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.

Mr.

Lankester also adduces the close resemblance of the parts on the right and left sides of the body, and in the successive segments of the same individual animal; and here we have parts commonly called homologous which bear no relation to the descent of distinct species from a common progenitor.