like that possessed by reptiles, is next
formed by the subdivision of the auricle previously existing; and
lastly, a fourth chamber is produced by the growth of a partition
across the ventricle; and in perfect harmony with these changes are
the metamorphoses presented by the system of vessels immediately
proceeding from the heart. In like manner, the evolution of the brain
in man is found to present conditions which may be successively
compared with those of the fish, reptile bird, lower mammalia, and
higher mammalia; but in no instance is there an exact identity
between any of these. It is to be remembered, that every animal must
pass through _some_ change in the progress of its development, from
its embryonic to its adult condition; and the correspondence is much
closer between the embryonic fish and the foetal bird, or mammal,
than between these and the adult fish."--(P. 196.)

And take, also, the following short passage from the preface of the same
work, where the author has been speaking of the latest discoveries of
physiologists on the development of the embryo.

"Thus, when we ascend the scale of being, in either of the two
organized kingdoms, we observe the principle of specialisation
remarkably illustrated in the development of the germ into the
perfect structure. In the lowest of each kind, the first-formed
membranous expansion has the same character throughout, and _the
whole enters into the fully-developed structure_. In higher grades
the whole remains, but the organs evolved from the centre have
evidently the most elevated character. _In the highest none but the
most central portion is persistent_; the remainder forming organs of
a temporary and subservient nature."

The fact that the animal kingdom exhibits a gradual progression from forms
the most simple to forms the most complex, is, of course, appropriated by
our author as a proof of his theory of successive development. It is well
known, that whilst this scale of being is an idea which occurs to every
observer, the naturalist finds insuperable difficulties in arranging the
several species of animals according to such a scale. To relieve himself
from these, the author has taken under his patronage what, in honour of
its founder, he calls the _Macleay System_, in which the animal kingdom is
"arranged along a series of close affinities, _in