Final answer:
Darwin did not understand the mechanisms of genetics, how new variations appear, and the genetic basis of natural selection. Genetics, which explains the inheritance and variation of traits, was only integrated into the understanding of evolution with the modern synthesis in the 20th century.
Step-by-step explanation:
The three aspects of the theory of natural selection that Darwin did not understand are:
- The mechanisms of inheritance or genetics.
- How new variations appear in a population.
- The exact way through which traits that confer a survival advantage increase in frequency in a population, also known as the genetic basis of natural selection.
Darwin and Wallace were able to describe the process of natural selection, but they lacked the understanding of genetics, which was vital to explain the inheritance and variation of traits. Indeed, it was the work of Gregor Mendel, which was published after Darwin's On the Origin of Species, that shed light on how traits are passed down through generations. However, Mendel’s work remained obscure until it was rediscovered in the early 20th century, which then contributed to the development of the modern synthesis integrating evolution and genetics.
Darwin assumed a blending inheritance mechanism, which could not account for how some traits disappear in one generation and reappear in another. It wasn’t until the rediscovery of Mendel's work that the particulate nature of inheritance was understood.