Final answer:
Darwin's theory of natural selection was incomplete because it lacked a genetic foundation, not incorporating the mechanisms of inheritance, which were later explained by Gregor Mendel's work. The discovery of particulate inheritance and the establishment of the modern synthesis eventually reconciled natural selection with genetic inheritance, offering a more complete understanding of evolution.
Step-by-step explanation:
The theory of natural selection as formulated by Charles Darwin was inherently incomplete because it did not address the underlying mechanisms by which heritable traits were passed from one generation to the next. Though Darwin recognized that certain traits were inherited and that variation within a species was essential for natural selection to operate, the actual genetic science required to explain these observations—in particular, the works of Gregor Mendel—was not known to him. Consequently, Darwin could not precisely explain how evolution occurs on a genetic level.
The limitations in Darwin's understanding were due to the acceptance of blending inheritance at the time, which was later disproved by the particulate nature of inheritance established by Mendel's work on pea plants. This lack of knowledge proved to be an obstacle to comprehending how evolution could take place over successive generations. It wasn't until the early 20th century that scientists rediscovered Mendel's work and eventually integrated it with evolutionary theory, leading to the modern synthesis. This synthesis, which emerged by the 1940s, bridges the gap between the processes of natural selection and genetics, explaining how evolutionary processes can result in the gradual evolution of populations and species.