CLASSIFICATION 57 
3. Mixtae. (A definite succession of time only in the 
production of sporangia.) 
Pterideae , sori marginal. 
Aspidiae , Aspleniae and Polypodiaceae , sori superficial. 
B. Heterosporous. (Spores and sporangia differ in size and form.) 
Salviniaceae ( Azollci ) and Marsiliaceae ( Marsilia ). 
The arrangement adopted by Christensen and which is 
followed in the present work deals with Cohorts IV. and V. of 
the above in a somewhat different manner, as will be seen 
later, but as Christensen does not include Cohorts I., II. and 
III. of the above I now follow Bower in regard to these, in pre- 
ference to the arrangement used in my first edition, which was : 
Order i. Filices, including Cohorts IV. and V., A, of the 
above. 
2. Equisetaceae, equal to Cohort II. of the above. 
3. Lycopodiaceae, sporangia uniform. Lyco- 
podium and Psilotum. 
4. Selaginellaceae, sporangia of two kinds. ' Sela- 
ginella and lsoetes. 
5. Rhizocarpeae, equal to Cohort V., B, of the 
above. Azolla and Marsilia . 
It will be seen from the above that each group has 
characters of its own, which, as referred to in a previous 
chapter (Reproduction and Propagation), appear not only in 
the plant which we describe as the species but also in the 
alternate generation of spore and sporeling; and as connecting 
links between some of the groups are known only as fossils 
(Lepidodendraceae, Sigillariaceae, etc.) and others are very 
probably extinct, the gradual evolution through long ages, 
and the consequent relationship of existing groups, can never 
be fully determined, and consequently any scheme of arrange- 
ment is simply the expression of an individual opinion on 
the most natural relationship, and may or may not obtain 
general support, while further investigations into the early 
life-history may lead to still further developments. 
