The significant scientific theory proposed by Charles Darwin during the 19th century is the theory of evolution by natural selection. In his landmark work, "On the Origin of Species," published in 1859, Darwin presented evidence that species evolve over time through a process driven by competition, environmental pressures, and varying traits advantageous for survival and reproduction. This theory challenged the prevailing views of creation, providing a scientific framework for understanding the biological diversity of life on Earth. Darwin's ideas sparked significant debate and laid the groundwork for modern evolutionary biology, deeply impacting the fields of genetics, anthropology, and ecology.