Sketch from Charles Darwin's Notebooks on Transmutation of Species (1837), arguably the first example of a phylogenetic tree. (1) represents a common ancestor, the barred branches represent extant descendants, and the unmarked branches represent extinct descendants. The notes read:

I think
[sketch]
Case must be that one generation then should be as many living as now. To do this & to have many species in same genus (as is) requires extinction.
Thus between A & B immense gap of relation, C & B the finest gradation, B & D rather greater distinction. Thus genera would be formed – bearing relation [page break] to ancient types – with several extinct forms[.]