What key element was missing from Darwin's theory of evolution?

In Darwin's theory of evolution, one significant aspect that was not adequately addressed was the sources of variation and mechanisms of heredity, which is fundamental to understanding how traits are passed down through generations. While Darwin proposed the idea of natural selection as a process driving evolution, he lacked the knowledge of genetics that would later provide insights into how variations arise and are inherited.

Darwin's theory emerged before the discovery of genes, DNA, and the understanding of Mendelian inheritance. Without this framework, it was challenging to explain how traits could be passed on with variability, which is essential for natural selection to act upon. It wasn't until the integration of Mendelian genetics into evolutionary theory, known as the modern synthesis, that a more comprehensive understanding emerged, illustrating how variation arises and is inherited, thus providing a solid foundation for evolutionary biology.