All living things change over time, according to Charles Darwin's theory of evolution by natural selection, which states that these changes are caused by four distinct factors:
1) heredity (also known as genetics),
2) an organism's living environment, and
3) mutations (or random variation)
4) Reproduction through sexual means
Extinct animal fossils that are lacking in levels below where they are located support his theory, indicating that animals evolved over time to be different from what they were previously.

