The significant scientific theory that Charles Darwin introduced in the 19th century was the theory of evolution by natural selection. In his landmark work, "On the Origin of Species" (1859), Darwin proposed that species evolve over time through a process where individuals with advantageous traits are more likely to survive and reproduce. This challenged prevailing scientific and theological views about the immutability of species and offered a naturalistic explanation for the diversity of life on Earth. Darwin's ideas laid the groundwork for modern evolutionary biology and profoundly influenced various fields, including genetics, ecology, and anthropology.