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Show 64 HIGH RATE OF INCREASE. (CHAP. III. favourable to them during two or three following se.ason~. Still more striking is the evidence from our domestic animals of many kinds which have run wild in sev~r.al parts of the world : if the statements of the rate of Increase of slow-breeding cattle and horses in South Ameri~a, and latterly in Australia had not been well authenticated, they would have be~n quite incredible. So it is with plants : cases could be given of introduced plants w.hich have become common throughout whole Islands In a period of less than ten years. Several of the plants now most numerous over the wide plains of La Plata, clothing square leagues of surface ahnost to the exclusion of all other plants, have .been introduc~d fro!ll Europe ; and there are plants wlnch now range Ill India, as I hear from Dr. Falconer, fron1 Cape Comorin ~o th~ Hi~ala:y:a, which have been imported fi·om Amenca since Its discovery. In such cases, and endless instances could be given, no one supposes that the fertility of these animals or plants has been suddenly and temporaTil:y in?reased in any .s?nsible degree. The obvious explanation IS that the conditions of life have been very favourable, and that there has consequently been less destruction of the old and young, and that n.early all the young have been enabled to breed. In such cases the geon1etrical rati~ ?f inc.rease, the r~sult of ·which never fails to be surpnsing, simply explains the extraordinarily rapid increase and wide diffusion of naturalised productions in their new homes. In a state of nature almost every plant produces seed, and amongst animals there are very few which do not annually pair. lienee we. may c?n:fidently assert, that .all pla~ts and animals are tending to Increase at a geo~etn?al ra~w, that all would most rapidly stock every station In whiCh they could any how exist, and that the ge~metrical tende~cy to increase must be checked by destruction at so~11e P.enod of life. Our familiarity with the larger domestic anur~als tends, I think, to mislead us : we see no great destructwn falling on them, and we forget that thousands are annually slaughtered for food, and that in a state of nature an equal number would have somehow to be disposed of. The only difference between organisms which annually OnAP. III.] ITIGH RATE OF INCREASE. 65 produce eggs or seeds by the thousand, and those which prod?-ce extren1ely few, is, that the slow-breeders 'vould require a few more years to people, under favourable conditions, a whole district, let it be ever so large. The cond?r lays a couple of eggs and the ostrich a score, and yet In the same country the condor 1nay be the more nuinerous of the two : the Fulmar petrel lays but one egg, yet it is believed to be the 1nost numerous bird in tho ':orld. One fly deposits hundreds of eggs, and another, hke the hippobosca, a single one; but this difference does not determine how 1nany individuals of the two species can be supported in a district. A large number of ego-s is of some importance to those species, which depend on::> a rapidly fluctuating amount of food, for it allows them rapidly to increase in number. But the real importance of a large number of eggs or seeds is to 1nake up for much destruction at some period of life; and this period in the great majority of cases is an early one. If an animal can in any way protect its own eggs or young, a s:mall number may be produced, and yet the average stock be fully kept up; but if many eggs or young are destroyed many Inust be produced, or the species will beQome ex~ tinct. It would suffice to keep up the full number of a tree, which lived on an average for a thousand years if a single seed were produced once in a thousand years 'supposing that this seed were never destroyed, and could be ensured to germinate in a fitting place. So that in all cases, the average number of any animal depends only indirectly on the number of its eggs or seeds. In looking at Nature, it is most necessary to keep the foregoing considerations always in n1ind-never to forget that every single organic being around us may be said to be striving to the utmost to increase in numbers ; that each lives by a struggle at some period of its life; that heavy destruction inevitably falls either on the young or old, during each generation or at recurrent intervals. Lighten any check, mitigate the destruction ever so little, and the number of the species will almost instantaneously increase to any amount. The face of Nature may be compared to a yielding surface, with ten thousand sharp |