PDF - Wallace Online
PDF - Wallace Online PDF - Wallace Online
NATURAL SELECTIONreason why woodpeckers are scarce with us, while in thetropics they are among the most abundant of solitary birds.Thus the house sparrow is more abundant than the redbreast,because its food is more constant and plentiful, seeds ofgrasses being preserved during the winter, and our farm-yardsand stubble-fields furnishing an almost inexhaustible supply.Why, as a general rule, are aquatic, and especially sea-birds,very numerous in individuals Not 1 because they are moreprolific than others, generally the contrary but because their;food never fails, the sea-shores and river-banks daily swarmingwith a fresh supply of small mollusca and Crustacea.Exactly the same laws will apply to mammals. Wild catsare prolific and have few enemies ; why then are they neveras abundant as rabbits ? The only intelligible answer is, thattheir supply of food is more precarious. It appears evident,therefore, that so long as a country remains physically unchanged,the numbers of its animal population cannotmaterially increase. If one species does so, some othersrequiring the same kind of food must diminish in proportion.The numbers that die annually must be immense and as the;individual existence of each animal depends upon itself, thosethat die must be the weakest the very young, the aged, andthe diseased while those that prolong their existence canonly be the most perfect in health and vigour those who arebest able to obtain food regularly, and avoid their numerousenemies. It is, as we commenced by remarking, " a strugglefor existence," in which the weakest and least perfectlyorganised must always succumb.The Abundance or Rarity of a Species dependent uponits more orless perfect Adaptation to the Conditions of ExistenceIt seems evident that what takes place among the individualsof a species must also occur among the several alliedspecies of a group, viz., that those which are best adaptedto obtain a regular supply of food, and to defend themselvesagainst the attacks of their enemies and the vicissitudes of theseasons, must necessarily obtain and preserve a superiority inpopulation ;while those species which, from some defect ofpower or organisation, are the least capable of counteractingthe vicissitudes of food-supply, etc., must diminish in numbers,
ii ON THE TENDENCY OF VARIETIES, ETC. 27and, in extreme cases, become altogether extinct. Betweenthese extremes the species will present various degrees ofcapacity for ensuring the means of preserving life ;and it isthus we account for the abundance or rarity of species. Ourignorance will generally prevent us from accurately tracingthe effects to their causes ;but could we become perfectlyacquainted with the organisation and habits of the variousspecies of animals, and could we measure the capacity of eachfor performing the different acts necessary to its safety andexistence under all the varying circumstances by which it issurrounded, we might be able even to calculate the proportionateabundance of individuals which is the necessaryresult.If now we have succeeded in establishing these two points1st, that the animal population of a country is generallystationary, being kept down by a periodical deficiency of food, andother checks ; and, 2d, that the comparative abundance or scarcityof the individuals of the several species is entirely due to theirorganisation and resulting habits, which, rendering it more difficultto procure a regular supply offood and to provide for their personalsafety l in some cases than in others, can only be balanced by adifference in the population which have to exist in a given areawe shall be in a condition to proceed to the consideration ofvarieties, to which the preceding remarks have a direct andvery important application.Useful Variations mil tend to Increase ; useless or hurtfulVariations to DiminishMost or perhaps all the variations from the typical formof a species must have some definite effect, however slight, onthe habits or capacities of the individuals. Even a change ofcolour might, by rendering them more or less distinguishable,affect their safety; a greater or less development of hairmight modify their habits. More important changes, such asan increase in the power or dimensions of the limbs or any ofthe external organs, would more or less affect their mode ofprocuring food or the range of country which they could in-1 ' 'And that of their offspring" should have been added. But it mustbe remembered that the writer had no opportunity of correcting the proofs ofthis paper.
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ii ON THE TENDENCY OF VARIETIES, ETC. 27and, in extreme cases, become altogether extinct. Betweenthese extremes the species will present various degrees ofcapacity for ensuring the means of preserving life ;and it isthus we account for the abundance or rarity of species. Ourignorance will generally prevent us from accurately tracingthe effects to their causes ;but could we become perfectlyacquainted with the organisation and habits of the variousspecies of animals, and could we measure the capacity of eachfor performing the different acts necessary to its safety andexistence under all the varying circumstances by which it issurrounded, we might be able even to calculate the proportionateabundance of individuals which is the necessaryresult.If now we have succeeded in establishing these two points1st, that the animal population of a country is generallystationary, being kept down by a periodical deficiency of food, andother checks ; and, 2d, that the comparative abundance or scarcityof the individuals of the several species is entirely due to theirorganisation and resulting habits, which, rendering it more difficultto procure a regular supply offood and to provide for their personalsafety l in some cases than in others, can only be balanced by adifference in the population which have to exist in a given areawe shall be in a condition to proceed to the consideration ofvarieties, to which the preceding remarks have a direct andvery important application.Useful Variations mil tend to Increase ; useless or hurtfulVariations to DiminishMost or perhaps all the variations from the typical formof a species must have some definite effect, however slight, onthe habits or capacities of the individuals. Even a change ofcolour might, by rendering them more or less distinguishable,affect their safety; a greater or less development of hairmight modify their habits. More important changes, such asan increase in the power or dimensions of the limbs or any ofthe external organs, would more or less affect their mode ofprocuring food or the range of country which they could in-1 ' 'And that of their offspring" should have been added. But it mustbe remembered that the writer had no opportunity of correcting the proofs ofthis paper.
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