184 ^Ir. A. R. Wallace on the Law which has regulated 



XVIII. — On the Law which has regulated the Introduction of 

 New Species. By Alfred R. Wallace, F.R.G.S. 



Every naturalist who has directed his attention to the subject 

 of the geographical distribution of animals and plants, must 

 have been interested in the singular facts which it presents. 

 Many of these facts are quite different from what would have 

 been anticipated, and have hitherto been considered as highly 

 enrious, but quite inexplicable. None of the explanations at- 

 tempted from the time of Linnaeus are now considered at all 

 satisfactorv ; none of them have given a cause sufficient to ac- 

 count for the facts known at the time, or comprehensive CJiough 

 to include all the new facts which have since been, and are daily 

 being added. Of late years, however, a great light has been 

 thrown upon the subject by geological investigations, which 

 have shown that the present state of the earth, and the organisms 

 now inhabiting it, are but the last stage of a long and uninter- 

 rupted series of changes which it has undergone, and conse- 

 quently, that to endeavour to explain and account for its pre- 

 sent condition without any reference to those changes (as has 

 frequently been done) must lead to very imperfect and erroneous 

 conclusions. 



The facts proved by geology are briefly these : — That during 

 an immense, but unknown period, the surface of the earth has 

 undergone successive changes ; land has sunk beneath the ocean, 

 while fresh land has risen up from it ; mountain chains have 

 been elevated; islands have been formed into continents, and 

 continents submerged till they have become islands ; and these 

 changes have taken place, not once merely, but perhaps hun- 

 dreds, perhaps thousands of times : — That all these operations 

 have been more or less continuous, but unequal in their progress, 

 and diu'ing the v»hole series the organic life of the earth has 

 undergone a corresponding alteration. This alteration also has 

 been gradual, but complete ; after a certain interval not a single 

 species existing which had lived at the commencement of the 

 period. This complete renewal of the forms of life ako appears 

 to have occurred several times : — That from the last of the Geo- 

 logical epoclis to the present or Historiial epoch, the change of 

 organic life has been gradual : the first appearance of animals 

 now existing can in many cases be traced, their numbers gradu- 

 ally increasing in the more recent forniivtions, while other species 

 continually die out and disajipear, so that the present condition 

 of the organic world is clearly derived by a natural process of 

 gradual extinction and creation of species from that of the latest 

 geological periods. We may therefore safely infer a like grada- 

 tion and natural sequence from one geological epoch to another. 



