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.
The notebook page where Charles Darwin first proposes the idea of evolution. A reminder that, like most breakthroughs, even the primary theory behind life on earth began with the seed of hesitation on a blank page.