e, and the whole creature becomes a particular kind of a
bird which picks its way out of the shell and shifts for itself as a
chick.

Only a few of the countless details have been mentioned which demonstrate
the resemblance of the successive stages first to fishes, and later to
amphibia and reptiles. We have a wide choice of materials, but even the
foregoing brief list of illustrations shows that the order in which the
stages follow is the one which comparative anatomy independently proves to
be the order of the evolution of fishes, amphibia, reptiles, and birds.
Why, now, should it be necessary for a developing bird to follow this
order? The answer has been found in the immense array of embryological
facts that investigators have verified and classified, that all tell the
same story. It is, that birds have arisen by evolution from ancestors
which were really as simple as the members of these lower classes. It
seems then that the only way a bird of to-day can become itself is to
traverse the path along which its progenitors had progressed in evolution.
Stating its conclusions precisely, science formulates the principle in the
following words: _individual development is a brief resume of the history
of the species in past times_, or, more technically, _ontogeny
recapitulates phylogeny_. To be sure, the full history is not reviewed in
detail, for the chick embryo does not actually swim in water and breathe
by means of gills. Only a condensed account of evolution of its kind is
presented by an embryo during its development; as Huxley and Haeckel have
put it, whole lines and paragraphs and even pages are left out; many false
passages of a later date are inserted as the result of peculiar larval and
embryonic needs and adjustments. But in its major statements and as a
general outline, the account is a trustworthy natural document submitted
as evidence that higher species of to-day have evolved from ancestors
which must have been like some of the present lower animals.

Coming now to the mammalia, it might seem that we have reached forms so
highly developed that they would not exhibit the same kind of
developmental history, but would have their own mode of growing up. This
is not so, for like the adult fish, the larval tadpole, and the embryo
chick, an embryo of a cat or a man is at one time constructed with a
series of gill-clefts and with blood-vessels and skeletal supports of
fishlike nature that are everywhere associated