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.

[Illustration:  FIG. 47.—­The arm of a monkey, a prehensile appendage.]

[Illustration:  FIG. 48.—­The arm of a bird, a flying appendage.  In life covered with feathers.]

[Illustration:  FIG. 49.—­The arm of an ancient half-bird half-reptile animal.  In life covered with feathers and serving as a wing.]