We now resume the history of a study which down to the end of the eighteenth century had yielded only meagre and uncertain results (see above, pp. 85-88). At the date in question it had been ascertained that the spores (then called "seeds") of ferns, and probably of other cryptogams, are capable of propagating the species, but no one knew precisely what part the spore played in the life-history, or could explain the true difference between a cryptogam and a flowering plant. The great improvements in the construction of the compound microscope which were effected between 1812 and 1830 rendered it possible to elucidate much more thoroughly the structure and development of the chief groups of cryptogams. The sexual reproduction of algæ was explored; moving filaments (spermatozoids) were seen to enter the chambers in which embryos afterwards formed; the conjugation of similar cells was observed in algæ and fungi, and recognised as a simple mode of sexual reproduction. The resemblance of the spermatozoids of mosses and ferns to animal spermatozoa was noted, and their participation in the process of fertilisation was more and more closely followed until at length Hofmeister in 1851 saw them fuse with the egg-cell of a fern. Suminski, whose full name, Lesczyc-Suminski, is unpronounceable by Englishmen, had discovered (in 1848) that the prothallus of a fern, which is the product of the germinated spore and had been hitherto taken for the cotyledon, bears two kinds of reproductive organs, one of which liberates spermatozoids, while an egg-cell is developed within the other. He did not correctly describe all the details, but he showed where the essential reproductive organs form, and where fertilisation is effected. The masterly researches of Hofmeister (1849-57) fused what had been a number of partial discoveries into a connected and luminous doctrine. He proved that the prothallus is one of two generations in the life-history; that it begins with a spore and ends with a fertilised egg-cell; that in the higher cryptogams there is a regular alternation of generations; that the prothallus of the fern answers to the leafy moss, while the leafy fern is the equivalent of the moss-capsule; that the egg-cell is the same structure in both cryptogams and flowering plants; that the pollen-tube and the seed are found to-day only in flowering plants; that the gymnosperms make a transition from the higher cryptogams to the angiosperms; that unity of plan pervades the whole series of mosses, ferns, fern-like plants, gymnosperms, and angiosperms. Before Darwin's _Origin of Species_ had appeared Hofmeister presented to evolutionists a clear example of a descent in which every principal term is well authenticated, while the extremes are far apart. The Enrichment of English Gardens.