Complex sentence: He was first influenced by associationist psychology as it was expounded by Alexander Bain and Herbert Spencer, the latter often considered the most important individual in the transition from associationist psychology to Darwinian functionalism.
Split into simpler sentences: He was first influenced by associationist psychology as it was expounded by Alexander Bain and Herbert Spencer. Spencer was the most important individual in the transition from associationist psychology to Darwinian functionalism.