Simplify the following text for a Grade 4 native English-speaking person.
Original: The population of England had almost doubled from 16.8 million in 1851 to 30.5 million in 1901 , due to the large immigration during the later part of the 20th century .
Simplified: The population of England had almost doubled from 16.8 million in 1851 to 30.5 million in 1901 .