# The Principles of Biology Vol. I/Chapter III.9

CHAPTER IX.
EXTERNAL FACTORS.
§ 148. When illustrating the rhythm of motion (First Principles, § 83) 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 coincidence which, though it causes in some planets no sensible alterations of climate, involves, in the case of the Earth, an epoch of 21,000 years during which each hemisphere goes through a cycle of temperate seasons, and seasons that are extreme in their heat and cold." Further, we saw that there is a variation of this variation. The slow rhythm of temperate and intemperate 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 in which the Earth's orbit has least eccentricity, the temperate and intemperate climates which repeat their cycle in 21,000 years, are 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 variations 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 responses in the changed functions of organisms. The whole vegetal and animal kingdoms, are subject to quadruply-compounded rhythms in the incidence of the forces on which life primarily depends—rhythms so involved in their slow working round that at no time during one of these vast epochs, can the incidence of these various 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; in some cases from salt water to fresh water; and in some cases from fresh water to salt 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 which are able to move about with some rapidity; so must the alterations of temperate and intemperate climates produce extensions and restrictions of habitats. These, though slow, must be universal—must affect the habitats of stationary organisms as well as those of locomotive ones. For if, during an astronomic era, there is going on at any limit to a plant's habitat, a diminution of the winter's cold or summer's heat, which ​had before stopped its spread at that limit; then, though the individual plants are fixed, yet the species will move: the seeds of plants living at the limit, will produce individuals which 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 one half of a long epoch, slowly flow into new regions, and then will slowly ebb away from them. Further, this rise and fall in the tide of each species will, during far longer intervals, undergo increasing rises and falls and then decreasing rises and falls. There will be an alteration of spring tides and neap tides, answering 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 carrying each species into the presence of new physical conditions, new soil and surface.
§ 149. The power of geological actions to modify everywhere 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 in course of formation, 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 modified; 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 different aspects. Extrusions of trap, wherever they take place, revolutionize the localities; both over the areas covered and over the areas on to which their detritus is carried. And where volcanoes are formed, the ashes they occasionally send out modify the character of the soil throughout large surrounding 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 deepened by subsidence, and there shallowed by upheaval. While the falling upon it of sediment brought down by neighbouring large rivers, is raising the sea-bottom in one place, in another the habitual rush of the tide is carrying away the sediment deposited in past times. 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 untouched 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 change of form in 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 characters of each environment, occur in ever-new combinations, and with ever-increasing complexity. As already shown (First Principles, § 158), 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. So that plants and animals of all kinds are, ​in the course of generations, subjected by alterations in the crust of the Earth, to sets of incident forces differing 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 slow alternations of elevation and subsidence which take place over immense areas, here producing a continent where once there was a fathomless ocean, and there causing wide seas to spread where in a long past epoch there stood snow-capped mountains, gradually work great atmospheric changes. While 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 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 approach the coast, 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 different 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 consequently an increase in the precipitation of water—a precipitation which takes the shape of snow where the elevation is very great, and of rain where it is not so great. The gatherings of clouds and descents 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 meteorological 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 snowline runs irregularly to different heights, in all the hollows and ravines of each mountain side.
As climatic variations thus geologically produced, are compounded with those which result from slow astronomical changes; and as no correspondence exists between 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 unparalleled 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 modification 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 particular 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 in the same way. The animals which eat the seeds or browse on the leaves, either of the plant primarily affected or those of its competitors, are severally altered in their states of nutrition and in their numbers; and this change presently tells on various predatory animals and parasites. And since each of these secondary and tertiary changes becomes itself a centre of others, the increase or decrease of each species produces waves of influence which spread and reverberate and re-reverberate throughout the whole Flora and Fauna of the locality.
More marked and multiplied still, are the ultimate effects of those causes which make possible the colonization of neighbouring areas. Each intruding plant or animal, besides the new inorganic conditions to which it is subject, is subject to organic conditions different from those to which it has been accustomed. It has to compete with some organisms unlike those of its preceding habitat. It must preserve itself from enemies not before encountered. Or it may meet with a species over which it has some advantage greater than any 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 both north and south, or east and west, 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 different groups of its members will thus expose themselves to the actions and reactions 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, on the average, the organic environments of organisms have been advancing in heterogeneity. As the number of species with which each species is directly or indirectly implicated, multiplies, each species is oftener subject to changes in the organic actions which influence it. These more frequent changes severally grow more involved. And the corresponding reactions affect larger Floras and Faunas, in ways increasingly complex and varied.
§ 152. When the astronomic, geologic, meteorologic, and organic agencies which are at work on each species of plant and animal are contemplated as becoming severally more complicated in themselves, and as co-operating in ways that are always partially 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 scarcely conceivable. Every kind of plant and animal may be regarded as for ever passing 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 complication in the incident forces, exists. All other things continuing the same, every additional faculty by which an organism is brought into relation with external objects, as well as every improvement in such faculty, becomes a means of subjecting the organism to a greater number and variety of external stimuli, and to new combinations of external stimuli. So that each advance in complexity of organization, itself becomes an added source of complexity in the incidence of external forces.
Once more, every increase in the locomotive powers of animals, increases both the multiplicity and the multiformity of the actions of things upon them, and of their reactions ​upon things. Doubling a creature's activity quadruples the area that comes within the range of its excursions; thus augmenting in number and heterogeneity, the external agencies which act on it during any given interval.
By compounding the actions of these several orders of factors, there is produced a geometric progression of changes, increasing with immense rapidity. And there goes on an equally rapid increase in the frequency with which the combinations of the actions are altered, and the intricacies of their co-operations enhanced.