Natural selection is a process by which organisms that are most suited to their environment survive and reproduce most successfully; also called survival of the fittest
The struggle for existence is a natural history. It refers to the competition between living things to survive. This, and the struggle for life, were used over 40 times by Charles Darwin in the Origin of Species, and the phrase is the title of chapter 3 of the Origin.
Extinct: describes a species that has completely disappeared from Earth.
Dinosaurs, and many other life forms from over a few million years ago, are all extinct.
Adaptation: is a structure, behaviour, or physiological process that helps an organism survive and reproduce in a particular environment. e.g.: Hibernation is an example of a physiological adaptation
Bears adapted the ritual of hibernation to survive long and cold winters by needing little to no food.
Survival of the fittest: The idea that species adapt and change by natural selection with the best suited mutations becoming dominant. This expression is often attributed to Charles Darwin and, although it appears in the fifth edition of his Origin of Species, 1869, it is there attributed to Herbert Spencer: "The expression often used by Mr. Herbert Spencer of the survival of the fittest is more accurate..." Spencer had published The principles of biology in 1864. In that he referred to 'survival of the fittest' twice: "This survival of the fittest, implies multiplication of the fittest." "This survival of the fittest... is that which Mr. Darwin has called 'natural selection, or the preservation of favoured races in the struggle for life'."