By mid-March 1837, barely six months after his return to England, Darwin was speculating in his "Red Notebook" on the possibility that "one species does change into another" to explain the geographical distribution of living species such as the rheas, and extinct ones such as the strange extinct mammal "Macrauchenia", which resembled a giant guanaco, a llama relative. Around mid-July, he recorded in his "B" notebook his thoughts on lifespan and variation across generations—explaining the variations he had observed in Galápagos tortoises, mockingbirds, and rheas. He sketched branching descent, and then a genealogical branching of a single evolutionary tree, in which "It is absurd to talk of one animal being higher than another", discarding Lamarck's idea of independent lineages progressing to higher forms.</ref>