In the mid 1800's the growth of the populations of California and the western territories was mainly a result of the?

In the mid-1800s, the growth of populations in California and the western territories was primarily driven by the Gold Rush, which began in 1848. This massive influx of prospectors and settlers seeking fortune led to rapid urbanization and the establishment of new communities. Additionally, the expansion of railroads and the promise of land through policies like the Homestead Act further attracted people to the West, fostering economic growth and migration.