Alfred W. Crosby was a man who studied history and wrote books about how people from different parts of the world interacted with each other a long time ago. He wanted to understand why and how people moved around the world and how it influenced things like culture and the environment.
Think of it like when you go to school and you learn about different countries and cultures. Alfred Crosby did that too but he studied how people from different countries and cultures came into contact with each other and what happened as a result. Some of the things he talked about were diseases, food, animals, and plants that were introduced to different places because of people moving around.
Alfred Crosby was also very interested in how humans interacted with the environment. That means he wanted to understand how people affected the world around them like changing the land or the climate. One of the things he talked about was how European explorers brought different animals and plants to the Americas and how this changed the environment there.
Overall, Alfred Crosby was a historian who studied how people from different parts of the world interacted with each other and how this changed things like culture, the environment, and the way people lived.