This problem is the one which has occupied such a prominent place in the scientific world during the last forty years, and which has contributed so largely toward making modern biology such a different subject from the earlier studies of natural history.  It is simply the evidence for organic evolution, or the theory of descent.  The subject has for forty years been thoroughly sifted and tested by every conceivable sort of test.  As a result of the interest in the question there has been disclosed an immense mass of evidence, relevant and irrelevant.  As the evidence has accumulated it has become more and more evident that the evolution theory must be recognized as the only one which is in accord with the facts, and the outcome has been a practical unanimity among thinkers that the theory of descent must be the foundation of our further study.  The evidence which has forced this conclusion upon scientists we must stop for a moment to consider, since it bears very directly upon the subject we are studying.

==Historical.==—­The first source of evidence is naturally a historical one.  This long history of the construction of the living machine has left its record in the rocks which form the earth’s surface.  During this long period the rocks of the earth’s crust have been deposited, and in these rocks have been left samples of many of the steps in this history of machine building.  The history can be traced by the study of these samples just as the history of any machine might be traced from a study of the models in a patent office.  One might very easily trace, with most strict accuracy and minute detail, the history of the printing machine from the models which are preserved in the patent offices and elsewhere.  So is it with the history of the living machine.  To be sure, the history is rather incomplete and at times difficult to read.  Many a period in the development has left no samples for our inspection and must be interpreted in our history between what went before and what comes after.  Many of the machines, especially the early ones, were made of such fragile material that they could not be preserved in the rocks.  In many a case, too, the rocks in which the specimens were deposited have been subjected to such a variety of heatings and pressures, that they have been twisted out of shape and even crushed out of recognizable form.  But in spite of this the record is showing itself more complete each year.  Our paleontologists are opening layer after layer of these rocks, and thus examining each year new pages in nature’s history.  The more recent epochs in the history have been already read with almost historic accuracy.  From them we have learned in great detail how the finishing touches were given to these machines, and are able to trace with accuracy how the somewhat more generalized forms of earlier days were changed to produce our modern animals.