| Letter from Charles Darwin to J.-H. Fabre of January 31, 1880 | 106 |
| Letter from Charles Darwin to J.-H. Fabre of February 20, 1880 | 61 |
| Letter from Charles Darwin to J.-H. Fabre of January 21st, 1881 | 67 |
| Letter of Charles Darwin to G.J. Romanes of April 16, 1881 | 71 |
| Chapter IV. More Enquiries Into Mason Bees | 200 |
