If one reads Darwin's Natural Selection, several difficulties might follow if the reader does not understand the scientific terminology or biological concept. Understanding it requires a history in Biology, and some parts require one to possess a certain amount of knowledge to interpret it. Natural Selection was an essay, an excerpt of the book "On the Origin of Species by Means of Natural Selection," and talks about sexual selection and illustration of actions.
Natural selection in whole talks about how nature "selects" the certain individual from a group or population – a group of the same or one species – of animals, insects, plants, and even prokaryotes (single-celled organism without nucleus, a membrane surrounding the DNA) like bacteria and archaea. For example, between a faster lion and a stronger lion, the faster lion tends to survive since the stronger lion cannot catch it. It also talks about the evolution and inheritance of a specific trait like a bird's bone. If one looks at a bird while it is flying, one will notice the shape of its wings. The bones inside the wings and the feathers around it, allows the bird to fly without falling. If one looks at a penguin, it is a kind of bird too, but instead of flying, it uses its powerful wings to swim, allowing it to swim faster. Different species are unique to each other, they may have similarities, but they are never the same.
The sexual selection in terms of natural selection provides insight on how an individual from a population chooses its "partner" which can provide unique or better traits than the others. This part requires the reader to know the basic behavior of animals, instinct and habits. Not knowing those behaviors makes it hard for the reader to accept the information since it doesn't give enough details to produce an image on how it works. Let us take lions for example. Female lions tend to choose a stronger and faster male lion because it can provide the necessary speed and strength to hunt a fast moving animal, like deer, and wards off potential enemies and predators.