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PDF - Wallace Online

PDF - Wallace Online PDF - Wallace Online

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NATURAL SELECTION6. Most of the larger and some small groups extendthrough several geological periods.7. In each period, however, there are peculiar groups,found nowhere else, and extending through one or severalformations.8. Species of one genus, or genera of one family occurringin the same geological time, are more closely allied than thoseseparated in time.9. As, generally, in geography no species or genus occursin two very distant localities without being also found inintermediate places, so in geology the life of a species orgenus has not been interrupted. In other words, no groupor species has come into existence twice.10. The following law may be deduced from these facts :Every species has come into existence coincident both in space andtime with a pre-existing closely allied species.This law agrees with, explains, and illustrates all the factsconnected with the following branches of the subject:1st,The system of natural affinities. 2d, The distribution ofanimals and plants in space. 3d, The same in time, includingall the phenomena of representative groups, and those whichProfessor Forbes supposed to manifest polarity. 4th, Thephenomena of rudimentary organs. We will briefly endeavourto show its bearing upon each of these.The Form of a true system ajby this LawIf the law above enunciated be true, it follows that thenatural series of affinities will also represent the order inwhich the several species came into existence, each one havinghad for its immediate antitype a closely allied species existingat the time of its origin. It is evidently possible that two orthree distinct species may have had a common antitype, andthat each of these may again have become the antitypes fromwhich other closely allied species were created. The effect ofthis would be, that so long as each species has had but onenew species formed on its model, the line of affinities will besimple, and may be represented by placing the several speciesin direct succession in a straight line. But if two or morespecies have been independently formed on the plan of a

THE INTRODUCTION OF NEW SPECIEScommon antitype, then the series of affinities will be compound,and can only be represented by a forked or manybranchedline. Now, all attempts at a Natural classificationand arrangement of organic beings show that both these planshave obtained in creation. Sometimes the series of affinitiescan be well represented for a space by a direct progressionfrom species to species or from group to group, but it isgenerally found impossible so to continue. There constantlyoccur two or more modifications of an organ or modificationsof two distinct organs, leading us on to two distinct series ofspecies, which at length differ so much from each other as toform distinct genera or families. These are the parallel seriesor representative groups of naturalists, and they often occurin different countries, or are found fossil in different formations.They are said to have an analogy to each other whenthey are so far removed from their common antitype as todiffer in many important points of structure, while theystillpreserve a family resemblance. We thus see how difficult itis to determine in every case whether a given relation is ananalogy or an affinity, for it is evident that as we go backalong the parallel or divergent series, towards the commonantitype, the analogy which existed between the two groupsbecomes an affinity.We are also made aware of the difficultyof arriving at a true classification, even in a small andperfect group in the actual state of nature it is almost;impossible, the species being so numerous and the modificationsof form and structure so varied, arising probably fromthe immense number of species which have served as antitypesfor the existing species, and thus produced a complicatedbranching of the lines of affinity, as intricate as thetwigs of a gnarled oak or the vascular system of the humanbody. Again, if we consider that we have only fragments ofthis vast system, the stem and main branches being representedby extinct species of which we have no knowledge,while a vast mass of limbs and boughs and minute twigs andscattered leaves is what we have to place in order, so as todetermine the true position which each originally occupiedwith regard to the others, the whole difficulty of the trueNatural System of classification becomes apparent to us.We shall thus find ourselves obliged to reject all those

THE INTRODUCTION OF NEW SPECIEScommon antitype, then the series of affinities will be compound,and can only be represented by a forked or manybranchedline. Now, all attempts at a Natural classificationand arrangement of organic beings show that both these planshave obtained in creation. Sometimes the series of affinitiescan be well represented for a space by a direct progressionfrom species to species or from group to group, but it isgenerally found impossible so to continue. There constantlyoccur two or more modifications of an organ or modificationsof two distinct organs, leading us on to two distinct series ofspecies, which at length differ so much from each other as toform distinct genera or families. These are the parallel seriesor representative groups of naturalists, and they often occurin different countries, or are found fossil in different formations.They are said to have an analogy to each other whenthey are so far removed from their common antitype as todiffer in many important points of structure, while theystillpreserve a family resemblance. We thus see how difficult itis to determine in every case whether a given relation is ananalogy or an affinity, for it is evident that as we go backalong the parallel or divergent series, towards the commonantitype, the analogy which existed between the two groupsbecomes an affinity.We are also made aware of the difficultyof arriving at a true classification, even in a small andperfect group in the actual state of nature it is almost;impossible, the species being so numerous and the modificationsof form and structure so varied, arising probably fromthe immense number of species which have served as antitypesfor the existing species, and thus produced a complicatedbranching of the lines of affinity, as intricate as thetwigs of a gnarled oak or the vascular system of the humanbody. Again, if we consider that we have only fragments ofthis vast system, the stem and main branches being representedby extinct species of which we have no knowledge,while a vast mass of limbs and boughs and minute twigs andscattered leaves is what we have to place in order, so as todetermine the true position which each originally occupiedwith regard to the others, the whole difficulty of the trueNatural System of classification becomes apparent to us.We shall thus find ourselves obliged to reject all those

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