lustration: 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.]

==Significance of these Sources of History.==--The real force of these
sources of evidence comes to us only when we compare them with each
other. They agree in a most remarkable fashion. The history as disclosed
by fossils and that told by embryology agree with each other, and these
are in close harmony with the history as it can be read from comparative
anatomy. If archaeologists were to find, in different countries and
entirely unconnected with each other two or more different records of a
lost nation, the belief in the actual existence of that nation would be
irresistible. When researches at Nineveh, for example, unearth tablets
which give the history of ancient nations, and when it proves that among
the nations thus mentioned are some with the same names and having the
same facts of history as those mentioned in the Bible, it is absolutely
impossible to avoid the conclusion that such a nation with such a
history did actually exist. Two independent sources of record could not
be false in regard to such a matter as this.

Now, our sources of evidence for this history of the living machine
prove to be of exactly this kind. We have three independent sources of
evidence which are so entirely different from each other that there is
almost no likeness between them. One is written in the rocks, one in
bone and muscle, while the third is recorded in the evanescent and
changing pages of embryology and metamorphosis. Yet each tells the same
story. Each tells of a history of this machine from simple forms to more
complex. Each tells of its greater and greater differentiation of labour
and structure as the periods of time passed. Each tells of a growing
complexity and an increasing perfection of the organisms as successive
periods pass. Each tells us of common points of origin and divergence
from these points. Each tells us how the more complicated forms have
arisen as the results of changes in and modifications of the simpler
forms. Each shows us how the individual parts of the organisms have been
enlarged or diminished or changed in shape to adapt them to new duties.
Each, in short, tells the same story of the gradual construction