In several animal and plant groups, enough fossils are known
to bridge the wide gaps between existing types [species]. In
mammals, for example, the gap between horses, asses and zebras
(genus _Equus_) and their closest living relatives, the
rhinoceroses and tapirs, is filled by an extensive series of
fossils extending back sixty million years to a small animal,
_Hyracotherium_, which can only be distinguished from the
rhinoceros-tapir group by one or two horse-like details of the
skull. There are many other examples of fossil 'missing links',
such as _Archaeopteryx_, the Jurassic bird which links birds with
dinosaurs , and _Ichthyostega_, the late Devonian amphibian which
links land vertebrates and the extinct choanate (having internal
nostrils) fishes."

Not to mention the advanced Cynodonts, which rather nicely bridge
the gap between reptiles and mammals. In fact this sequence of
about 8 to 10 distinct forms is nearly as elegent a demonstration
of transition as the Neogene equids (horses).

The core transitional forms differ from one another by less than
the differences within a single famly of living insectivores, yet
the earliest is a classical Cynodont, and the last is a stem-mammal
called Morganucodon.