 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, which led to him becoming convinced about the transmutation of species and hence evolution.In opposing transmutation, Lyell had proposed that varieties arose due to changes in 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".