An amoeba can do an unusual mathematical trick: It multiplies by dividing. After it has grown to a certain size, the amoeba’s single cell divides in half to produce two amoebas. In about a day, the two amoebas have grown to the point at which they are ready to divide and form four; the day after that, there are eight amoebas, and so forth. How many amoebas will there be at the end of the week?

The institution of the foreign migrant worker appears around the world when a relatively rich country is near a relatively poor one and there is access between them. Mexican farm workers in the United States, Algerian farm workers in France, Turkish laborers in Germany, and Ethiopian domestic servants in Kuwait are all examples. Each example shows the combined effects of overpopulation and unequal distribution of employment opportunity and wealth. In all cases, there are dangers that migrant workers will be exploited by local employers who recognize their vulnerability and that they will be resented by local workers who see them as competitors. Cesar Chavez (1927–1994) organized the United Farm Workers union in California in response to such problems in 1962 and after a multiyear strike was able to obtain better wages and working conditions for grape pickers. Similar unions have developed in many places around the world.

Drama languished in Europe after the fall of Rome during the fifth and sixth centuries. From about A.D. 400 to 900 almost no record of dramatic productions exists except for those of minstrels and other entertainers, such as acrobats and jugglers, who traveled through the countryside. The Catholic church was instrumental in suppressing drama because the theater—represented by the excesses of Roman productions— was seen as subversive. No state-sponsored festivals brought people together in huge theaters the way they had in Greek and Roman times.

One form of distraction is imagery. Creating a vivid mental image can help control pain or other unpleasant physical symptoms. Usually people create a pleasant and progressive scenario, such as walking along the beach, hiking in the mountains, or enjoying a gathering of friends. Aggressive or arousing imagery can also be useful, such as imagining a heated argument, fighting off an enemy, or driving a race car at high speeds. Whatever imaginary scenario you use, try to visualize all the different sensations involved, including the sights, sounds, aromas, touches, and tastes. The goal is to become so absorbed in your fantasy that you distract yourself from the pain sensations.

Before the twentieth century, women had a single role, that of wife and mother. Occasionally accidents of royal succession placed a woman in a position of great power, and the eighteenth century saw two amazingly long-lasting cases: Catherine the Great, Empress of Russia, who ruled from 1762 to 1796, and Maria Theresa, de facto [in fact] empress of the Austrian Empire from 1740 to 1780. But what we now think of as “careers” were simply not open to women, with very few exceptions.

In 1985, scientists discovered that the high-altitude ozone layer, which shields organisms from harmful ultraviolet radiation, had thinned greatly over Antarctica. Ozone (O3) is produced by the action of sunlight on atmospheric oxygen (O2) in tropical regions and is transported to high latitudes, where it is destroyed. Chlorine compounds, produced mainly by humans, appear to be the main cause of the unusually high rates of ozone destruction. Ozone is now seriously depleted at very high latitudes, where conditions favor its destruction.