LAMARCK’S THEORY OF DESCENT 291 
, 
degraded organization, and are nearer the fishes’ 
(p. 155). 
Referring on the next page to the fishes, he re- 
marks :— 
“Without doubt their general form, their lack of 
a constriction between the head and the body to 
form a neck, and the different fins which support 
them in place of legs, are the results of the influence 
of the dense medium which they inhabit, and not 
that of the dégradation of their organization. But 
this modification (dégradation) is not less real and 
very great, as we can convince ourselves by examin- 
ing their internal organs; it is such as to compel us 
to assign to the fishes a rank lower than that of the 
reptiles.” 
He then states that the series from the lamprey 
and fishes to the mammals is not a regularly gradated 
one, and accounts for this “ because the work of 
nature has been often changed, hindered, and di- 
verted in direction by the influences which singu- 
larly different, even contrasted, circumstances have 
exercised on the animals which are there found ex- 
posed in the course of a long series of their renewed 
generations.” 
Lamarck thus accounts for the production of the 
radial symmetry of the medusz and echinoderms, 
his Radiaires. At the present day this symmetry is 
attributed perhaps more correctly to their more or less 
fixed mode of life. 
“Tt is without doubt by the result of this means 
which nature employs, at first with a feeble energy 
with polyps, and then with greater developments in 
the Radzata, that the radial form has been acquired; 
