And the mammals and amphibians are classed together as terrestrial vertebrates which are separate from fish. A hierarchical system of classification can be portrayed in a tree-like diagram. Such tree diagrams are commonly thought to correspond to a historical process of descent by evolution from common ancestors. However, in an evolution tree all of the existing species end up as leaves at the ends of the outer twigs of the tree. No species, either living or extinct, are intermediate so that they fit down on the branches, at the connecting nodes, or in the trunk of the tree.
Nothing is necessarily ancestral to anything else.
Therefore, contrary to the claim of some vocal apologists for evolution, this nested box or hierarchical system of classification really offers no proof for a succession of evolved species. It can properly be viewed as circumstantial evidence for either common ancestry or special creation. Extinction poses another serious problem for the evolutionary interpretation of the system of classification. In order for the species living today all to fit so neatly as they do into an orderly hierarchical system of classification, it is necessary for all of the ancient evolutionary intermediate types to have died out completely. For if any of them still existed, no such orderly system of classification of separate types would be possible today.