442 LETTERS TO DARWIN, 1843-1859 



\ 



checked in growth which is much affected by want of 



moisture. You are right then to query that bit about the 



plants developing spines in bad soil ; for they only lose the 



power of nourishing the new leaf buds sufficiently and do 



not develop a new organ. (Hence hairiness is of more 



importance than spininess in distrib.). The Persicaria 



becoming hairy when removed from moist places is natural : 



hairs are believed to be provided as hygrometric appendages, 



to modify respiration and transpiration, water plants don't 



want them. It is facts such as the Irish Yew presents that 



afford fair ground for argument on such a topic. Noting 



instances by tens or hundreds of variation in individual 



species is nothing new ; few have an idea of the labour 



required to establish or destroy a species of a mundane genus. 



You have a Senehiera from Tres Montes, its capsules are 



much larger than the common S. finnatipda, but that is so 



universally diffused a plant and so variable in the size of 



its leaves that at first sight no one would be inclined to 



grant specific dignity to the Tres Montes plant from the 



capsules. It struck me to put this subject to a Geographical 



test, the result is, that the S. finnatifida is probably a native 



of the Plate alone, w^hence it has spread by ships all over 



East and West America, all West Europe near the coast, 



in fact both shores of the Atlantic, from Britain to the Cape 



and from Patagonia to Canada, wherever ships touch and 



cultivation ensues, and on W. from Valparaiso to California, 



wherever ships go, but through many hundreds of specimens 



there is no variation whatever in the size of the pods, and I 



therefore conclude that the Tres Montes plant is the W. 



coast representative of the E. coast plant. Now though 



De Candolle had hinted that S. finn. was an American 



plant, he did not define its limits and retained two or three 



identical plants as different species which came from out 



of the way localities : to define its limits I had not only to 



consult all floras where it was described, but all where it 



was not, for such a mundane plant creeps into every flora. 



My troubles did not end here, for I had no Valparaiso 



Senehiera, and Bertero has an undescrfbed one from that 



port, which is aUuded to as S. diffusa, Bf rt. MSS. I naturally 



concluded yours was this, but thoug,ht I would write to 



Brit. Mus. to confirm it, for fear of accident, but Bertero's 



