I now propose to resume this subject on the most extensive scale, since I see that it has the most direct bearing upon the transmutation theory. . .First let me submit to you my plan.
Rivers and lakes are isolated by the land and sea from one another. The question is, then, how they came to be peopled with inhabitants differing both from those on land and those in the sea, and how does it come that every hydrographic basin has its own inhabitants more or less different from those of any other basin? Take the Ganges, the Nile, and the Amazons. There is not a living being 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 scientific doctrines at present under discussion, it is essential to know the facts in detail, with reference to every fresh-water basin on earth. I have already taken means to obtain the tenants 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 Tormes River, one at Leon from the Esla River, one at Burgos and Palencia from the northern tributaries, one at Soria and Segovia from the southern tributaries. If this could be done on such a scale as I propose, it would in itself be a work worthy of the Spanish government, and most creditable to any man who should undertake it. The fact is that nothing of the kind has ever been done yet anywhere. A single collection from the Minho would be sufficient, say from Orense or Melgaco. From the northern rivers along the gulf of Biscay all that would be necessary would be one thoroughly complete collection from one of the little rivers that come down from the mountains of Asturias, say from Oviedo.
The Ebro would require a more elaborate survey. From its upper course, one collection would be needed from Haro or Frias or Miranda; another from Saragossa, and one from its mouth, including the minnows common among the brackish waters near the mouth of large rivers. In addition to this, one or two of the tributaries of the Ebro, coming down from the Pyrenees, should be explored in the same manner; say one collection from Pampeluna, and one from Urgel, or any other place on the southern slope of the Pyrenees. A collection made at Barcelona from the river and the brackish marshes would be equally desirable; another from the river at Valencia, and, if possible, also from its head-waters at Ternel; another from the river Segura at Murcia, and somewhere in the mountains from its head-waters. Granada would afford particular interest as showing what its mountain streams feed. A collection from the Almeria River at Almeria, or from any of the small rivers of the southern coast of Spain, would do; and it would be the more interesting if another from the river Xenil could be obtained at or near Granada, to compare with the inhabitants of the waters upon the southern slope of the Sierra Nevada.
Next would come the Guadalquivir, from which a collection should be made at San Lucar, with the brackish water species; another at Seville or Cordova, one among the head-waters from the Sierra Nevada, and another from the mountains of the Mancha. From the Guadiana a collection from Villa Real, with the brackish species; one from Badajoz, and one from the easternmost headwaters, and about where the river is lost under ground.
The Tagus would again require an extensive exploration. In the first place a thorough collection of all the species found in the great estuary ought to be made with the view of ascertaining how far marine Atlantic species penetrate into the river basin; then one from Santarem, and another either from Talavera or Toledo or Aranjuez, and one from the head-waters in Guadalaxara, and another in Molina.
The collections made at different stations ought carefully to be kept in distinct jars or kegs, with labels so secure that no confusion or mistake can arise. But the specimens collected at the same station may be put together in the same jar. These collections require, in fact, very little care. (Here some details about mode of putting up specimens, transportation, etc.) If the same person should collect upon different stations, either in the same or in different hydrographic basins, the similarity of the specimens should not be a reason for neglecting to preserve them. What is aimed at is not to secure a variety of species, but to learn in what localities the same species may occur again and again, and what are the localities which nourish different species, no matter whether these species are in themselves interesting or not, new to science or known for ages, whether valuable for the table or unfit to eat. The mere fact of their distribution is the point to be ascertained, and this, as you see, requires the most extensive collections, affording in themselves comparatively little interest, but likely to lead, by a proper discussion of the facts, to the most unexpected philosophical results. . .Do, please, what you can in this matter. Spain alone might give us the materials to solve the question of transmutation versus creation. I am going to make a similar appeal to my friends in Russia for materials from that country, including Siberia and Kamschatka. Our own rivers are not easily accessible now.
Ever truly your friend,