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Show 40 DARWINIAN A. species can increase; but very frequently it is ~ot the o~taining of food · but the serving as prey to other ammals, whlCh de~ ' termine' s the average numbers of spec1• es. 1)- ( p. 68.) "Climate plays an important part in determining the average numbers of a species, and periodical seasons of extreme cold or drought I believe to be the most effective of aU checks. I estimated that the winter of 1854-'55 destroyed four~fifths of the birds in my own grounds; and this is a tremendous destruc~ tion when we remember that ten per cent. is an extraordinarily sev~re mortality from epidemics with man. The action of climate seems at first sight to be quite independent of the struggle for existence ; but, in so far as climate chiefly acts in reducing food, it brings on the most severe struggle between the individuals, whether of the same or of distinct species, which subsist on the same kind of food. Even when climate, for in~ stance extreme cold, acts directly, it will be the least vigorous, or those which have got least food through the advancing winter, which will suffer most. When we travel from south to north, or from a damp region to a dry, we invariably see some species gradually getting rarer and rarer, and finally disappearing; and, the change of climate being conspicuous, we are tempted to at~ tribute the whole effect to its direct action. But this is a very false view; we forget that each species, even where it most abounds, is constantly suffering enormous destruction at some period of its life, from enemies or from competitors for the same place and food; and if these enemies or competitors be in the least degree favored by any slight change of climate, they will increase in numbers, and, as each area is already stocked with inhabitants, the other species will decrease. When we travel southward and see a species decreasing in numbers, we may feel sure that the c:mse lies quite as much in other species being favored as in this one being hurt. So it is when we travel northward, but in a somewh.at lesser degree, for the number or species of all kinds, and therefore of competitors, decreases north ward; hence, in going north ward, or in . ascending a mountain we far oftener meet with stunted forms, due to the directly in' jurious action of climate, than we do in procee dm' g THE ORIGIN OF SPECIES. 41 southward or in descending a mountain. When we reach the arctic regions, or snow~capped summits, or absolute deserts, the struggle for life is almost exclusively with the elements. "That climate acts in main part indirectly by favoring other species, we may clearly see in the prodigious number of plants in our gardens which can perfectly well endure our climate, but which never become naturalized, for they cannot compete with our native plants, nor resist destruction by our native animals.'' -(pp. 68, 69.) After an instructive instance in which ''cattle absolutely determine the existence of the Scotch fir," ' we are referred to cases in which insects determine the existence of cattle: "Perhaps Paraguay offers the most curious instance of this; for here neither cattle, nor horses, nor dogs, have ever run wild, though they swarm southward and northward in a feral state; and Azara and Rengger have shown that this is caused by the greater number in Paraguay of a certain fly, which lays its eggs in the na~els of these animals when first born. The in~ crease of these flies, numerous as they are, must be habitually checked by some means, probably by birds. Hence, if certain insectivorous birds (whose numbers are probably regulated by hawks or beasts of prey) were to increase in Paraguay, the flies would decrease-then cattle and · horses would become feral, and this would certainly greatly alter (as indeed I have observed in .parts of South America) the vegetation; this, again, would, largely affect the insects; and this, as we have just seen in Staffordshire, the insectivorous birds, and so onward in ever~ increasing circles of complexity. We began this series by in~ sectivorous birds, and we bad ended with them. Not that in Nature the relations can ever be as simple as this. Battle within battle must ever be recurring with varying success ; and yet in the long~run tho forces are so nicely balanced that the face of Nature remains uniform for long periods of time, though assuredly the mei·est trifle would often give the victory to one organic being over another . . Nevertheless, so profound is our |