found to be particularly difficult. When I made the first attempt in
my General Morphology (1866) to work out the theory and apply it to
classification, I found no problem of phylogeny that gave me so much
trouble as the linking of the vertebrates with the invertebrates.

But just at this time the true link was discovered, and at a point
where it was least expected. Towards the end of 1866 two works of the
Russian zoologist, Kowalevsky, who had lived for some time at Naples,
and studied the embryology of the lower animals, were issued in the
publications of the St. Petersburg Academy. A fortunate accident had
directed the attention of this able observer almost simultaneously to
the embryology of the lowest vertebrate, the Amphioxus, and that of an
invertebrate, the close affinity of which to the Amphioxus had been
least suspected, the Ascidia. To the extreme astonishment of all
zoologists who were interested in this important question, there
turned out to be the utmost resemblance in structure from the
commencement of development between these two very different
animals--the lowest vertebrate and the mis-shaped, sessile
invertebrate. With this undeniable identity of ontogenesis, which can
be demonstrated to an astounding extent, we had, in virtue of the
biogenetic law, discovered the long-sought genealogical link, and
definitely identified the invertebrate group that represents the
nearest blood-relatives of the vertebrates. The discovery was
confirmed by other zoologists, and there can no longer be any doubt
that of all the classes of invertebrates that of the Tunicates is most
closely related to the vertebrates, and of the Tunicates the nearest
are the Ascidiae. We cannot say that the vertebrates are descended
from the Ascidiae--and still less the reverse--but we can say that of
all the invertebrates it is the Tunicates, and, within this group, the
Ascidiae, that are the nearest blood-relatives of the ancient