482 :\lr. A. R. Wallace on the Natural History 



In a former Number of this periodical we endeavoured to show 

 that the simple law, of every new creation being closely allied to 

 some species already existing in the same country, would explain 

 all these anomalies, if taken in conjunction with the changes of 

 surface and the gradual extinction and introduction of species, 

 which are facts proved by geology. At the period when Xew 

 Guinea and North Australia were united, it is probable that their 

 physical features and climate were more similar, and that a con- 

 siderable proportion of the species inhabiting each portion of the 

 country were found over the whole. After the separation took 

 place, we can easily understand how the climate of both might 

 be considerably modified, and this might perhaps lead to the ex- 

 tinction of certain species. During the period that has since 

 elapsed, new species have been gradually introduced into each, 

 but in each closely allied to the pre-existing species, many of 

 which were at first common to the two countries. This process 

 would evidently produce the present condition of the two faunas, 

 in which there are many allied species, — few identical. The 

 great well-marked grouj)s absent from the one would necessarily 

 be so from the other also, for however much they might be 

 adapted to the country, the law of close affinity would not allow 

 of their appearance, except by a long succession of steps occu- 

 pying an immense geological interval. The species which at the 

 time of separation were found only in one country, would, by the 

 gradual introduction of species allied to them, give rise to groups 

 peculiar to that countiy. This separation of New Guinea from 

 Australia no doubt took place while Aru yet formed part of the 

 former island. Its separation must have occurred at a very re- 

 cent period, the number of species common to the two showing 

 that scarcely any extinctions have since taken place, and pro- 

 bably as few^ introductions of new species. 



If we now suppose the Aru Islands to remain undisturbed 

 during a period equal to about one division of the Tertiary epoch 

 of geologists, we have reason to believe that the change of species 

 of Vertebrata will become complete, an entirely new race having 

 gradually been introduced, but all more or less closely allied to 

 those now existing. During the same period a new fauna will 

 also have arisen in New Guinea, and then the two will present 

 the same comparative features that North Australia and New 

 Guinea do now. Let the process of gradual change still go on 

 for another period regulated by the same laws. Some species 

 will then have become extinct in the one country, and unre- 

 placed, while in the other a numerous series of modified species 

 may have been introduced. Then the faunas will come to differ 

 not in species only, but in generic groups. There would be then 

 the resemblance between them that there is between the West 



