ery encouraging progress has been made in the solution of these
problems. All brachiopods form first a tiny, embryonic shell, called
the protegulum, which is believed to represent the ancestral form of the
whole group, and in the more advanced genera the developmental stages
clearly indicate the ancestral genera of the series, the succession
of adult forms in time corresponding to the order of the ontogenetic
stages. The transformation of the delicate calcareous supports of the
arms, often exquisitely preserved, are extremely interesting. Many of
the Palaeozoic genera had these supports coiled like a pair of spiral
springs, and it has been shown that these genera were derived from types
in which the supports were simply shelly loops.

The long extinct class of crustacea known as the Trilobites are likewise
very favourable subjects for phylogenetic studies. So far as the known
record can inform us, the trilobites are exclusively Palaeozoic in
distribution, but their course must have begun long before that era, as
is shown by the number of distinct types among the genera of the
lower Cambrian. The group reached the acme of abundance and relative
importance in the Cambrian and Ordovician; then followed a long, slow
decline, ending in complete and final disappearance before the end of
the Permian. The newly-hatched and tiny trilobite larva, known as
the protaspis, is very near to the primitive larval form of all the
crustacea. By the aid of the correlated ontogenetic stages and the
succession of the adult forms in the rocks, many phylogenetic series
have been established and a basis for the natural arrangement of the
whole class has been laid.

Very instructive series may also be observed among the Echinoderms and,
what is very rare, we are able in this sub-kingdom to demonstrate the
derivation of one class from another. Indeed, there is much reason to
believe that the extinct class Cystidea of the Cambrian is the ancestral
group, from which all the other Echinoderms, star-fishes, brittle-stars,
sea-urchins, feather-stars, etc., are descended.

The foregoing sketch of the palaeontological record is, of necessity,
extremely meagre, and does not represent even an outline of the
evidence, but merely a few illustrative examples, selected almost
at random from an immense body of material. However, it will perhaps
suffice to show that the geological record is not so hopelessly
incomplete as Darwin believed it to be. Since "The