The term "would" before "undermine" had been added after writing what is now noted as the first expression of his doubts about species being immutable. That led to him becoming convinced about the transmutation of species and hence evolution.   </ref> In opposing transmutation, Lyell had proposed that varieties arose due to changes in the environment, but these varieties lived in similar conditions though each on its own island. Darwin had just reviewed similar inconsistencies with mainland bird genera such as "Pteroptochos". Though his suspicions about the Falkland Island fox may have been unsupported, the differences in Galápagos tortoises between islands were remembered, and he later wrote that he had been greatly struck from around March 1836 by the character of South American fossils and species on the Galapagos Archipelago, noting "These facts origin (especially latter) of all my views".