Final answer:
The Pleasure Principle is a concept proposed by Sigmund Freud that explains humans' drive for pleasure and avoidance of pain. Around the time Darwin published 'The Origin of Species' in 1859, significant developments were happening in science. Sex is a conscious activity for humans, and its primary purpose is to pass on genes and enhance reproductive fitness.
Step-by-step explanation:
The Pleasure Principle refers to the concept that humans are driven to seek pleasure and avoid pain. It was proposed by Sigmund Freud, a psychologist, in the early 20th century.
During the time when Darwin published his book 'The Origin of Species' in 1859, significant developments were taking place in the field of science. Darwin's theory of evolution through natural selection revolutionized the biological sciences and sparked debates about the origin of species.
In terms of the question about why humans consciously have sex, it can be explained by the concept of reproductive fitness. The primary purpose of sex is to pass on our genes and increase our chances of reproductive success.