The first two stages in the development of civilized man were probably the invention of primitive weapons and the discovery of fire, al though nobody knows exactly when acquired the use of (1)_____. The (2)_____ of language is also obscure. No doubt it began very gradually. Animals have a few cries that serve (3)_____ signals, (4)_____ even the highest apes have not been found able to pronounce words (5)_____ with the most intensive professional instruction. The superior brain of man is apparently (6)_____ for the mastering of speech. When man became sufficiently intelligent, we must suppose that he (7)_____ the number of cries for different purposes. It was a great day (8)_____ he discovered that speed could be used for narrative. There are those who think that (9)_____ picture language preceded oral language. A man (10)_____ a picture on the wall of his cave to show (11)_____ direction he had gone, or (12)_____ prey he hoped to catch. Probably-picture language and oral language developed side by side. I am inclined to think that language (13)_____ the most important single factor in the development of man. Two important stages came not (14)_____ before the dawn of written history. The first was the domestication of animals; the second was agriculture. Agriculture was (15)_____ in human progress to which subsequently there was nothing comparable (16)_____ our own machine age. Agriculture made possible (17)_____ immense increase in the number of the human species in the regions where it could be successfully practiced. (18)_____ were, at first, only those in which nature fertilized the soil (19)_____ each harvest. Agriculture met with violent resistance from the pastoral nomads, but the agricultural way of life prevailed in the end (20)_____ the physical comforts it provided.