onsisting of Mammals, Birds,
Fishes, and Reptiles. II. Invertebrata, consisting of Molluscs,
Centipedes, Annelids, Crustacea, Arachnids, Insects, Worms, Radiata,
Polyps, Infusoria.

"The degradation of organism," he concludes, "in this descending scale
is not perfectly even, and cannot be made so by any classification,
nevertheless there is such evidence of sustained degradation in the
principal groups as must point in the direction of some underlying
general principle."[263]

Lamarck's sixth chapter is headed "Degradation and Simplification of the
Animal Chain as we proceed downwards from the most complex to the most
simple Organisms."

"This is a positive fact, and results from the operation of a constant
law of nature; but a disturbing cause, which can be easily recognized,
varies the regular operation of the law from one end to the other of the
chain of life.[264]

"We can see, nevertheless, that special organs become more and more
simple the lower we descend; that they become changed, impoverished, and
attenuated little by little; that they lose their local centres, and
finally become definitely annihilated before we reach the lowest
extremity of the chain.[265]

"As has been said already, the degradation of organism is not always
regular; such and such an organ often fails or changes suddenly, and
sometimes in its changes assumes forms which are not allied with any
others by steps that we can recognize. An organ may disappear and
reappear several times before being entirely lost: but this is what we
might expect, for the cause which has led to the evolution of living
organisms has evolved many varieties, due to external influences.
Nevertheless, looking at organization broadly, we observe a descending
scale."[266]

"If the tendency to progressive development was the only cause which had
influenced the forms and organs of animals, development would have been
regular throughout the animal chain; but it has not been so: Nature is
compelled to submit her productions to an environment which acts upon
them, and variation in environment will induce variation in organism:
this is the true cause of the sometimes strange deviations from the
direct line of progression which we shall have to observe.[267]

"If Nature had only called aquatic beings into existence, and if these
beings had lived always in the same climate, in the same kind of water,
and at the same depth, the organization of these animals would doubtles