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. pinnatifida, but that is so 
universally diffused a plant and so variable in the size of 
its leaves that at first sight no one w^ould 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. yinjiatifida is probably a native 
of the Plate alone^ whence 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 undescribed one from that 
port, which is alluded to as S. diffusa, Bert. MSS. I naturally 
concluded yours was this, but thought I would write to 
Brit. Mus. to confirm it, for fear of accident, but Bertero's 
