PDF - Wallace Online
PDF - Wallace Online PDF - Wallace Online
24 NATURAL SELECTION nthe more luxuriant prairies and pampas of America. Thegreater or less fecundity of an animal is often considered tobe one of the chief causes of its abundance or scarcity ;but aconsideration of the facts will show us that it really has littleor nothing to do with the matter. Even the least prolific ofanimals would increase rapidly if unchecked, whereas it isevident that the animal population of the globe must bestationary, or perhaps, through the influence of man, decreasing.Fluctuations there may be but ; permanent increase, except inrestricted localities, is almost impossible. For example, ourown observation must convince us that birds do not go onincreasing every year in a geometrical ratio, as they woulddo were there not some powerful check to their naturalincrease. Very few birds produce less than two young oneseach year, while many have six, eight, or ten ;four willcertainly be below the average ;and if we suppose that eachpair produce young only four times in their life, that willalso be below the average, supposing them not to die eitherby violence or want of food. Yet at this rate how tremendouswould be the increase in a few years from a single pair!simple calculation will show that in fifteen years each pair ofbirds would have increased to nearly ten millions lwhereas!we have no reason to believe that the number of the birds ofany country increases at all in fifteen or in one hundred andfifty years. With such powers of increase the populationmust have reached its limits, and have become stationary, ina very few years after the origin of each species. It isevident, therefore, that each year an immense number ofbirds must perish as many in fact as are born and as on;the lowest calculation the progeny are each year twice asnumerous as their parents, it follows that, whatever be theaverage number of individuals existing in any given country,twice that number must perish annually, a striking result, butone which seems at least highly probable, and is perhapsunder rather than over the truth. It would therefore appearthat, so far as the continuance of the species and the keepingup the average number of individuals are concerned, largebroods are superfluous. On the average all above one become1 This is under estimated. The number would really amount to morethan two thousand millions !A
ii ON THE TENDENCY OF VARIETIES, ETC. 25food for hawks and kites, wild cats or weasels, or perish ofcold and hunger as winter comes on. This is strikinglyproved by the case of particular species; for we find thattheir abundance in individuals bears no relation whatever totheir fertility in producing offspring.Perhaps the most remarkable instance of an immense birdpopulation is that of the passenger pigeon of the UnitedStates, which lays only one, or at most two eggs, and is saidto rear generally but one young one. Whyis this bird soextraordinarily abundant, while others producing two .or threetimes as many young are much less plentiful The ? explanationis not difficult. The food most congenial to this species,and on which it thrives best, isabundantly distributed overa very extensive region, offering such differences of soil andclimate, that in one part or another of the area the supplynever fails. The bird is capable of a very rapid and longcontinuedflight, so that it can pass without fatigue over thewhole of the district it inhabits, and as soon as the supply offood begins to fail in one place is able to discover a freshfeeding-ground. This example strikingly shows us that theprocuring a constant supply of wholesome food is almost thesole condition requisite for ensuring the rapid increase of agiven species, since neither the limited fecundity nor the unrestrainedattacks of birds of prey and of man are heresufficient to check it. In no other birds are these peculiarcircumstances so strikingly combined. Either their food ismore liable to failure, or they have not sufficient power ofwing to search for it over an extensive area, or during someseason of the year it becomes very scarce, and less wholesomesubstitutes have to be found ;and thus, though more fertilein offspring, they can never increase beyond the supply offood in the least favourable seasons.Many birds can only exist by migrating, when their foodbecomes scarce, to regions possessing a milder, or at least adifferent climate, though, as these migrating birds are seldomexcessively abundant, it is evident that the countries theyvisit are still deficient in a constant and abundant supply ofwholesome food. Those whose organisation does not permitthem to migrate when their food becomes periodically scarce,can never attain a large population. This is probably the
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ii ON THE TENDENCY OF VARIETIES, ETC. 25food for hawks and kites, wild cats or weasels, or perish ofcold and hunger as winter comes on. This is strikinglyproved by the case of particular species; for we find thattheir abundance in individuals bears no relation whatever totheir fertility in producing offspring.Perhaps the most remarkable instance of an immense birdpopulation is that of the passenger pigeon of the UnitedStates, which lays only one, or at most two eggs, and is saidto rear generally but one young one. Whyis this bird soextraordinarily abundant, while others producing two .or threetimes as many young are much less plentiful The ? explanationis not difficult. The food most congenial to this species,and on which it thrives best, isabundantly distributed overa very extensive region, offering such differences of soil andclimate, that in one part or another of the area the supplynever fails. The bird is capable of a very rapid and longcontinuedflight, so that it can pass without fatigue over thewhole of the district it inhabits, and as soon as the supply offood begins to fail in one place is able to discover a freshfeeding-ground. This example strikingly shows us that theprocuring a constant supply of wholesome food is almost thesole condition requisite for ensuring the rapid increase of agiven species, since neither the limited fecundity nor the unrestrainedattacks of birds of prey and of man are heresufficient to check it. In no other birds are these peculiarcircumstances so strikingly combined. Either their food ismore liable to failure, or they have not sufficient power ofwing to search for it over an extensive area, or during someseason of the year it becomes very scarce, and less wholesomesubstitutes have to be found ;and thus, though more fertilein offspring, they can never increase beyond the supply offood in the least favourable seasons.Many birds can only exist by migrating, when their foodbecomes scarce, to regions possessing a milder, or at least adifferent climate, though, as these migrating birds are seldomexcessively abundant, it is evident that the countries theyvisit are still deficient in a constant and abundant supply ofwholesome food. Those whose organisation does not permitthem to migrate when their food becomes periodically scarce,can never attain a large population. This is probably the
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