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PDF - Wallace Online PDF - Wallace Online

wallace.online.org
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NATURAL SELECTIONduring an immense but unknown period the surface of theearth has undergone successive changes land has sunk beneaththe ocean, while fresh land has risen up from it ;;mountain chains have been elevated ;islands have beenformed into continents, and continents submerged till theyhave become islands ;and these changes have taken place,not once merely, but perhaps hundreds, perhaps thousands oftimes. That all these operations have been more or lesscontinuous but unequal in their progress, and during thewhole series the organic life of the earth has undergone acorresponding alteration. This alteration also has beengradual, but complete after a certain interval not a; singlespecies existing which had lived at the commencement of theperiod. This complete renewal of the forms of life alsoappears to have occurred several times. That from the lastof the geological epochs to the present or historical epoch,the change of organic life has been gradual : the first appearanceof animals now existing can in many cases be traced,their numbers gradually increasing in the more recent formations,while other species continually die out and disappear,so that the present condition of the organic world is clearlyderived by a natural process of gradual extinction and creationof species from that of the latest geological periods.and naturalWe may therefore safely infer a like gradationsequence from one geological epoch to another.Now, taking this as a fair statement of the results ofgeological inquiry, we see that the present geographical distributionof lifeupon the earth must be the result of all theprevious changes, both of the surface of the earth itself andof its inhabitants. Many causes, no doubt, have operated ofwhich we must ever remain in ignorance, and we may, therefore,expect to find many details very difficult of explanation,and in attempting to give one, must allow ourselves to callinto our service geological changes which it is highly probablemay have occurred, though we have no direct evidence oftheir individual operation.The great increase of our knowledge within the last twentyyears, both of the present and past history of the organicworld, has accumulated a body of facts which should afforda sufficient foundation for a comprehensive law embracing and

THE INTRODUCTION OF NEW SPECIESexplaining them all, and giving a direction to new researches.It is about ten years since the idea of such a law suggesteditself to the writer of this essay, and he has since taken everyopportunity of testing it by all the newly-ascertained factswith which he has become acquainted, or has been able toobserve himself. These have all served to convince him ofthe correctness of his hypothesis. Fully to enter into such asubject would occupy much space, and it is only in consequenceof some views having been lately promulgated, hebelieves, in a wrong direction, that he now ventures to presenthis ideas to the public, with only such obvious illustrations ofthe arguments and results as occur to him in a place farremoved from all means of reference and exact information.A Law deduced from well-known Geographical andGeological FactsThe following propositions in Organic Geography and Geologygive the main facts on which the hypothesis is founded.GKOGRAPHY1.Large groups, such as classes and orders, are generallyspread over the whole earth, while smaller ones, such asfamilies and genera, are frequently confined to one portion,often to a very limited district.2. In widely distributed families the genera are oftenlimited in range ; in widely distributed genera well-markedgroups of species are peculiar to each geographical district.3. When a group is confined to one district, and is rich inspecies, it is almost invariably the case that the most closelyallied species are found in the same locality or in closelyadjoining localities, and that therefore the natural sequenceof the species by affinityis also geographical.4. In countries of a similar climate, but separated by awide sea or lofty mountains, the families, genera, and speciesof the one are often represented by closely allied families,genera, and species peculiar to the other.5. The distribution of the organic world in time is verysimilar to its present distribution in space.

NATURAL SELECTIONduring an immense but unknown period the surface of theearth has undergone successive changes land has sunk beneaththe ocean, while fresh land has risen up from it ;;mountain chains have been elevated ;islands have beenformed into continents, and continents submerged till theyhave become islands ;and these changes have taken place,not once merely, but perhaps hundreds, perhaps thousands oftimes. That all these operations have been more or lesscontinuous but unequal in their progress, and during thewhole series the organic life of the earth has undergone acorresponding alteration. This alteration also has beengradual, but complete after a certain interval not a; singlespecies existing which had lived at the commencement of theperiod. This complete renewal of the forms of life alsoappears to have occurred several times. That from the lastof the geological epochs to the present or historical epoch,the change of organic life has been gradual : the first appearanceof animals now existing can in many cases be traced,their numbers gradually increasing in the more recent formations,while other species continually die out and disappear,so that the present condition of the organic world is clearlyderived by a natural process of gradual extinction and creationof species from that of the latest geological periods.and naturalWe may therefore safely infer a like gradationsequence from one geological epoch to another.Now, taking this as a fair statement of the results ofgeological inquiry, we see that the present geographical distributionof lifeupon the earth must be the result of all theprevious changes, both of the surface of the earth itself andof its inhabitants. Many causes, no doubt, have operated ofwhich we must ever remain in ignorance, and we may, therefore,expect to find many details very difficult of explanation,and in attempting to give one, must allow ourselves to callinto our service geological changes which it is highly probablemay have occurred, though we have no direct evidence oftheir individual operation.The great increase of our knowledge within the last twentyyears, both of the present and past history of the organicworld, has accumulated a body of facts which should afforda sufficient foundation for a comprehensive law embracing and

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