Final answer:
Darwin didn't understand genetics, which was later explained by Gregor Mendel's work, culminating in the modern synthesis of evolution and genetics.
Step-by-step explanation:
Charles Darwin didn't understand genetics, a problem solved by Gregor Mendel's work on inheritance. Darwin, along with Alfred Russel Wallace, developed the theory of evolution by natural selection without knowledge of how traits were passed down through generations. The then-prevailing theory of blending inheritance didn't support the idea of natural selection. It wasn't until Mendel's principles of inheritance were rediscovered in the early 20th century that scientists could explain how evolutionary changes were inherited, leading to the modern synthesis which combines Darwin's and Wallace's ideas with genetics.