Final answer:
Darwin's theory of evolution lacked the understanding of genetics, specifically how traits were inherited and varied over time. The missing keys in Darwin's theories were later provided by the rediscovery of Gregor Mendel's principles of genetics, which explained the inheritance and variation of traits through genes.
Step-by-step explanation:
Charles Darwin's theory of evolution by natural selection, outlined in his 1859 publication Origin of the Species, was groundbreaking yet incomplete. While Darwin provided substantial evidence for evolution, such as the Galápagos finches and the Giant Galápagos tortoises having adaptations suited to their specific environments, he lacked the understanding of genetics necessary to explain how traits were inherited and how new ones appeared. This missing piece of the puzzle was the concept of genes and their role in heredity and variation, which was unknown to Darwin at the time.
The gap in Darwin's knowledge was later filled by the rediscovery of Gregor Mendel's principles of genetics, which explained the mechanism behind the inheritance and variation of traits. Mendel's work, initially overshadowed and forgotten, described how traits were passed from parents to offspring through discrete units known as genes. His experiments with pea plants demonstrated how these units of heredity could lead to the emergence of new traits and influence the evolutionary process. The integration of Mendel's genetic principles with Darwin's theory of evolution shed light on the missing keys of Darwin's evolutionary theory, providing a comprehensive understanding of how species change over time.