43

80

much more alike than are fishes or mammals;
and that will strengthen any
induction about birds. Orchids, on the
other hand, are extraordinarily various.

In the second place a character may
have a greater or less tendency to be
present or absent throughout the whole
of certain kinds of groups. Thus, coloration
often differs within one species,
while the number of the principal bones
of the skeleton, and almost all characters
which are developed early in individual
life and which persist to maturity
are common to all the members
of large classes.