What is Darwin's Descent with Modification?
Darwin perceived unity in life, which he attributed to the descent of all organisms from an ancestor that lived in the remote past. He also thought that as the descendants of that ancestral organism lived in various habitats over millions of years, they had accumulated diverse modifications, or adaptations, that fit them to specific ways of life.
Darwin reasoned that over long periods of time, descent with modification eventually led to the rich diversity of life we see today.
Darwin viewed the histroy of life as a tree, with multiple branchings from a common trunk out of the tips of the youngest twigs. The tips of the twigs represent the diversity of organisms living in the present. Each fork of the tree represents an ancestor of all the lines of evolution that subsequently branch from that point.
In some cases, there are no living species that fill the gap between some lineagess and their closest relatives today. In fact, many brnaches of evolution, even some major ones, are dead ones.
To Darwin, the Linnaean hierarchy reflected the branching history of the tree of life, with organisms at the various levels related through descent from common ancestors.