Complex sentence: Amongst the most significant and recurring themes of the text is that of man's relationship with his environment, featuring strongly in Letter II, III and in the letters accounting James' visits to Nantucket and Martha's Vineyard.
Split into simpler sentences: Amongst the most significant and recurring themes of the text is that of man's relationship with his environment. The theme appears strongly in Letter II, III and in the letters describing Nantucket and Martha's Vineyard.