==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 of the living machine by slow steps and through long ages of time.  When these three sources of history so accurately agree with each other, it is as impossible to disbelieve in the existence of such history as it is to disbelieve in the existence of the ancient Hittite nation, after its history has been told to us by two different sources of record.

Now all this is very germane to our subject.  We are trying to learn how this living machine, with its wonderful capabilities, was built.  The history which we have outlined is undoubtedly the history of the building of this machine, and the knowledge that these complicated machines have been produced as the result of slow growth is of the utmost importance to us.  This knowledge gives us at the very start some idea of the nature of the forces which have been