a common standard, and the intermarriages of individuals
of the most remote varieties are not less fruitful than between those of
the same tribe.

_Tiedemann on the brain of the foetus in vertebrated animals._--There
is yet another department of anatomical discovery to which I must
allude, because it has appeared to some persons to afford a distant
analogy, at least, to that progressive development by which some of the
inferior species may have been gradually perfected into those of more
complex organization. Tiedemann found, and his discoveries have been
most fully confirmed and elucidated by M. Serres, that the brain of the
foetus, in the highest class of vertebrated animals, assumes, in
succession, forms, bearing a certain degree of resemblance to those
which belong to fishes, reptiles, and birds, before it acquires the
additions and modifications which are peculiar to the mammiferous tribe;
so that, in the passage from the embryo to the perfect mammifer, there
is a typical representation, it is said, of all those transformations
which the primitive species are supposed to have undergone, during a
long series of generations, between the present period and the remotest
geological era.

"If you examine the brain of the mammalia," says M. Serres, "at an early
stage of uterine life, you perceive the cerebral hemispheres
consolidated, as in fish, in two vesicles, isolated one from the other;
at a later period, you see them affect the configuration of the cerebral
hemispheres of reptiles; still later again, they present you with the
forms of those of birds; finally they acquire, at the era of birth, and
sometimes later, the permanent forms which the adult mammalia present.

"The cerebral hemispheres, then, arrive at the state which we observe in
the higher animals only by a series of successive metamorphoses. If we
reduce the whole of these evolutions to four periods, we shall see, that
in the first are born the cerebral lobes of fishes; and this takes place
homogeneously in all classes. The second period will give us the
organization of reptiles; the third, the brain of birds; and the fourth,
the complex hemispheres of mammalia.

"If we could develop the different parts of the brain of the inferior
classes, we should make, in succession, a reptile out of a fish, a bird
out of a reptile, and a mammiferous quadruped out of a bird. If, on the
contrary, we could starve this organ in the mammalia, we might reduce it
succe