were tree-living animals. These
were the occurrence of an opposable big toe (when this was present at
all), the great development of the fourth toe, the reduction and partial
syndactylism of the second and third toes, and in some cases the
regression of the nails. These characters were shown to be typical of
arboreal Vertebrates, and their occurrence in forms not arboreal
indicated that these were descended from tree-living ancestors. Traces
of an arboreal ancestry could be demonstrated even in the marsupial mole
_Notoryctes_.

These are only two examples out of hundreds that might be given. Present
day structure was interpreted in the light of past history; the common
element in organic form was seen to be due to common descent; the
existence of vestigial and non-functional organs was no longer a riddle.

There was even a tendency to concentrate attention upon the historical
side of structure, upon what the animal passively inherited rather than
upon what it personally achieved. Homologies were considered more
interesting than analogies, vestigial organs more interesting than
foetal and larval adaptations. Convergence was anathema. The dead-weight
of the past was appreciated at its full and more than its full value;
and the essential vital activity of the living thing, so clearly shown
in development and regeneration, was ignored or forgotten.

But evolutionary morphology for all practical purposes was a development
of pure or idealistic morphology, and was powerless to bring to fruit
the new conception with which evolution-theory had enriched it. The
reason is not far to seek. Pure morphology is essentially a science of
comparison which seeks to disentangle the unity hidden beneath the
diversity of organic form. It is not immediately concerned with the
causes of organic diversity--that is rather the task of the sciences of
the individual, heredity and development. To take an example--the
recapitulation theory may legitimately be used as a law of pure
morphology, as stating the abstract relation of ontogeny to phylogeny,
and the probable line of descent of any organism may be deduced from it,
as a mere matter of the ideal derivation of one form from another; but
an explanation of the reason for the recapitulation of ancestral history
during development can clearly not be given by pure morphology unaided.
From the fact that the common starfish shows in the course of its
development distinct traces of a stalk[463] it i