1900: Patients battle illness, while doctors can do little more than counsel and comfort them and keep them clean. Today: Doctors treat and often cure patients with a vast array of medicines and medical technologies, but some diseases are still incurable.

1900: We know the distances to 70 of the nearest stars. There are no galaxies except for the Milky Way, although some astronomers do believe that some of the fuzzy "nebulae" seen in the night sky are separate, very distant galaxies. The universe is unchanging. Newton's laws are used to explain all physical phenomena. The elements that have been identified are believed to be built of indivisible atoms that are the "essence" of the element. Physicists believe there is little more to be discovered in their field. However, there are a few nagging issues, such as the behavior of light, which will spur a revolution in the field as the new century begins.

1900: Many people suffer from the stresses the industrial, urbanizing society, experiencing a variety of nervous disorders, such as insomnia, headaches, anxiety, and exhaustion. Some doctors think this is a disease they call "neurasthenia," and there are many patent medicines claiming to be cures. Some think that you are born with a temperament toward this condition, as are those who succumb to serious mental illness and are confined to asylums for the insane. A few doctors begin to look for a way to treat the mind, rather than the body, to cure these conditions. Today: Science has come to understand that our mental condition is part of a complex interplay between our biology and our environment. We have identified brain structures and chemistry which control behavior and have produced medicines which affect our behavior. We have begun to identify the genes that control the molecules of the brain.

1900: A few hand-built automobiles are found among the steam, electric, and horse-drawn vehicles used for transportation. The industrial revolution and an age of invention -- most recently, electric light, telephones, and motion pictures -- have begun to change the face of the world. But factories and symphony orchestras still provide the loudest sounds around, few people have running water, ice keeps our food cold, and we can travel off the ground only by balloon. Today: We live in a world that is tied together by technology. We can travel to any part of the globe in a matter of hours and communicate by television, telephone, radio, or over computer networks nearly instantaneously. The world is awash in consumer goods, many of which are becoming even smaller, energy-efficient, and "smarter." We continue to use technology to explore outer space and to learn more about our world. Yet scientists and policy makers are still unsure how best to control technology to benefit humankind and to preserve and restore our environment

1900: As the twentieth century dawns, scientists have few hard facts about the origins of Earth, humans, or life itself. Most believe the Earth was formed from material torn from the sun, and that as it cooled, its surface contracted, buckled, and cracked, creating mountains and oceans and causing earthquakes and volcanoes. We know little about our prehistoric ancestors. Our best clues are in the few ancient fossils recovered in Europe and Asia. The mechanism of evolution, by which physical traits are passed from one generation to the next, is unknown. Today: Our planet is not a static platform for life. The Earth is dynamic and unpredictable. Its surface is composed of large moving plates that consist of both the continents and oceanic crust. The movement is only a few centimeters a year, but it's enough to reshape the Earth, build mountains, spawn earthquakes, and create volcanoes. The accumulation of fossil finds has shown that the birthplace of the human species is in Africa. With the discovery of the structure of DNA has come an understanding of the mechanism by which chemical molecules pass on the code of heredity, creating the basis for evolution and the diversity of life.