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Show 66 CHECKS TO INCREASE. [CHAP, III. wedges packed close together and driv~n inwards by incessant blows, somethnes one wedge bmng struck, and then another with greater force. What checks the natural tendency of each species to increase in nun1ber is n1ost obscure. Look at the Inost vio-orous species · by as much as it swarms in numbers, by so much will 'its tendency to increase be still furth~r increased. We know not exactly what the checks are In even one sin o-le instance. Nor will this surprise any one who · reflects 0 how ignorant we are on this head, even in reo-ard to mankind, so incomparably better known than any other animal. This subje?t has been ably treat~d by several authors and I shall, In my future work, discuss some of the che~ks at considerable length, 1nore especially in regard to the feral anhnals of South America. Here I will make only a few remarks, just to recall to the reader's mind soine of the chief points. Eggs or very young animals seem generally to suffer most, b~t this is not invariably the case. With plants the:e IS a. vast destruction of seeds, but, from some observations whwh I have made I believe that it is the seedlings which suffer Inost fron1 germinating in ground already thickly stocked with other plants. Seedlings, also,. are destr~yed ~n vast numbers by various enemies; for Instance, on a piece of ground three feet long and two wide, dug and cleared, and where there could be no choking from other plants, I marked all the seedlings of our native weeds as they came up and out of the 357, no less than 295 were destroyed, chiefly by slugs and insects. If turf which has long been mown and the case would be the same with turf closely brows~d by quadrupeds, be let to grow, the more vigorous plants gradually kill the less vigorous, though fully grown, plants: thus out of twenty species growing on a little spot of turf (three feet by four) nine species perished from the other species being allowed to gro'! up freely. . The amount of food for each spemes of course gives the extreme limit to which each can increase; but very frequently it is ~ot the o~taining fo?d, but the serving as prey to other animals, which determines th~ average numbers of a species. Thus, there seems to be httle doubt that CnAP. III.] CHECKS TO INCREASE. 61 the stock of partridges, grouse, and hares, on any large estate, depends chiefly on the destruction of vermin. _(f not on~ head of game were shot during the next twentyyears In England, and, at the same time, if no vermin were destroyed, there would, in all probability, be less game than at present, although hundreds of thousands of game animals are now annually killed. On the other hand, in some cases, as with the elephant and rhinoceros, non~ are destroyed by beasts of prey : even the tiger in lnd1a n1ost rarely dares to attack a young elephant protected by its dam. 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 rnost effective of all checks. I estimated that the winter of 1854-55 destroyed four-fifths of the birds in my own ()'rounds· and this is a tremendous destruction, when vve r~rr1embe;. that ten. per. cent: is an extraordina;·ily severe 1nortality from epidemics With man. The action of climate seems at .:first sight t~ be quite ind.ependent of the struggle for existence; but 1n so far as chmate chiefly acts in reducing ~oo~, _it brings 0~1 the most severe struggle between the Individuals, whether of the same or of distinct species "rhich subsist on the same kind of food. Even when eli~ 1nate, for instance 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. Wl~en we travel fr?m s?uth to north, or from a damp !'egi?n to a dry, we Invariably see some species O'radually getting rarer and rarer, and finally disappearing C: and the cha~ge of climate being conspicuous, we are te~pted to attribute 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 iife, fron1 enemies or from co1npetitors for the same place and food ; and if these enemies or c01npetitors be in the least degree favoured by any slight change of climate, they will increase in numbers, and, as each area is already fully stocked with inhabitants, the other species will decrease. When |