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.