THE LIVING MACHINE BUILDING FACTORS. 139 



Historical. The first source of evidence is 

 naturally a historical one. This long history of the 

 construction of the living machine has left its rec- 

 ord 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 his- 

 tory of machine building. The history can be 

 traced by the study of these samples just as the his- 

 tory 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 inter- 

 preted in our history between what went before 

 and what comes after. Many of the machines, 

 especially the early ones, were made of such frag- 

 ile 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 



