state."

As this passage shows, Darwin held that embryology was of interest
because of the light it seems to throw upon ancestral history
(phylogeny) and because of the help it would give in enabling us to
arrive at a natural system of classification. With regard to the latter
point, he quotes with approval the opinion that "the structure of
the embryo is even more important for classification than that of the
adult." What justification is there for this view? The phase of life
chosen for the ordinary anatomical and physiological studies, namely,
the adult phase, is merely one of the large number of stages of
structure through which the organism passes. By far the greater number
of these are included in what is specially called the developmental
or (if we include larvae with embryos) embryonic period, for the
developmental changes are more numerous and take place with greater
rapidity at the beginning of life than in its later periods. As each of
these stages is equal in value, for our present purpose, to the adult
phase, it clearly follows that if there is anything in the view that
the anatomical study of organisms is of importance in determining their
mutual relations, the study of the organism in its various embryonic
(and larval) stages must have a greater importance than the study of the
single and arbitrarily selected stage of life called the adult.

But a deeper reason than this has been assigned for the importance of
embryology in classification. It has been asserted, and is implied by
Darwin in the passage quoted, that the ancestral history is repeated
in a condensed form in the embryonic, and that a study of the latter
enables us to form a picture of the stages of structure through which
the organism has passed in its evolution. It enables us on this view to
reconstruct the pedigrees of animals and so to form a genealogical tree
which shall be the true expression of their natural relations.

The real question which we have to consider is to what extent the
embryological studies of the last 50 years have confirmed or rendered
probable this "theory of recapitulation." In the first place it must
be noted that the recapitulation theory is itself a deduction from the
theory of evolution. The facts of embryology, particularly of vertebrate
embryology, and of larval history receive, it is argued, an explanation
on the view that the successive stages of development are, on the
whole, records of adult stages of struc