Complex sentence: In 1961, McKim, Mead & White was succeeded by the firm Steinman, Cain, and White, which by 1971 had become Walker O. Cain and Associates.
Split into simpler sentences: In 1961, McKim, Mead & White was succeeded by the firm Steinman, Cain, and White. By 1971 it had become Walker O. Cain and Associates.