26 THE PATH OF SCIENCE 



type of pottery was examined and compared with the other 

 examples. This process of comparison resuked in bringing 

 the thousand graves into a connected order in time, each 

 grave as a general rule containing some of the pottery of the 

 graves near it in the order but not containing pots of those 

 that were more distant in the order. The whole series of 

 graves could be divided into fifty parts, and these were num- 

 bered arbitrarily from 30 to 80 in order to leave space for 

 later discoveries of graves that might not fit into the sequence 

 and that might have to be placed before or after those that 

 had been examined. In this way, a definite sequence dating 

 could be made for the graves and, therefore, for the pottery 

 and other material found in the graves, ending ^vith the 

 graves of the historical dynasties for which chronological 

 dates were kno^vn. The same method has been applied to 

 the dating of the different levels of excavation in Mesopo- 

 tamia and Syria. Indeed, our knowledge of prehistoric Meso- 

 potamia is almost entirely dependent on dating by means of 

 pottery. 



At this stage in the history of civilization, when men had 

 the good tools of the neolithic age and pots hardened in the 

 fire, a new factor of fundamental importance appeared— the 

 second of the great inventions of mankind. 



Agiiculture was probably discovered by the women, who 

 gathered the seeds of plants while their men hunted animals. 

 One day they must have realized that seeds could be sown 

 artificially and that, if they waited long enough, seeds pro- 

 duced a crop. With the coming of agriculture came real 

 civilization. Men ceased to be nomads. They settled in 

 villages; and those villages were naturally along the river 

 valleys, where there was mud, in which the seeds could be 

 planted, and water, necessary for plant growth. 



There, in the villages or, rather, in the to^vns into which 

 the villages had grown, came the third great invention— writ- 

 ing. And with writing, the period of prehistory ends and 

 history commences. Man began to write five or six thousand 

 years ago. Those who study the river valleys of Mesopotamia 



