PDF - Wallace Online
PDF - Wallace Online PDF - Wallace Online
14 NATURAL SELECTION jNow it appears highly probable that aformations now exposed to our researches was elevated at theend of the Palaeozoic period, and remained so through theinterval required for the organic changes which resulted inthe fauna and flora of the Secondary period. The records ofthis interval are buried beneath the ocean which covers threefourthsof the globe.long period of quiescence or stability in the physical conditionsof a district would be most favourable to the existenceof organic life in the greatest abundance, both as regardsindividuals and also as to variety of species and generic group,just as we now find that the places best adapted to the rapidgrowth and increase of individuals also contain the greatestprofusion of species and the greatest variety of forms, thetropics in comparison with the temperate and arctic regions.On the other hand, it seems no less probable that a change inthe physical conditions of a district, even small in amount ifrapid, or even gradual if to a great amount, would be highlyunfavourable to the existence of individuals, might cause theextinction of many species, and would probably be equallyunfavourable to the creation of new ones. In this too wemay find an analogy with the present state of our earth, forit has been shown to be the violent extremes and rapidchanges of physical conditions, rather than the actual meanstate in the temperate and frigid zones, which renders themless prolific than the tropical regions, as exemplified by thegreat distance beyond the tropics to which tropical formspenetrate when the climate is equable, and also by the richnessin species and forms of tropical mountain regions whichprincipally differ from the temperate zone in the uniformityof their climate. However this may be, it seems a fairassumption that during a period of geological repose the newspecies which we know to have been created would haveappeared, that the creations would then exceed in number theextinctions, and therefore the number of species would increase.In a period of geological activity, on the other hand, it seemsprobable that the extinctions might exceed the creations, andthe number of species consequently diminish. That sucheffects did take place in connection with the causes to whichwe have imputed them, is shown in the case of the Coalformation, the faults and contortions of which show a period
i THE INTRODUCTION OF NEW SPECIES 15of great activity and violent convulsions, and it is in theformation immediately succeeding this that the poverty offorms of life is most apparent. We have then only to supposea long period of somewhat similar action during the vastunknown interval at the termination of the Palaeozoic period,and then a decreasing violence or rapidity through theSecondary period, to allow for the gradual repopulation ofthe earth with varied forms, and the whole of the facts areexplained. 1 We thus have a clue to the increase of the formsof life during certain periods, and their decrease during others,without recourse to any causes but those we know to haveexisted, and to effects fairly deducible from them. The precisemanner in which the geological changes of the earlyformations were effected is so extremely obscure, that whenwe can explain important facts by a retardation at one timeand an acceleration at another of a process which we knowfrom its nature and from observation to have been unequal,a cause so simple may surely be preferred to one so obscureand hypothetical as polarity.I would also venture to suggest some reasons against thevery nature of the theory of Professor Forbes. Our knowledgeof the organic world during any geological epoch isnecessarily very imperfect. Looking at the vast numbers ofspecies and groups that have been discovered by geologists,this may be doubted but we should ; compare their numbersnot merely with those that now exist upon the earth, but witha far larger amount. We have no reason for believing thatthe number of species on the earth at any former period wasmuch less than at present ;at all events the aquatic portion,with which geologists have most acquaintance, was probablyoften as great or greater. Now we know that there havebeen many complete changes of species new sets of ;organismshave many times been introduced in place of old ones whichhave become extinct, so that the total amount which haveexisted on the earth from the earliest geological period musthave borne about the same proportion to those now living, asthe whole human race who have lived and died upon the1 Professor Ramsay has since shown that a glacial epoch probably occurredat the time of the Permian formation, which will more satisfactorily accountfor the comparative poverty of species.
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14 NATURAL SELECTION jNow it appears highly probable that aformations now exposed to our researches was elevated at theend of the Palaeozoic period, and remained so through theinterval required for the organic changes which resulted inthe fauna and flora of the Secondary period. The records ofthis interval are buried beneath the ocean which covers threefourthsof the globe.long period of quiescence or stability in the physical conditionsof a district would be most favourable to the existenceof organic life in the greatest abundance, both as regardsindividuals and also as to variety of species and generic group,just as we now find that the places best adapted to the rapidgrowth and increase of individuals also contain the greatestprofusion of species and the greatest variety of forms, thetropics in comparison with the temperate and arctic regions.On the other hand, it seems no less probable that a change inthe physical conditions of a district, even small in amount ifrapid, or even gradual if to a great amount, would be highlyunfavourable to the existence of individuals, might cause theextinction of many species, and would probably be equallyunfavourable to the creation of new ones. In this too wemay find an analogy with the present state of our earth, forit has been shown to be the violent extremes and rapidchanges of physical conditions, rather than the actual meanstate in the temperate and frigid zones, which renders themless prolific than the tropical regions, as exemplified by thegreat distance beyond the tropics to which tropical formspenetrate when the climate is equable, and also by the richnessin species and forms of tropical mountain regions whichprincipally differ from the temperate zone in the uniformityof their climate. However this may be, it seems a fairassumption that during a period of geological repose the newspecies which we know to have been created would haveappeared, that the creations would then exceed in number theextinctions, and therefore the number of species would increase.In a period of geological activity, on the other hand, it seemsprobable that the extinctions might exceed the creations, andthe number of species consequently diminish. That sucheffects did take place in connection with the causes to whichwe have imputed them, is shown in the case of the Coalformation, the faults and contortions of which show a period
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