eories and
strongly rooted prejudices regarding his own position in nature,
and his relations to the underworld of life; while that which
remains a dim suspicion for the unthinking, becomes a vast
argument, fraught with the deepest consequences, for all who are
acquainted with the recent progress of the anatomical and
physiological sciences."

Huxley then proceeded to elaborate the argument from development for
the essential identity of man and the apes. This argument has now
become more or less familiar to us all, as it has gained additional
support from recent extension of embryological knowledge, and as it
has been used in every work on evolution since Huxley first laid
stress on it. The adult forms of animals are much more complex than
their embryonic stages, and the series of changes passed through in
attaining the adult condition make up the embryological history of the
animal. Huxley took the embryology of the dog as an example of the
process in the higher animals generally, and as it had been worked out
in detail by a set of investigators. The dog, like all vertebrate
animals, begins its existence as an egg; and this body is just as much
an egg as that of a fowl, although, in the case of the dog, there is
not the accumulation of nutritive material which bloats the egg of the
hen into its enormous size. Since Huxley wrote, it has been shewn
clearly that among the mammalian animals there has been a gradual
reduction in the size of the egg. The ancestors of the mammals laid
large eggs, like those of birds or reptiles; and there still exist two
strange mammalian creatures, the Ornithorhynchus and Echidna of
Australia, which lay large, reptilian-like eggs. The ancestors of most
living mammalia acquired the habit of retaining the eggs within the
body until they were hatched; and, as a result of this, certain
structures which grow out from the embryo while it is still within the
egg and become applied to the inner wall of the porous shell for the
purpose of obtaining air, got their supply of oxygen, not from the
outer air, but from the blood-vessels of the maternal tissues. When
this connection (called the placenta) between embryo and mother
through the egg-shell became more perfect, not only oxygen but
food-material was obtained from the blood-vessels of the mother; and,
in consequence, it became unnecessary for the eggs to be provided with
a large supply of food-yolk. Among existing mar