288 SCIENCE PROGRESS. 



that there have been two distinct lines of development, 

 perhaps more, among the Pteridophytes, arising quite 

 independently of one another, but from similar ancestral 

 forms. In one of these, the Lycopodineae, the leaves are 

 small, the axis much elongated, and regularly each 

 sporophyte bears but a single sporangium. In the other, 

 the Filicineae, the leaves are large and the axis usually 

 much less developed ; sporangia are found usually in large 

 numbers upon the, sometimes, profoundly modified sporo- 

 phylls. Whether the third group of living Pteridophytes, 

 the Equisetineae, form a third entirely distinct series, or 

 whether they are more nearly related to one or the other 

 of the two other Classes, is by no means easy to determine. 

 The parallelisms in development found in the three 

 groups do not necessarily imply a genetic connection, as 

 perhaps the most remarkable case of all, that of heterospory, 

 has beyond question arisen independently in at least five 

 widely remote groups where the question of relationship 

 cannot for a moment be maintained. The development of 

 heterospory in unrelated groups of Pteridophytes also 

 suggests the possible multiple origin of the " Flowering 

 Plants," or, more correctly, " Seed Plants ". That hetero- 

 spory was the first step in this process no botanist would 

 think of disputing, and the homologies of the macro- 

 sporangium of the Pteridophytes and the ovule of the 

 Phanerogams are universally admitted, as well as those of 

 the microspores and pollen cells. When, however, the 

 question is raised as to which group of Pteridophytes has 

 given rise to the Phanerogams, and whether all of the 

 latter belong to a common stock, the matter is not so 

 simple. Hofmeister first showed that the homologies 

 between the Gymnosperms and the heterosporous Lyco- 

 podineae were very striking, and that the former, except in 

 the absence of spermatozoids, were really nearer to the 

 Pteridophytes than to the Angiosperms. It was assumed 

 that the latter were derived from the Pteridophytes through 

 the Gymnosperms and that the Phanerogams formed a 

 single series ; but later many botanists have dissented 

 from this view and are inclined to look for the orioin of 



