Lyell summarises the letters as:
1) Darwin's theory, pigeons not arboreal, 1860
2) Mull tertiary leaves, 1861
3) Non-arboreal habits of pigeons, March 22 1865
4) Darwin on correlation of growth, June 2 1865
5) Variety of Humming Bird, April 24 1867, Gould on ... of species in Humming Bird
6) Humming Birds, not opposed to Darwin's May 1 1867
7) Applicable to Humming Birds May 17 1867
8) Elephant in ivory of Perigord Cave, June 8th 1867
9) Enquiries as to ... skeletons of Stone Age in Paris, May 28 1865
10) Leslie's 100 perforated skeletons, May 31 1868
11) Wallace on nudification, July 27 1868
12) Reply to Wallace on nudification, (.... book) 9 - p26, Aug 21 - '68
13) On Wallace on nudification, Sept 28 1868.