Complex sentence: This evidence shows that the labiovelar series was distinct from the plain velar series in PIE, and cannot have been a secondary development in the Centum languages; but it says nothing about the palatovelar vs. plain velar series.
Split into simpler sentences: This evidence shows that the labiovelar series was distinct from the plain velar series in PIE, and cannot have been a secondary development in the Centum languages. However, it says nothing about the palatovelar vs. plain velar series.