PHYLOGENY. 77 



Incidentally they are, however, essential to the exist- 

 ence of the animal kingdom, since they alone elaborate 

 protoplasm and proteids from inorganic nature. In 

 the simplest plants there is no sexuality, and repro- 

 duction is effected by spores which are mere fragments 

 of the parental protoplasm (Protophyta). In the next 

 stage sexual conjugation is necessary, but the sexes 

 do not differ from each other in characters (Zygo- 

 phyta). In the third stage (Oophyta) the sexes are 

 distinct, and the reproductive elements are distin- 

 guished as female germ-cell and male antheridium. 

 In the remaining types of plants a distinct set of indi- 

 viduals, the prothallia, is produced by cell-division, 

 whose function is sexual reproduction, thus constitut- 

 ing an alternation of generations. These plants may 

 be entirely cellular (Carpophyta), or may be furnished 

 with vascular canals. Of the latter the male and fe- 

 male prothallia may be naked and free (Pteridophyta 

 or ferns, etc.), or may be enclosed in modified leaves, 

 or flowers, the Phaenogamia or flowering plants. 



For the reasons already mentioned the order of 

 "successional relation " above pointed out, is likely to 

 prove to be the order of appearance of plants in time, 

 and that such is the fact is demonstrated by their pa- 

 leontology. In the earliest beds in which plants are 

 positively known to occur, the Ordovician, we have 

 only Algae (Zygophyta and Oophyta). In the Siluric 

 we have a great predominance of the same classes, a 

 very few species of which appear to have formed great 

 erect stems. In the next period, the Devonic, prob- 

 able Carpophyta are present, while the vascular Pte- 

 ridophyta appear for the first time, and in consider- 

 able numbers. A few members of the gymnospermous 

 Phaenogamia (Coniferae) appear. In the Carbonic pe- 



