Final answer:
Charles Darwin could not address the mechanism of inheritance in evolutionary theory because the concept of genes was not understood during his lifetime. His inability to explain how traits were passed down and changed through generations was later resolved by the modern synthesis of genetics and natural selection.
Step-by-step explanation:
One aspect of evolutionary theory that Charles Darwin could not successfully address during his lifetime was the mechanism of inheritance, or how traits are passed from parents to offspring. Darwin was unable to explain how evolution occurs at a genetic level because he was unaware of the concept of genes, an area of science that was not well-understood until Gregor Mendel's principles of inheritance were rediscovered at the turn of the 20th century. The lack of genetic knowledge made it difficult for Darwin to understand how new traits appeared and how, through natural selection, advantageous traits could be permanently incorporated into a species' genetic makeup. This missing piece of Darwin's theory was later filled by the 'modern synthesis', which integrated the principles of genetics with the concept of natural selection, to form a more complete understanding of evolutionary processes.