Hundreds of such embryonic adaptations are known to
embryologists. These were explained as adaptations and as falsifications of
the ancestral records.

[Illustration: FIG. 7. Diagram of chick showing relations of amnion,
allantois and yolk. (After Lillie.)]

At the end of the last century Weismann injected a new idea into our views
concerning the origin of variations. He urged that variations are germinal,
i.e. they first appear in the egg and the sperm as changes that later bring
about modifications in the individual. The idea has been fruitful and is
generally accepted by most biologists today. It means that the offspring of
a pair of animals are not affected by the structure or the activities of
their parents, but the germ plasm is the unmodified stream from which both
the parent and the young have arisen. Hence their resemblance. Now, it has
been found that a variation arising in the germ plasm, no matter what its
cause, may affect any stage in the development of the next individuals that
arise from it. There is no reason to suppose that such a change produces a
new character that always sticks itself, as it were, on to the end of the
old series. This idea of germinal variation therefore carried with it the
death of the older conception of evolution by superposition.

In more recent times another idea has become current, mainly due to the
work of Bateson and of de Vries--the idea that variations are
discontinuous. Such a conception does not fall easily into line with the
statement of the biogenetic "law"; for actual experience with discontinuous
variation has taught us that new characters that arise do not add
themselves to the end of the line of already existing characters but if
they affect the adult characters they change them without, as it were,
passing through and beyond them.

[Illustration: FIG. 8. Diagram of head of chick A and B, showing gill
slits, and aortic arches; and head of fish C showing aortic arches. (After
Hesse.)]

[Illustration: FIG. 9. Human embryo showing gill slits and aortic arches.
(After His; from Marshall.)]

I venture to think that these new ideas and this new evidence have played
havoc with the biogenetic "law". Nevertheless, there is an interpretation
of the facts that is entirely compatible with the theory of evolution. Let
me illustrate this by an example.

[Illustration: FIG. 10. Young fish, dorsal view, and side view, showing
gill slits. (After Kopsch.)]

The embryos of t