xxii] CELEBES, THE MOLUCCAS, ETC. 377 



subjects were then uppermost in my mind. The first from 

 which I will quote is that to Mr. Bates, and referring to a 

 paper on the Papilios of the Amazon which he had sent 

 me I make some remarks on the distribution of animals in 

 South America, which I do not think I have published 

 anywhere. 



"Your paper is in every respect an admirable one, and 

 proves the necessity of minute and exact observation over 

 a wide extent of country to enable a man to grapple with 

 the more difficult groups, unravel their synonymy, and mark 

 out the limits of the several species and varieties. All this 

 you have done, and have, besides, established a very interest- 

 ing fact in zoological geography, that of the southern bank 

 of the lower river having received its fauna from Guayana, 

 and not from Brazil. There is, however, another fact, I think, 

 of equal interest and importance which you have barely 

 touched upon, and yet I think your own materials in this 

 very paper establish it, viz. that the river, in a great many 

 cases, limits the range of species or of well-marked varieties. 

 This fact I considered was proved by the imperfect materials 

 I brought home, both as regards the Amazon and Rio Negro. 

 In a paper I read on 'The Monkeys of the Lower Amazon 

 and Rio Negro' I showed that the species were often dif- 

 ferent on the opposite sides of the river. Guayana species 

 came up to the east bank, Columbian species to the west 

 bank, and I stated that it was therefore important that 

 travellers collecting on the banks of large rivers should note 

 from which side every specimen came. Upon this Dr. 

 Gray came down upon me with a regular floorer. 'Why,' 

 said he, ' we have specimens collected by Mr. Wallace him- 

 self marked " Rio Negro " only.' I do not think I answered 

 him properly at the time, that those specimens were sent from 

 Barra before I had the slightest idea myself that the species 

 were different on the two banks. In mammals the fact was 

 not so much to be wondered at, but few persons would credit 

 that it would apply also to birds and winged insects. Yet 

 I am convinced it does, and I only regret that I had not 

 collected and studied birds there with the same assiduity as I 



