Descent with Modification

Darwin (1859): Evolution as Descent with Modification

This is the only illustration in the Origin of Species. Compare this with his 1837 sketch, "I think ...". Darwin postulates eleven species lineages (A - L) at Time 0. If distance apart on the horizontal axis is taken as an indication of morphological variability, and considering differences among ancestors implied before Time 0, these might be grouped in three genera, (ABCD), (EF), & (GHIKL). Of these, all but three (A, F, & I) have gone extinct not later than Time X. Species A has seven descendant species at Time XIV: note that these are more morphologically diverse than the six descendant species of I, and might be placed in three different subgenera versus two for the descendants of I. Species F survives unchanged as what might be called a "living fossil". Thus at Present (Time XIV) there are 14 living species descended from an original three at Time 0: these species have originated by processes of lineage splitting, are more diverse than their ancestors, and remain morphologically distinct. They have arisen by Descent with Modification.