150 SOCIAL EVOLUTION 



the land stood the drain of thousands of years of culti- 

 vation required to support the thick population of the 

 valley. The date-palm, easy of cultivation, offered 

 nourishing food. The vast stretches of the desert be- 

 yond the valley wall protected the inhabitants from ex- 

 ternal foes. Warm climate, fertile soil, constant water 

 supply and protection from invasion made easily pos- 

 sible production beyond the necessities of life. Such 

 surplus is absolutely essential to the development of 

 civilization. So it was that the Nile valley became one 

 of the earliest culture centers. The formation of the 

 valley, making the river always accessible, facilitated the 

 development of trade and commercial relations between 

 the many little states first appearing along the Nile. 

 The disposable wealth created by this combination of 

 happy circumstances led in later centuries to the rise of 

 non-laboring classes rulers, courtiers, soldiers, priests, 

 landlords, and merchant princes at times serviceable, 

 at other times merely parasitic. The leisure made pos- 

 sible by slave labor on a gigantic scale gave time for the 

 development of art, literature, science and philosophy. 

 Civilization resulted from surplus production depend- 

 ing in turn upon the existence of certain natural resources 

 and favorable conditions of climate and location. 



"Egypt affords an excellent example of the value of 

 climatic study. . . . Here we have a hot, dry climate 

 where the main dependence for the crops is not on the 

 rains but on the rise of the Nile. This rise, regular as 

 the seasons, the comparatively small change in tempera- 

 ture among the seasons themselves, the almost complete 

 absence of rainfall, taken in connection with the fertility 

 of the soil and the small number of staple crops, has pro- 

 duced a condition of affairs in which all that is demanded 



