be homologous. The homology of appendicular organs is suggested in the phrase "principium florum et foliorum idem est," and he developed his ideas in two memoirs, Prolepsis Plantarum (1760-63); only homologous parts, he said, can change into one another; "the liver cannot become the heart, nor the heart the stomach."
In 1751 Linnæus published his famous Philosophia Botanica—a work of the greatest importance; and in the previous year he constructed his floral clock an arrangement of flowers opening and closing with regular periodicity; and he described the somnus plantarum, or the nocturnal changes of positions in flowers and leaves.
With all his ceaseless toil, impetuosity, and hardships, he was always enthusiastic in his studies and researches. In the words of Ovid: "Scribentem juvat ipse favor minuitque laborem; cumque suo crescens pectore fervet opus."
Owing to the financial difficulties of Cliffort, Linnæus was obliged to leave the beautiful and historic gardens of Hartecamp; but shortly afterwards he obtained employment in the botanical gardens at Leyden. While there he published two other works, which greatly enhanced his reputation.
At this period in his career, the Government of the Dutch Republic desired to send him on a botanical expedition to South Africa, and they promised to give him