of very different Fungi. There was a difficulty also in drawing the boundary-line between the lower animals and plants; the difficulty was solved by classing all objects capable of independent movement with animals; thus whole families of Algae (the Volvocineae, Bacillariaceae, and others) were claimed by the zoologists, and when the swarmspores of a genuine Alga were seen for the first time in the act of escaping, the phenomenon was described as the changing of the plant into an animal. Trentepohl in 1807, and Unger in 1830, explained in this way the escape of the zoospores of Vaucheria. The remarkable thing is, not that such views were entertained, but that the majority of botanists combined with them a belief in the constancy of species. But this dogma rendered good service to the science in this instance, for the botanists, who at a later time applied themselves to the systematic examination of the Algae and Fungi, confided in the constancy of the processes of development in each species, which they expected would assert itself in these forms as in the Mosses and higher plants. With much that was obscure and doubtful, the result of occasional observation accompanied by uncritical interpretation, the literature of the subject had contained for some time a certain number of single well-established facts of real importance, which were well adapted to serve as starting-points for earnest and exact investigation. Among the Algae the genera Spirogyra and Vaucheria especially had supplied remarkable phenomena; Joseph Gärtner observed the formation of zygospores in Spirogyra in 1788, Hedwig saw in the mode of their production at least a suggestion of sexuality (1798), and Vaucher[58], in his ‘Histoire de Conferves d’eau douce,’ which appeared in 1803 and was far in advance of its time, called conjugation distinctly a sexual process; the optical means at his disposal did not enable him to observe the fertilisation in Vaucheria (Ectosperma), which was named after him, though he described the sexual organs accurately; the movement also of the zoospores in this genus escaped him, and Trentepohl first observed their escape and swarming in 1807[59]. Vaucher had also observed the formation of new nets in the old cells of Hydrodictyon, and Areschoug repeated the observation in 1842, when he saw the swarming of young cells in the old ones. Bischoff, as early as 1828, saw the spermatozoids of Chara, though without understanding them. Observations on conjugating Algae were multiplied; Ehrenberg in 1834 saw corresponding phenomena in Closterium, and Morren described them more exactly in 1836. The formation of swarmspores in fresh-water and salt-water Algae was frequently observed between 1820 and 1830, and in his ‘Neues System,’ iii, which appeared in 1839, Meyen gave a summary of all that was known up to that time of the propagation of the Algae. But a new aspect was given to the knowledge of the Algae by