Darwin also introduces the concept of sexual selection, which is the idea that certain traits, such as bright plumage or elaborate courtship displays, are favored by members of the opposite sex and therefore more likely to be passed on to future generations. He argues that sexual selection is responsible for many of the physical and behavioral differences between males and females in the animal kingdom.In Part I of the book, Darwin also explores the similarities and differences between humans and other primates, arguing that humans have evolved from a common ancestor with apes and monkeys. He notes that humans share many physical and behavioral traits with other primates, such as opposable thumbs and the ability to use tools, but also emphasizes the unique characteristics that distinguish humans from other animals, such as language and the ability to reason.