Complex sentence: Tokarev was a follower of Aleksandr Nikolaevich Maksimov, who had undertaken early studies of Siberian and Australian hunter-gatherers, but who believed that an ethnologist should not retrict himself to "primitive" people, but should study people at all stages of development.
Split into simpler sentences: Tokarev was a follower of Aleksandr Nikolaevich Maksimov, who had undertaken early studies of Siberian and Australian hunter-gatherers. Maksimov believed that an ethnologist should not restrict himself to "primitive" people, but should study people at all stages of development.