MEMOIR. XXXV 
touched upon, and yet I think your own materials in this very paper establish 
it, viz.: that the river, ina great many cases, limits the range of species, or of 
well-marked varieties. . . . In mammals this fact was not so much to be 
wondered at, but few persons would credit that it would extend to birds and 
winged insects. . . . It would seem that Guyana forms having once crossed 
the river, have a great tendency to become modified, and then never recross. 
Why the Brazilian species should not first have taken possession of their own 
side of the river is the mystery. I should be inclined to think that the river 
bed is comparatively new, and that the south plains were once continuous 
with Guyana; in fact that Guyana is olderthan North Brazil, and after it had 
pushed out its alluvial plains into what is now North Brazil, an elevation on 
the Brazilian side made the river cut a new channel to the northward, leaving 
the Guyana species isolated, exposed to competition with a new set of species, 
and thus led to their becoming modified as we now findthem. The pheno- 
menon of a tract of country having been peopled from one now separated 
from it, and not from that of which it forms a part, is too extraordinary not 
to require some special and extraordinary cause, and the one I have men- 
tioned seems capable of producing the effects, and by no means improbable 
(however unexpected) in itself... . 
‘‘T suppose you will turn now to the Coleoptera, and give us the 
Cicindelid@@ on the same plan... . 
‘‘ Yours very sincerely, 
‘* ALFRED R. WALLACE.”’ 
But for Darwin’s persistent urgency, it is probable that the 
delightful narrative of Bates’s Wanderjahre would not have been 
written. The following letters—notably that from Bates given 
on p. lxiii—go to prove this, and, moreover, are of interest as 
showing how high an estimate of Bates was formed by Darwin 
even before their personal intercourse began. 
Charles Darwin to H. W. Bates. 
‘“‘DOowN, BROMLEY, KENT, AZrél 4th, 1860. 
**“My DEAR SIR, 
‘‘T have been unwell, so have delayed thanking you for your 
admirable letter.* I hope you will not think me presumptuous in saying how 
much I have been struck with your varied knowledge, and with the decisive 
manner in which you bring it to bear on each point—a rare and most high 
quality, as far as my experience goes. I cannot hope you will find time to 
publish largely ; before the Linnean Society you might bring boldly out your 
views on species. 
‘‘ Have you ever thought of publishing your travels and working in them 
the less abstruse parts of your natural history? You must also have seena 
* The absence of this and other letters from Bates to which reference is made, is due, 
as Mr. Francis Darwin informs me, to Mr. Darwin having destroyed all his corre- 
spondence dated prior to 1862. 
