Darwin's theory was controversial in the 1800's because it was very hard for the people at that time to believe in such theories which questioned their religious beliefs.
Step-by-step explanation:
It was hard to convince the scientist at that time to believe in the Darwin's theory. People at that time believed that God has created all the animals and plants, and all the living things are a creation of the Almighty.
But gradually, after around 50 years, Darwin's Theory was again published, and it was discovered that genes are passed to the offspring's.