(41/1. The following letter illustrates Darwin's work on aberrant genera. In the "Origin," Edition I., page 429, he wrote: "The more aberrant any form is, the greater must be the number of connecting forms which, on my theory, have been exterminated and utterly lost. And we have some evidence of aberrant forms having suffered severely from extinction, for they are generally represented by extremely few species; and such species as do occur are generally very distinct from each other, which again implies extinction.") Down, November 15th [1855?]. In Schoenherr's Catalogue of Curculionidae (41/2. "Genera et Species Curculionidum." (C.J. Schoenherr: Paris, 1833-38.)), the 6,717 species are on an average 10.17 to a genus. Waterhouse (who knows the group well, and who has published on fewness of species in aberrant genera) has given me a list of 62 aberrant genera, and these have on an average 7.6 species; and if one single genus be removed (and which I cannot yet believe ought to be considered aberrant), then the 61 aberrant genera would have only 4.91 species on an average. I tested these results in another way. I found in Schoenherr 9 families, including only 11 genera, and these genera (9 of which were in Waterhouse's list) I found included only 3.36 species on an average. This last result led me to Lindley's "Vegetable Kingdom," in which I found (excluding thallogens and acrogens) that the genera include each 10.46 species (how near by chance to the Curculionidae), and I find 21 orders including single genera, and these 21 genera have on average 7.95 species; but if Lindley is right that Erythroxylon (with its 75 species) ought to be amongst the Malpighiads, then the average would be only 4.6 per genus. But here comes, as it appears to me, an odd thing (I hope I shall not quite weary you out). There are 29 other orders, each with 2 genera, and these 58 genera have on an average 15.07 species: this great number being owing to the 10 genera in the Smilaceae, Salicaceae (with 220 species), Begoniaceae, Balsaminaceae, Grossulariaceae, without which the remaining 48 genera have on an average only 5.91 species. This case of the orders with only 2 genera, the genera notwithstanding having 15.07 species each, seems to me very perplexing and upsets, almost, the conclusion deducible from the orders with single genera. I have gone higher, and tested the alliances with 1, 2, and 3 orders; and in these cases I find both the genera few in each alliance, and the species, less than the average of the whole kingdom, in each genus.