A miscellaneous collection of letters written by naturalist Louis Jean Rodolph Agassiz concerning a wide range of topics: natural history and naturalists, geology, mineralogy, fossils, publications, expeditions, and the Museum of Comparative Zoology at Harvard, which he founded.

This is primarily a collection of letters, with some additional documents, concerning Young's interest in botany, geology, mineralogy, and natural history. There is information about Bowdoin College, where he studied under Parker Cleaveland. There is also much on the natural history of Maine, where he was the State Botanist in 1847-1849, and also on the Bangor Natural History Society. There are materials on Brazil in relation to Young's service there as the U.S. consul to Rio Grande do Sul from 1863 to 1873. There are also letters from his brother, John C. Young, and his sister, Sarah Augusta Young.

Alexander Dallas Bache (1806-1867) was an important scientific reformer during the early nineteenth century. From his position as superintendent of the United States Coast Survey, and through leadership roles in the scientific institutions of the time, Bache helped bring American science into alignment with the professional nature of its European counterparts. In addition, Bache fostered the reform of public education in America. The Alexander Dallas Bache Collection consists of 91 letters written primarily by Bache. In most cases, these are brief notes replying to letters that are not part of the collection. The majority of items relate to Bache's work as superintendent of the U.S. Coast Survey, including letters of recommendation and introduction, and there is minor correspondence regarding the National Academy of Sciences, scientific matters, his travels in Europe, or personal matters. Of particular interest is the letter from Louis Agassiz in 1856 expressing his view of what the natural history museum of the future should be.