Final answer:
Critics of Darwin's natural selection argued against the randomness of nature and lack of a genetic inheritance mechanism. The blending inheritance theory complicated the understanding of natural selection. The modern synthesis eventually reconciled natural selection with genetics, reinforcing Darwin's concepts.
Step-by-step explanation:
Charles Darwin's theory of natural selection faced criticism due to it contradicting the biblical account where creation is seen as static, and the perception that nature was inherently hostile and random in its adaptations. Moreover, during Darwin's era, the mechanisms of inheritance were not understood, thanks to the prevalent but mistaken theory of blending inheritance which confused how natural selection might work. Darwin and his contemporary Alfred Russel Wallace did not know about Gregor Mendel's work on genetics, which only gained attention in the early 20th century, leading to the modern synthesis that reconciled genetics with the theory of evolution.
Darwin's ideas about evolution being a slow and gradual process through descent with modification were also criticized due to the lack of genetic understanding. The modern synthesis that arose by the 1940s, however, integrated Mendelian genetics with natural selection, bringing clarity to how small variations could indeed be selected for and lead to significant evolutionary changes over time.