WHEN in my book, Les Colonies animales et la formation
des organismes, I attempted to explain how the different
types of the Animal Kingdom had been evolved, it was not
difficult for me after having given a history of the branched and
segmented animals, to show, as other authors had pointed out
for each group in particular, that the Annelid Worms, in all
probability, were the progenitors of the Echinoderms, Molluscs,
Vertebrates, and, in consequence, of the degenerate derivatives
of the latter, the Tunicates. But although I had at that time
already pointed out the importance of tachygenesis, I had not
as yet realized the full consequences of this mode of hereditary
action, nor had I perceived one particularly powerful cause for
the modification of organisms, namely, the changes of posture
that have taken place in each species in the course of ages.

To convince ourselves of the reality of these changes, we
need but cast our eye over the existing series of living forms.
Among the Crustaceans, Apus and other Branchiopods swim
with their ventral side uppermost and their dorsal side down-
wards; the same is true of Notonectes among the Insects, and
its name indicates this position; among the Cirripedes, Lepas
and its allies suspend themselves from floating objects by their
head, which is drawn out into a long peduncle; while forms like
Badanus, closely related to them, obliterate, so to speak, this
same extremity against the rocks to which they closely adhere.
In the subdivisions of the Tunicates we observe the same
contrast between Boltenia and the other Ascidiacea, while the
Tunicates that have reverted to swimming retain the
normal position. Among the Echinoderms the common
sea-urchin has its mouth below and the anus above,
so that the five radial areas, bearing their organs of
locomotion, are erected vertically like the petals of a flower,
all five arising from the mouth and capable of reaching the
orifice opposite. But there are some species which dig cease-

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