sentences:  In "On the Origin of Species" (1859) Chapter IV he presented an abstract diagram of a theoretical tree of life for species of an unnamed large genus (see figure).On the horizontal base line hypothetical species within this genus are labelled A – L and are spaced irregularly to indicate how distinct they are from each other, and are above broken lines at various angles suggesting that they have diverged from one or more common ancestors.On the vertical axis divisions labelled I – XIV each represent a thousand generations.