quite recently. For many
years physiologists held that cells were units of function just as
much as they are units of structure; but in the last ten years there
has been a strong return to the opinion of Huxley.

In 1851 two very important memoirs were published in the _Transactions
of the Royal Society_, which contained the results of Huxley's
observations of the interesting animals known as "tunicates." The
first of these papers begins as follows:

"The Salpae, those strange gelatinous animals, through masses of
which the voyager in the great ocean sometimes sails day after
day, have been the subject of a great controversy since the time
of the publication of the celebrated work of Chamisso, _De
Animalibus Quibusdam e Classe Vermium Linnaeana_. In this work
there were set forth, for the first time, the singular phenomena
presented by the reproductive processes of these
animals,--phenomena so strange, and so utterly unlike anything
then known to occur in the whole province of zooelogy, that
Chamisso's admirably clear and truthful account was received with
almost as much distrust as if he had announced the existence of a
veritable Peter Schlemihl."

According to Chamisso, salps appeared in two forms: solitary forms,
and forms in which a number of salps are united into a long chain.
Each salp of the aggregate form contains within it an embryo receiving
nutrition from the mother by a connection similar to the placenta by
which the embryo of a mammal receives nourishment from the blood of
the mother. These embryos grow up into the solitary form, and the
solitary form gives rise to a long chain of the aggregate form which
developes in the interior of the body. Chamisso compared this progress
to the development of insects. "Supposing," he said, "caterpillars did
not bodily change into butterflies, but by a process of sexual
breeding produced young which grew into the ordinary adults, and that
these adults, as indeed they do, gave rise to caterpillars by sexual
reproduction, then there would be a true alternation of generations."
The first generation would give rise to a second generation totally
unlike itself, and this second generation would reproduce, not its
kind, but the first generation; such an alternation of generations he
stated to occur among the salps. Huxley had an excellent opportunity
to study this question at Cape York in November, 1849. "For a time