Which concept did Darwin not incorporate into his theory of evolution, as it was unknown in his time?

Genetics

At the time of Darwin, the mechanisms of heredity remained a mystery, which limited Darwin's ability to explain how traits were transmitted from one generation to the next. It was only with the rediscovery of Gregor Mendel's work at the turn of the 20th century that scientists began to understand the laws of heredity, laying the foundation for modern genetics. This convergence of Mendelian genetics and natural selection strengthened the theory of evolution, allowing for the development of evolutionary biology as we know it today. The integration of genetics into the study of evolution thus filled a crucial gap in understanding biological diversity.