176 EVOLUTION OF THE EARTH 



less common there than in colder climates. In warm regions 

 cooking is apt to be done over small fires that do not make 

 people uncomfortable, or else in pits dug in the hot ashes; 

 and the temperature is rarely high enough to melt iron ore. 

 Farther north, however, where the winters are chilly, huge 

 fires must have been common from the earliest times. There, 

 too, in the fairly dry areas many stones lie on the surface 

 almost everywhere. The next necessity is that when some bits 

 of ore happened to be in a particularly hot fire a man of 

 unusual genius should be present and observe the molten 

 metal. This might occur anywhere, but we have already seen 

 reasons for believing that quickness of intellect is fostered in 

 some climates more than in others. Finally, the fourth requi- 

 site, and much the most important, is that the man of genius 

 have the zeal and determination, as Darwin puts it, to bring 

 to fruition the ideas engendered by his observations. A single 

 mind, however, was not enough to consummate the great dis- 

 covery. It was necessary that the generation of men who 

 lived with the genius should be "in a mental mood to receive 

 th^j3w_Jnvention or discovery." That mood, together with 

 the necessary energy olTtfe-prnrT'oTliHc^^ found 



within the tropics. It is common in region^BTessecTwith a 

 stimulating climate. Thus each of the conditions controlling 

 the discovery of the art of smelting is so much stronger outside 

 the tropics than within them that it seems highly probable 

 that the art arose in a stimulating extra-tropical climate. It 

 is generally supposed that the use of iron originated in North 

 "Africa perhaps 6,000 years ago. This is eminently consistent 

 with our conclusions. At that time, as is almost universally 

 admitted by geologists, the climate of the world was inter- 

 mediate between that of the Glacial Period and the present. 

 Therefore North Africa was then a decidedly more stimulat- 

 ing place than it is today. This same line of reasoning applies 

 to other great steps in the early development of civilization, 





