In the early 1870s, Crépin exchanged his teaching position for a job at the Musée royal d’Histoire naturelle (today the Royal Belgian Institute of Natural Sciences) in Brussels, where he studied the fossil flora of Belgium. A few years later he was appointed as director of the State Botanic Garden in Brussels (today’s Meise Botanic Garden). None of these curricular turns, however, could distract him from the preparation of his world monograph.
The task of preparing such a monograph proved very demanding. In the second half of the 19th century, interest in botany increased. For Crépin, the swelling stream of publications and herbarium collections, supplemented by useful information, as well as trivia, from thousands of incoming letters, was both a blessing and a challenge. Field botany offered amateur naturalists an attractive pastime. In their private herbaria, they amassed large amounts of dried plants. Often they collected a series of specimens of a single plant species on a location, exchanging the duplicates with other naturalists.
Some offered their valuable collections for sale in the form of exsiccatae – publications of collections of dried and correctly identified specimens. Each specimen of each species in a series of exsiccatae had to be collected at the same location and at the same date. Used as reference collections, exsiccatae were important tools in a time when books with high quality illustrations were scarce. These books were also quite expensive, which explains why Crépin sometimes made carefully pencilled copies of drawings from the materials which he received on loan.
Based on expanding herbarium collections and observations of fresh material from cultivated plants, the number of newly-described species increased every year, especially in Europe. Each species described by the splitters required careful scrutiny by the monographer.
In France, Pierre Déséglise accepted 329 species in section Caninae in 1877 alone. Another Frenchman, Jean Michel Gandoger, had praise for the framework Crépin had established for classification of roses, yet at the same time he rejected the Belgian rhodologist’s efforts to reduce the number of species. Taking an opposing approach, Gandoger listed no less than 4266 species of roses from Europe, North Africa and the Orient in 1881 in Tabulae Rhodologicae. In return, Crépin scorned this kind of uncontrolled fragmentation.
But despite substantial collecting activity, the study of roses was hampered by the lack of sufficient materials from Asia and America. This slowed down Crépin’s work on his rose monograph.