But morphology is a much more complex subject than it at first appears, as has lately been well shown in a remarkable paper by Mr. E. Ray Lankester, who has drawn an important distinction between certain classes of cases which have all been equally ranked by naturalists as homologous. He proposes to call the structures which resemble each other in distinct animals, owing to their descent from a common progenitor with subsequent modification, “homogenous”; and the resemblances which cannot thus be accounted for, he proposes to call “homoplastic”. For instance, he believes that the hearts of birds and mammals are as a whole homogenous — that is, have been derived from a common progenitor; but that the four cavities of the heart in the two classes are homoplastic — that is, have been independently developed.