"marsupial bones," though very like an opossum's, would by no
means have warranted the prediction that the skull would turn
out to be that of the opossum. It might just as well have been
like that of some other marsupial; or even like that of the
totally different group of Monotremes, of which the only living
representatives are the Echidna and the
Ornithorhynchus.

For all practical purposes, however, the empirical laws of co-
ordination of structures, which are embodied in the
generalisations of morphology, may be confidently trusted, if
employed with due caution, to lead to a just interpretation of
fossil remains; or, in other words, we may look for the
verification of the retrospective prophecies which are based
upon them.

And if this be the case, the late advances which have been
made in palaeontological discovery open out a new field for such
prophecies. For it has been ascertained with respect to many
groups of animals, that, as we trace them back in time, their
ancestors gradually cease to exhibit those special modifications
which at present characterise the type, and more nearly embody
the general plan of the group to which they belong.

Thus, in the well-known case of the horse, the toes which are
suppressed in the living horse are found to be more and more
complete in the older members of the group, until, at the bottom
of the Tertiary series of America, we find an equine animal
which has four toes in front and three behind. No remains of the
horse tribe are at present known from any Mesozoic deposit.