Placental animals, e.g. might be at each period less variable than Marsupials, and nevertheless have undergone more DIFFERENTIATION and development than marsupials, owing to some advantage, probably brain development. I am surprised, but do not pretend to form an opinion at Hooker's statement that higher species, genera, etc., are best limited. It seems to me a bold statement. Looking to the 'Origin,' I see that I state that the productions of the land seem to change quicker than those of the sea (Chapter X., page 339, 3d edition), and I add there is some reason to believe that organisms considered high in the scale change quicker than those that are low. I remember writing these sentences after much deliberation... I remember well feeling much hesitation about putting in even the guarded sentences which I did. My doubts, I remember, related to the rate of change of the Radiata in the Secondary formation, and of the Foraminifera in the oldest Tertiary beds... Good night, C. DARWIN. CHARLES DARWIN TO C. LYELL. Down, October 1 [1862]. ... I found here (On his return from Bournemouth.) a short and very kind note of Falconer, with some pages of his 'Elephant Memoir,' which will be published, in which he treats admirably on long persistence of type. I thought he was going to make a good and crushing attack on me, but to my great satisfaction, he ends by pointing out a loophole, and adds (Falconer, "On the American Fossil Elephant," in the 'Nat. Hist. Review,' 1863, page 81. The words preceding those cited by my father make the meaning of his quotation clearer. The passage begins as follows: "The inferences which I draw from these facts are not opposed to one of the leading propositions of Darwin's theory. With him," etc. etc.) "with him I have no faith that the mammoth and other extinct elephants made their appearance suddenly... The most rational view seems to be that they are the modified descendants of earlier progenitors, etc." This is capital. There will not be soon one good palaeontologist who believes in immutability. Falconer does not allow for the Proboscidean group being a failing one, and therefore not likely to be giving off new races. He adds that he does not think Natural Selection suffices. I do not quite see the force of his argument, and he apparently overlooks that I say over and over again that Natural Selection can do nothing without variability, and that variability is subject to the most complex fixed laws...