586 LOUIS AGASSIZ. 



in the one alike to any one in the others, etc. 

 Now to advance the investigation to the point 

 where it may tell with reference to the scien- 

 tific doctrines at present under discussion, it is 

 essential to know the facts in detail, with ref- 

 erence to every fresh-water basin on earth. I 

 have already taken means to obtain the ten- 

 ants of all the rivers of Brazil, and partly of 

 Russia, and I hope you may be able to put me 

 in the way of getting those of Spain, if not 

 of some other country beside. The plan I 

 propose for that country would be worthy of 

 the Doctors of Salamanca in her brightest 

 days. If this alone were carried out, it would 

 be, I believe, sufficient to settle the whole 

 question. 



My idea is to obtain separate collections 

 from all the principal rivers of Spain and 

 Portugal, and even to have several separate 

 collections from the larger rivers, one from 

 their lower course, one from their middle 

 course, and another from their head -waters. 

 Take, for instance, the Douro. One collection 

 ought to be made at Oporto, and several 

 higher up, among its various tributaries and 

 in its upper course ; say, one at Zamora and 

 Valladolid, one at Salamanca from the Tonnes 

 River, one at Leon from the Esla River, one 



