After two years in the academy, he enrolled at Dickinson College where his interest in natural history was encouraged by his professor, Henry Darwin Rogers, who would later become a distinguished geologist.

[1][2] Two years after entering Dickinson, the college was forced to close temporarily and Peck left without earning a diploma.

[3] After leaving school, Haldeman took over management of his father's new sawmill and became a silent partner with two of his brothers who started an iron manufacturing business in the area.

"[1] In 1833–1834, he attended lectures in the medical department at the University of Pennsylvania in Philadelphia in order to better prepare himself for the study of natural history.

In 1840 he began the publication of his Monograph of the Freshwater Univalve Mollusca of the United States, issued in nine parts with the final volume not appearing until 1866.

In 1844 he wrote a paper, "Enumeration of the Recent Freshwater Mollusca Which are Common to North America and Europe", where he laid out in detail the case for Lamarckian evolution and transmutation of species.

Haldeman's participation in the society put him in regular contact with other leading American entomologists including Frederick E. Melsheimer and John G.

His ear was remarkably sensitive, and he discovered a new organ of sound in lepidopterous insects, which was described by him in Benjamin Silliman's American Journal of Science in 1848.

On his return trip from Texas, he was offered the position of president of Masonic College in Selma, Alabama, which he accepted and held from January to October 1852.

In 1858, Haldeman was awarded the Trevelyan Prize, given by the Phonetic Society of Great Britain, for his article entitled "Analytic Orthography".