able, while Mr. Darwin simply says they _are_ most variable,
and that, this being so, the favourable variations will be preserved and
accumulated--an assertion which Lamarck would certainly not demur to.

"Irregular degrees of perfection," says Lamarck, "and degradation in the
less essential organs, are due to the fact that these are more liable
than the more essential ones to the influence of external circumstances:
these induce corresponding differences in the more outward parts of the
animal, and give rise to such considerable and singular difference in
species, that instead of being able to arrange them in a direct line of
descent, as we can arrange the main groups, these species often form
lateral ramifications round about the main groups to which they belong,
and in their extreme development are truly isolated."[259]

In his summary of the second chapter of his 'Origin of Species,' Mr.
Darwin well confirms this when he says, "In large genera the species are
apt to be closely, but unequally, allied together, forming little
clusters round other species."

"A longer time," says Lamarck, "and a greater influence of surrounding
conditions, is necessary in order to modify interior organs.
Nevertheless we see that Nature does pass from one system to another
without any sudden leap, when circumstances require it, provided the
systems are not too far apart. Her method is to proceed from the more
simple to the more complex.[260]

"She does this not only in the race, but in the individual." Here
Lamarck, like Dr. Erasmus Darwin, shows his perception of the importance
of embryology in throwing light on the affinities of animals--as since
more fully insisted on by the author of the 'Vestiges of Creation,' and
by Mr. Darwin,[261] as well as by other writers. "Breathing through
gills is nearer to breathing through lungs than breathing through
trachea is. Not only do we see Nature pass from gills to lungs in
families which are not too far apart, as may be seen by considering the
case of fishes and reptiles; but she does so during the existence of a
single individual, which may successively make use both of the one and
of the other system. The frog while yet a tadpole breathes through
gills; on becoming a frog it breathes through lungs; but we cannot find
that Nature in any case passes from trachea to lungs."[262]

Lamarck now rapidly reviews previous classifications, and propounds his
own, which stands thus:--I. Vertebrata, c