate and intemperate climates produce extensions and restrictions of habitats. These extensions and restric- tions, though slow, will be universal — will aifect the habitats of stationary organisms as well as those of locomotive ones. For if during an astronomic era, there is going on at any limit to a plant's habitat, a diminution of the winter's cold or summer's heat, which had before stopped its spread at EXTERNAL FACTORS. 413 that limit; then, though the individual plants arc fixed, yet the species will move: the seeds of plants living at the limit, will produce individuals that survive beyond the limit. The gradual spread so effected, having gone on for some ten thousand years, the opposite change of climate will begin to cause retreat: the tide of each species will during the one half of a long epoch, slowly flow into new regions, and then will slowly ebb away from them. Further, this rise and fall in the tide of each sj)ecies, will, during far longer intervals, undergo increasing rises and falls and then de- creasing rises and falls. There will be an alternation of spring tides and neap tides, answering in its period to the changing eccentricity of the Earth's orbit.
These astronomical rhythms, therefore, entail on organisms unceasing changes in the incidence of forces in two ways. They directly subject them to variations of solar influences, in such a manner that each generation is somewhat differently affected. in its functions; and they indirectly bring about complicated alterations in the environing agencies, by carry- ing each species into the presence of new physical conditions.
§ 149. The power of geological actions to modify every- where the circumstances in which plants and animals are placed, is conspicuous. In each locality, denudation slowly uncovers different deposits; and slowly changes the exposed areas of deposits already uncovered. Simultaneously, the alluvial beds that are being formed, are qualitatively affected by these progressive changes in the natures and proportions of the strata denuded. The inclinations of surfaces and their directions with respect to the Sun, are at the same time altered; and the organisms existing on them are thus having their thermal conditions continually altered, as well as their drainage. Igneous action, too, complicates these gradual modifications. A flat region cannot be step by step thrust up into a protuberance, without unlike climatic changes being produced in its several parts, by their exposures to dif- 414 THE EVOLUTION OF LIFE.
ferent aspects. Extrusions of trap, wherever they take place, revolutionize the localities; both over the areas covered, and over the areas on which their detritus is left. And where volcanoes are formed, the ashes they occasionally send out, modify the character of the soil throughout large sur- rounding tracts.
In like manner alterations in the Earth's crust, cause the ocean to be ever subjecting the organisms it contains to new combinations of conditions. Here the water is being deep- ened by subsidence, and there shallowed by upheaval. While the falling iipon it of sediment brought down by neighbour- ing large rivers, is raising the sea-bottom in one place; in another, the habitual rush of the tide is carrying away the sedunent previously deposited. The mineral character of the submerged surface on which sea- weeds grow and molluscs crawl, is everywhere occasionally changed: now by the bringing away from an adjacent shore some previously un- touched strata; and now by the accumulation of organic remains, such as the shells of pteropods or of foraminifera. A further series of alterations in the circumstances of marine organisms, is entailed by changes in the movements of the water. Each modification in the outlines of neighbouring shores, makes the tidal streams vary their directions or velocities, or both. And the local temperature is from time to time raised or lowered, because some far-distant re-ar- rangement of the Earth's crust, has wrought a divergence in those circulating currents of warm and cold water which pervade the ocean.
These geologically-caused changes in the physical charac- ters of each environment, occur in ever-new combinations, and with ever-increasing complexity. As already shown {First Principles, § 118), it follows from the law of the multiplication of effects, that during long periods, each tract of the Earth's surface increases in heterogeneity of both form and substance. Hence plants and animals of all kinds, are, in the course of generations, subject by these alterations in the crust of the EXTERNAL FACTORS. 415 Earth, to sots of incident forces which dilTcr from previous sets, both by changes in the proportions of the factors, and, occasionally, by the addition of new factors.
§ 150. Variations in the astronomical conditions joined with variations in the geological conditions, bring about variations in the meteorological conditions. Those extremely slow alternations of elevation and subsidence, which there is reason to believe take place over immense areas, here pro- ducing a continent where once there was a fathomless ocean, and there causing wide seas to spread where in a long past epoch there stood snow-capped mountains, gradually work great atmospheric changes. AVliile yet the highest parts of an emerging surface of the Earth's crust, exist as a cluster of islands, the plants and animals which in course of time migrate to them, have climates that are peculiar to small tracts of land surrounded by large tracts of water. As, by successive upheavals, greater areas are exposed, there begin to arise sensible contrasts between the states of their peripheral parts and their central parts: the sea and land breezes, which daily moderate the extremes of temperature near the shores, cease to affect the interiors; and the interiors, less qualified too in their heat and cold by such ocean-currents as bathe the shores, acquire more decidedly the characters due to their latitudes. Along with the further elevations which unite the members of the archipelago into a continent, there come new meteorologic changes, as well as exacerbations of the old. The winds, which were comparatively uniform in their directions and periods when only islands existed, grow involved in their distribution, and widely- different in dif- ferent parts of the continent. The quantities of rain which they discharge and of moisture which they absorb, vary everywhere according to the proximity to the sea and to surfaces of land having special characters.
Other complications result from variations of height above the sea: elevation producing a decrease of heat and conse- 410 THE EVOLUTION OF LIFE.
quently an increase in the precipitation of water — a precipit- ation that takes the shape of snow where the elevation is very great, and of rain where it is not so great. The gather- ing of clouds and descent of showers around mountain tops, are familiar to every tourist. Inquiries in the neighbouring valleys, prove that within distances of a mile or two the recurring storms differ in their frequency and violence. Nay, even a few yards off, the meteorologic conditions vary in such regions: as witness the way in which the condensing vapour keeps eddying round on one side of some high crag, while the other side is clear; or the way in which the snow- line runs irregularly to many different heights, in all the minor valleys and ravines and hollows of each mountain side.
Climatic variations that are thus geologically produced, being compounded with those which result from the slow astronomical changes; and no correspondence existing be- tween the geologic and the astronomic rhythms; it results that the same plexus of actions never recurs. Hence the incident forces to which the organisms of every locality are exposed by atmospheric agencies, are ever passing into un- paralleled combinations; and these are on the average ever becoming more complex.
§ 151. Besides changes in the incidence of inorganic forces, there are equally continuous, and still more involved, changes in the incidence of forces which organisms exercise on one another. As before pointed out (§ 105), the plants and animals inhabiting each locality, are held together in so entangled a web of relations, that any considerable modifica- tion which one species undergoes, acts indirectly on many other species; and eventually changes, in some degree, the circumstances of nearly all the rest. If an increase of heat, or modification of soil, or decrease of humidity,' causes a par- ticular kind of plant either to thrive or to dwindle; an unfavourable or favourable effect is wrought on all such competing kinds of plants, as are not immediately influenced EXTERNAL FACTORS. 417 in the same way. The animals which eat the seeds or browse ' on the leaves either of the plant primarily afFccted or those of its competitors, are severally altered in their states of nutri- tion and in their numbers; and this change presently tells on various predatory animals and parasites. And since each of these secondary and tertiary changes, becomes itself h centre of others; the increase or decrease of each species, produces waves of influence which spread and reverberate and re-reverberate, throughout the whole Flora and Fauna of the locality.
More marked and multiplied still, are the ultimate effects of those causes which make possible the colonization of neigh- bouring areas. Each intruding plant or animal, besides the new inorganic conditions to which it is subject, is subject to organic conditions considerably different from those to which it has been habituated. It has to compete with some organ- isms unlike those of its preceding habitat. It must preserve itself fi-om enemies not before encountered. Or it may meet with a species over which it has some advantage greater than any that it had over the species it was previously in contact with. Even where migration does not bring it face to face with new competitors or new enemies or new prey, it inevitably experiences new proportions among these. Further, an expanding species is almost certain to invade more than one adjacent region. Spreading north or south, it will come among the plants and animals, here of a ^ level district and there of a hilly one — ^here of an inland tract, and there of a tract bordered by the sea. And while differ- ent groups of its members will thus expose themselves to the actions and re-actions of different Floras and Faunas, these different Floras and Faunas will simultaneously have their organic conditions changed by the intruders.
This process becomes gradually more active and more complicated. Though in particular cases, a plant or animal may fall into simpler relations with the living things around, than those it was before placed in; yet it is manifest that, 418 THE EVOLUTION OF LIFE.
on the average, the organic environmerLts of organisms have been increasing in heterogeneity. As the niunber of species with which each species is directly or indirectly implicated, multiplies, each species is oftener subject to changes in the organic actions which influence it. These more frequent changes severally grow more involved. And the corre- sponding reactions affect larger Floras and Faunas, in ways increasingly complex and varied.
§ 152. When the astronomic, geologic, meteorologic, and organic agencies that are at work on each species of organ- ism, are contemplated as becoming severally more compli- cated in themselves, and at the same time as co-operating in ways that are always more or less new; it will be seen that throughout all time, there has been an exposure of organisms to endless successions of modifying causes which gradually acquire an intricacy that is scarcely conceivable. Every kind of plant and animal may be regarded as for ever pass- ing into a new environment — as perpetually having its relations to external circumstances altered, either by their changes with respect to it when it remains stationary, or by its changes with respect to them when it migrates, or by both.
Yet a further cause of progressive alteration and compli- cation in the incident forces, exists. * All other things con- tinuing the same, every additional faculty by which an organism is brought into relation with external objects, as well as every improvement in such faculty, becomes a means of subjecting the organism to a greater nimiber and variety of external stimuli, and to new combinations of external stimuli. So that each advance in complexity of organization, itself becomes an added source of complexity in the incidence of external forces.
Once more, every increase in the locomotive powers of animals, increases both the multiplicity and the multiformity of the actions of things upon them, and of their reactions EXTERNAL FACTORS. 419 upon things. Doubling a creature's activity, quadruples the area that comes >vithin the range of its excursions: thus augmenting in number and heterogeneity, the external agencies which act on it during any given interval.
By compounding the actions of these several orders of factors, there is produced a geometric progression of changes, increasing with, immense rapidity. And there goes on an equally rapid increase in the frequency with which the combinations of the actions are altered, and the intricacies of their co-operations enhanced.
CHAPTER X.
CHAPTER X.
INTERNAL FACTORS.
§ 153. We saw at the outset (§§ 10 — 16), tliat organic matter is built up o£ molecules so extremely unstable, that tbe slightest variation in their conditions destroys their equilibrium; and causes them either to assume altered structures or to decompose. But a substance which is beyond all others changeable by the actions and reactions of the forces liberated from instant to instant within its own mass, must be a substance that is beyond all others change- able by the forces acting on it from without. If their composition fits organic aggregates for undergoing with special facility and rapidity those re- distributions of matter and motion whence result individual organization and life; then their composition must make them similarly apt to undergo those permanent re-distributions of matter and mo- tion which are expressed by changes of structure, in corre- spondence with permanent re-distributions of matter and motion in their environments.
Already in First Principles, when considering the phe- nomena of Evolution in general, the leading characters and causes of those changes which constitute organic evolution, were briefly traced. Under each of the derivative laws of force to which the passage from an incoherent, indefinite homogeneity to a coherent, definite heterogeneity, conforms, were given illustrations drawn from the metamorphoses of INTERNAL FACTORS. 421 living bodies. Here it will be needful to contemplate the several resulting* processes as going on at once, in both individuals and species.
§ 154. Our postulate being that organic evolution in ge- neral commenced with homogeneous organic matter, just as the evolution of indi\'idual organisms commences, we have first to remember that the state of homogeneity is an un- stable state (First Principles, § 109). In any aggregate "the relations of outside and inside, and of comparative nearness to neighbouring sources of influence, imply the re- ception of influences that are unlike in quantity or quality, or both; and it follows that unlike changes will be produced in the parts thus dissimilarly acted upon." Further, ''if any given whole, instead of being absolutely uniform through- out, consists of parts distinguishable from each other — if each of these parts, while somewhat unlike other parts, is uniform within itself; then, each of them being in unstable equilibrium, it follows that while the changes set up within it must render it multiform, they must at the same time render the whole more multiform than before; " and hence, '' whether that state with which we commence be or be not one of perfect homogeneity, the process must equally be towards a relative heterogeneity." This loss of homogeneity which the special instability of organic aggre- gates fits them to display more promptly and variously than any other aggregates, must be shown in more numerous ways in proportion as the incident forces are more numerous. Every differentiation of structure being a result of some difference in the relations of the parts to the agencies acting on them, it follows that the more multiplied and more unlike the agencies, the more varied must be the differentiations wrought. Hence the gravitation from a state of homogeneity to a state of heterogeneity, will be conspicuously sho^ii in proportion as the environment is complex. This transition from a uniform to a multiform state, must con- 422 THE EVOLUTION OF LIFE.
tinue through successive individuals. Given a series of or- ganisms, each of which is developed from a portion of a preceding organism, and the question is, whether, after exposure of the series for a million years to changed incident forces, one of its members will be the same as though the incident forces had only just changed. To say that it will, is implicitly to deny the persistence of force. In relation to any cause of divergence, the whole series of such organisms may be considered as fused together into a continuously- existing organism; and when so considered, it becomes manifest that a continuously-acting cause will go on working a continuously-increasing effect, until some counteracting cause prevents any further effect.
But now if any primordial organic aggregate, must, in itself and through its descendants, gravitate from uniformity to multiformity, in obedience to the more or less multiform forces acting on it; what must happen if these multiform forces are themselves ever undergoing slow variations and complications? Clearly the process, ever- advancing towards a temporary limit but ever having its limit removed, must go on unceasingly. On those structural changes wrought in the once homogeneous aggregate by an original set of in- cident forces, will be superposed further changes wrought by a modified set of incident forces; and so on throughout all time. Omitting for the present those circumstances which check and qualify its consequences, the instability of the homogeneous must be recognized an ever-acting cause of organic evolution, as of all other evolution.
While it follows that every organism, considered as an in- dividual and as one of a series, tends thus to pass into a more heterogeneous state; it also follows that every species, con- sidered as an aggregate of individuals, tends to do the like. Throughout the area it inhabits, the conditions can never be absolutely uniform: its members must, in different parts of its area, be exposed to different sets of incident forces. Still more decided must this difference of exposure be when INTERNAL FACTORS. 423 its members spread into other habitats. Those expansive and repressive energies which set to each species a limit that perpetually oscillates from side to side of a certain mean, are, as ^we lately saw, frequently changed by new combinations of the external factors — astronomic, geologic, meteorologic, and organic. Hence there from time to time arise lines of di- minished resistance, along which the species flows into new localities. Such portions of the species as thus migrate, are subject to circumstances markedly contrasted with its average circumstances. And from multiformity of the circumstances, must come multiformity of the species.