Mainly his disregard of heredity as a cauative factor. He was largely dependent on Lamarcks’s notion of the inheritance of acquired characteristics. He rejected Weisman’s work on cells, and of course was ignorant of Mendel’s work. His followers accepted this as dogma, and forty years after Origin of Species , Haeckel set the whole theory in concrete in his Die Weltraetzel. As I said, scholars who have become convinced of a master theory are unlikely to deviate from or accept anything different.