Complex sentence: Welby and Peirce had something in common, namely approaches to the problem of meaning that were not devoid of similarity, but most of the correspondence consists of Peirce elaborating his related theory of semiotics.
Split into simpler sentences: Welby and Peirce had something in common, namely approaches to the problem of meaning that were not devoid of similarity. But most of the correspondence consists of Peirce elaborating his related theory of semiotics.