Mr. A. R. Wallace on the Genus Pitta. 1 1 1 



7io])tera and P. muelleri) are both common to the great islands of 

 Sumatra and Borneo. This is an arrangement totally opposed 

 to our ideas of putting the right bird in the right place. It is 

 exactly as if the Isle of Man possessed two peculiar species of 

 Thrush, while the allied species were common to Britain and 

 Ireland. 



Those naturalists (and I fear they are many) who consider that 

 the Darwinian school attempts to explain too much of the mys- 

 tery of nature will perhaps think that I should give some idea 

 of how this anomalous state of things came about, and, if I 

 neglect to do so, will lay claim to it as a fact in op])osition to my 

 own doctrines. Now though I entirely object to judgment 

 being passed on a theory of nature by its power to explain all 

 mysteries — seeing that the most important data for solving such 

 problems as this are almost always wanting — yet in the present 

 case it is by no means difficult to give a fair conjectural expla- 

 nation. Modification of form is admitted to be a matter of time. 

 The amount of diversity in the organic remains of two beds or 

 strata is a measure of the time between the deposition of those 

 strata. So the amount of diversity in the species of two adja- 

 cent islands is the measure of the time those islands have been 

 separated. In the present case, therefore, as the island of Banca, 

 close to Sumatra, presents in this genus a greater diversity from 

 it than does Borneo, it would follow that Banca was separated 

 and became an island at a time when Sumatra and Borneo were 

 still united. Looking at the position of these islands on the 

 map, this seems hard to believe ; but it is in reality by no means 

 improbable. The whole coast of Sumatra opposite Banca is 

 barely raised above the level of the sea, and is a network of tidal 

 channels through a soft alluvial soil. Evidently this part of 

 Sumatra is newly formed land, the result of the action of tro- 

 pical rains on the mountains and high lands more than a hundred 

 and fifty miles back in the interior. The nearness of Banca to 

 Sumatra is therefore recent and illusory. The south-west coast 

 of Borneo is almost equally low, and has been increasing in a 

 similar manner. The sea immediately between Sumatra and 

 Borneo has thus been lately filled up by alluvial deposits : it 

 was formerly deeper; and the connexion between those islands 



