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Show 130 CAUSES WlliCII DETERMINE TilE [Cl1. VIII. its foes, w1. th every other extrm. s1•c cause, remaine.d unaltered ' then might we have some groun d f,o r suspectm.g that the infirmi.t i.e s of age creep on as na t u rally on specie. s as upon m. dI' VI'd ua 1s . B u t I. n tl1 e ab sence of such observa. tions, let .u s turn to another class of f acts, an d exa mine attent.i vely the. ctr-cums t ances w1 1 I.C h d et ermi'ne the stations of p. articula. r . am. ma]s and plants, and perhaps we shall discover, m the VlClSSltudes to which these stations are exposed, a cause fully adequate to e~plain the phenomena under consideration. . Stations comprehend all the circumstances, whether relatmg ta the animate or inanimate world, which determine whether a given plant or animal can exist in a given. locality, .so that if it be shown that stations can become essentially modified by the influence of known causes, it will follow that speeies, as well as individuals, are mortal. Every naturalist is familiar with the fact, that although in a particular country, such as Great Britain, there m~y be more than three thousand species of plants, ten thousand msects, and a great variety in each of the other classes, yet there ~ill n.ot be more than a hundred, perhaps not half that number, tnhalnt. in(J' any O'iven locality. There may be no want of space in the b b • supposed tract; it may be a large mountain, or an extenstve moor, or a great river-plain, containing room enough f~r individuals of every species in our island; yet the spot w11l be occupied by a few to the exclusion of many~ an.d the~ f~w are enabled, throughout long periods, to ma1~tam t~e1r ground successfully against every int.ruder, not~1thstandmg the facilities which species enjoy, by virtue of their powers of diffusion, of invading adjacent· territories. The principal causes which enable a certain assemblage of plants thus to maintain their ground against all others .dcpe~d, as is well known, on the relations between the phys10logtcal nature of each species, and the climate, exposure, soil, a~d other physical conditions of the locality. Some plants hve only on rocks, others in meadows, a third class in marshe.s. Of the latter, some delight in a fresh-wa~er morass,-others 111 Ch. VIII.] STATIONS OF ANIMALS AND PLANTS. 131 salt marshes, where their roots may copiously absorb saline particles. Some prefer an alpine region in a warm latitude, where, during the heat of summer, they are constantly irrigated by the cool waters of melting snows. To others loose sand, so fatal to the generality of species, affords the most proper station, The Carex arenaria and the Elymtts arena- 1·ius acquire their full vigour on a sandy dune, obtaining an ascendency over the very plants which in a stiff clay would immediately stifle them. Where the soil of a district is of so peculiar a nature that it is extremely favourable to certain species, and agrees ill with every other, the former get exclusive possession of the ground, and, as in the case of heaths, live in societies. In like manner, the Bog moss (lfypnum palustre) is fully developed in peaty swamps, and becomes, like the heath, in the language of botanists, a social plant. Such monopolies would be very frequent, if the powers of ~ great number of species were not equally balanced, and if animals did not interfere most actively to preserve an equilibrium in the vegetable kingdom. "All the plants of a given country," says Decandolle in his usual spirited style, "are at war one with another. The first which establish themselves by chance in a particular spot, tend, by the mere occupancy of space, to exclude other species-the greater choke the smaller, the longest livers replace those which last for a shorter period, the more prolific gradually make themselves masters of the ground, which species mu1ti .. plying more slowly would otherwise fill." In this continual strife, it is not always the resources of the plant itself which enable it to maintain or extend its ground. Its success depends, in a great measure, on the number of its foe~ or alli~s among the animals and plants inhabiting the same reg10.n. ~bus, for example, a herb which loves the shade may multiply, If some tree with spreading bouO'hs and dense foliage fl . b our1sh in the neighbourhood. Another, which, if unassisted, wo~ld b~ overpowered by the rank growth of some hardy competltoJ ·, IS secure, because its leaves are unpalatable to cattle, K2 |
