4 



Mr Alfred K. Wallace on some Anomalies 



to work out the details of Geographical distribution. There 

 is no doubt but that the six regions marked out by Dr 

 Sclater are generally characterised by distinct forms of in- 

 sects. There is one case, however, which has come under 

 my own observation, in which the entomological would not 

 correspond with the ornithological regions. The Moluccas 

 and New Guinea in their birds and mammals are most de- 

 cidedly Australian, while the insects show a general corre- 

 spondence with the Indian type. It has also been pointed 

 out that the insects of Chili and of south temperate South 

 America have little affinity with Neotropical forms. 



Land shells, I am informed by the Eev. H. B. Tristram, 

 generally agree very well with the ornithological regions. 

 The subdivisions or provinces are, however, often very 

 strongly marked. 



In Plants, I am informed by Dr Hooker, the regions will 

 in many cases not at all correspond. 



In order to arrive at the cause and meaning of these 

 singular differences in the Geographical distribution of the 

 various classes, we must inquire how Zoological and Bota- 

 nical regions are formed, or why organic existences come 

 to be grouped geographically at all. 



It appears to me that this can be explained by a few 

 simple principles — 



Is^, The tendency of all species to diffuse themselves over 

 a wide area, some one or more in each group being actually 

 found to have so spread, and to have become, as Mr Darwin 

 terms them, dominant species. 



2(i, The existence of barriers checking, or absolutely 

 forbidding that diffusion. 



3(i, The progressive change or replacement of species, 

 by allied forms, w^hich has been continually going on in the 

 organic world. 



4^/i, A corresponding change in the surface, which has 

 led to the destruction of old and the formation of new 

 barriers. 



5^A, Changes of climate and physical conditions, which 

 will often favour the diffusion and increase of one gro'up, 

 and lead to the extinction or decrease of another. 



By means of these principles we will endeavour to ex- 



