160 THE STORY OF LIFE'S MECHANISM. 



At the outset we have the question of fact, and 

 we must notice the grounds upon which we 

 stand in assuming this history to be as outlined. 

 This problem is the one which has occupied 

 such a prominent place in the scientific world 

 during the last forty years, and which has contri- 

 buted 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 thor- 

 oughly 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 his- 

 torical one. This long history of the construc- 

 tion 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 



