Complex sentence: He became, in the 1880s, one of the most important art critics dealing with sculpture (writing mainly for the "Saturday Review") with an interest spurred on by his intimate friendship with the sculptor Hamo Thornycroft.
Split into simpler sentences: He became, in the 1880s, one of the most important art critics dealing with sculpture (writing mainly for the "Saturday Review"). His interest in sculpture was spurred on by his intimate friendship with the sculptor Hamo Thornycroft.