In On the Origin of Species, Charles Darwin described the evolutionary relationships between organisms as branches on a tree, a diagrammatic representation of all species that have ever existed connected by common descent.
The affinities of all the beings of the same class have sometimes been represented by a great tree. I believe this simile largely speaks the truth. The green and budding twigs may represent existing species; and those produced during each former year may represent the long succession of extinct species.
