Complex sentence: In 1774, appeared "Elementa Ornithologica" where he proposed a system of classification of the birds based on the structure of their legs, the work was followed by "Museum Ornithologicum" in 1789 when he described the birds of his collection.
Split into simpler sentences: In 1774, he wrote "Elementa Ornithologica", in which he proposed a system of classification of the birds based on the structure of their legs. This work was followed by "Museum Ornithologicum" in 1789, in which he described the birds in his collection.