lution, though it supplied much useful confirmation of
the theory--you may read Kowalevsky's earlier memoirs and not realise
that they were written several years after the publication of the
_Origin of Species_.

His first paper of evolutionary importance was a note in Russian on the
development of Amphioxus, published in 1865. This subject was followed
up in two papers which appeared in 1867[387] and 1877.[388] In his
papers on Amphioxus Kowalevsky made out the main features in the
development of this primitive form, and showed that the chief organs
were formed in essentially the same way as in Vertebrates; he described
the formation of the archenteron by invagination, the appearance of the
medullary folds, which coalesced to form the neural canal, the formation
of the notochord and of the gill-slits. At first he made the mistake of
supposing that the body-cavity arose from the segmentation-cavity, but
in his later paper he rightly surmised that it was formed from the
cavities of the "primitive vertebrae," or mesodermal segments. The origin
of the notochord from the endoderm was also not made out by Kowalevsky
in his paper of 1867.

Although many important details remained to be discovered by later
investigators,[389] Kowalevsky's work at once made the development of
Amphioxus the key to vertebrate embryology, the typical ontogeny with
which all others could be compared.

Meanwhile, in 1866 and 1871, Kowalevsky had communicated memoirs of even
greater interest,[390] in which he showed that the simple Ascidians
developed in an extraordinarily similar way to Amphioxus and hence to
Vertebrates in general. His proof that Ascidians also develop on the
vertebrate type aroused great interest at the time, and was naturally
acclaimed by the evolutionists as a striking piece of evidence in favour
of their doctrine. The systematic position of the Ascidians was at that
time quite uncertain; they were grouped, as a rule, with the Mollusca,
and certainly no one suspected that their well-known tailed larvae, first
seen by Savigny, showed any but the most superficial analogy with the
tadpoles of Amphibia. Kowalevsky's papers put a different complexion on
the matter. In the first of them he showed how the nervous system of the
simple Ascidian developed from ectodermal folds just as it did in
Amphioxus and Vertebrates, how gill-slits were formed in the walls of
the pharynx, and how there existed in the ascidian larva a structure
whic