reement occurring between
some member or other of a large group a and some member or other of a
large group b, the matter is very different when by such an initial
coincidence the two members have been particularized. The chances
against these two selected members exhibiting _another_ really
independent homoplastic agreement are enormous: let us say 10,000 to 1.
The chances against yet another coincidence are a hundred million to
one, and against yet one more "coincidence" they are the square of a
hundred million to one. Homoplasy can only be assumed when the
coincidence is of a simple nature, and is such as may be reasonably
supposed to have arisen by the action of like selective conditions upon
like material in two separate lines of descent.[4]

So, too, degeneration is not to be lightly assumed as the explanation of
a simplicity of structure. There is a very definite criterion of the
simplicity due to degeneration, which can in most cases be applied.
Degenerative simplicity is never uniformly distributed over all the
structures of the organism. It affects many or nearly all the structures
of the body, but leaves some, it may be only one, at a high level of
elaboration and complexity. Ancestral simplicity is more uniform, and
does not co-exist with specialization and elaboration of a single organ.
Further: degeneration cannot be inferred safely by the examination of an
isolated case; usually we obtain a series of forms indicating the steps
of a change in structure--and what we have to decide is whether the
movement has been from the simple to the more complex, or from the more
complex to the simple. The feathers of a peacock afford a convenient
example of primitive and degenerative simplicity. The highest point of
elaboration in colour, pattern and form is shown by the great
eye-painted tail feathers. From these we can pass by gradual transitions
in two directions, viz. either to the simple lateral tail feathers with
a few rami only, developed only on one side of the shaft and of uniform
metallic coloration--or to the simple contour feathers of small size,
with the usual symmetrical series of numerous rami right and left of the
shaft and no remarkable colouring. The one-sided specialization and the
peculiar metallic colouring of the lateral tail feathers mark them as
the extreme terms of a degenerative series, whilst the symmetry,
likeness of constituent parts _inter se_, and absence of specialized
pigment, as well as