ndo-European races, meaning thereby that back in the history of man
these races had a common point of origin. We never speak of any real
relation of objects unless thereby we mean to imply historical
connection. We are therefore justified in interpreting the manifest
relationships of organisms as pointing to history. Particularly are we
justified in this conclusion when we find that the relationships which
we draw between the types of life now in existence run parallel to the
history of these types as revealed to us by fossils and at the same time
disclosed by the study of embryology.

This subject of comparative anatomy includes a consideration of what is
called homology, and perhaps a concrete example may be instructive both
in illustration and as suggesting the course which nature adopts in
constructing her machines. We speak of a monkey's arm and a bird's wing
as homologous, although they are wonderfully different in appearance and
adapted to different duties. They are called homologous because they
have similar parts in similar relations. This can be seen in Figs. 47
and 48, where it will be seen that each has the same bones, although in
the bird's wing some of the bones have been fused together and others
lost. Their similarity points to a relationship, but their dissimilarity
tells us that the relationship is a distant one, and that their common
point of origin must have been quite far back in history. Now if we
follow back the history of these two kinds of appendages, as shown to us
by fossils, we find them approaching a common point. The arm can readily
be traced to a walking appendage, while the bird's wing, by means of
some interesting connecting links, can in a similar way be traced to an
appendage with its five fingers all free and used for walking. Fig. 49
shows one of these connecting links representing the earliest type of
bird, where the fingers and bones of the arm were still distinct, and
yet the whole formed a true wing. Thus we see that the common point of
origin which is suggested by the likenesses between an arm and a wing is
no mere imaginary one, for the fossil record has shown us the path
leading to that point of origin. The whole tells us further that
nature's method of producing a grasping or flying organ was here, not to
build a new organ, but to take one that had hitherto been used for other
purposes, and by slow changes modify its form and function until it was
adapted to new duties.

[Il