Complex sentence: His works became darker and more loosely painted, with an emphasis on mood and shadow, in the spirit of the Barbizon school--an artistic evolution that mirrored the general trend in American art as the Hudson River School declined in popularity.
Split into simpler sentences: His works became darker and more loosely painted, with an emphasis on mood and shadow, in the spirit of the Barbizon school. His evolution mirrored the general trend in American art as the Hudson River School declined in popularity.