end, within the limits
of the group, from their most complex to their simplest form. But each
organ, considered individually, does not descend by equally regular
gradation; the gradations are less and less regular according as the
organ is of less importance, and is more susceptible of modification by
the conditions which surround it. Organs of small importance, and not
essential to existence, are not always either perfected or degraded at
an equal rate, so that in observing all the species of any class we find
an organ in one species in the highest degree of perfection, while
another organ, which in this same species is impoverished or very
imperfect, is highly developed in another species of the same
group."[257]

The facts maintained in the preceding paragraph are in great measure
supported by Mr. Charles Darwin, who, however, assigns their cause to
natural selection.

Mr. Darwin writes, "Ordinary specific characters are more variable than
generic;" and again, a little lower down, "The points in which all the
species of a genus resemble each other, and in which they differ from
allied genera, are called generic characters; and these characters may
be attributed to inheritance from a common progenitor, for it can rarely
happen that natural selection will have modified several distinct
species fitted to more or less widely different habits, in exactly the
same manner; and as these so called generic characters have been
inherited from before the period when the several species first branched
off from their common progenitor, and subsequently have not varied or
come to differ in any degree, or only in a slight degree, it is not
probable that they should vary at the present day. On the other hand,
the points in which species differ from other species of the same genus
are called specific characters; and as these specific characters have
varied and come to differ since the period when the species branched off
from a common progenitor, it is probable that they should still often be
in some degree variable, or at least more variable than those parts of
the organization which have for a very long time remained
constant."[258]

The fact, then, that it is specific characters which vary most is agreed
upon by both Lamarck and Mr. Darwin. Lamarck, however, maintains that it
is these specific characters which are most capable of being affected by
the habits of the creature, and that it is for this reason they will be
most vari