636 GENERAL COMPARISON OF THE FILICALES 
continued. The Mixtae have commonly (though not always) a flat re- 
ceptacle, It is not a constant morphological feature, as shown by the 
fact that a line of transition from an elongated receptacle to a flat one 
has been demonstrated in the genera Dennstaedtia and Microlepia, and it 
is possible that other transitions have also taken place elsewhere. 
The term zzdusium has been applied to -bodies of the most various 
“form, position, and structure borne in relation to the Fern-sorus, and 
serving for the protection of the sporangia. It is hardly necessary to 
point out that these, however similar in their function, cannot be regarded 
as homogenetic throughout: they are often only examples of homoplasy. 
We call the basal indusium of Cyathea by the same name as the umbrella- 
like indusium of Matonia or olystichum, or the marginal flap of an 
Adiantum. It may be possible, by regarding the indusium as having had 
a high degree of plasticity, to accept some of its different forms as being 
modifications of one another, and a reasonable case can be made out 
sometimes, such as that in the transition from the Dennstaedtiineae to the 
Pterideae. But such cases as Cyathea and Matonia seem incompatible 
with any opinion of homogeny of the two structures, especially when it is 
remembered that in A/sophila and Gleichenia, genera which are respectively 
allied to the above, an indusium may be entirely absent: and this is in 
fact the usual condition among the Simplices. We shall then hold the 
indusium to be an inconstant body, varying in occurrence and in position, 
and the term will be used to designate outgrowths protective of the sorus, 
whatever their position and whatever their evolutionary history may have 
been. 
It has been demonstrated in many cases that the indusium is formed 
before the earliest sporangia appear: this is probably a case where 
physiological opportunism, rather than any recapitulatory habit, determines 
the order of succession. It is impossible to believe that those constant 
bodies, the sporangia, are of later evolutionary origin than the less 
constant body, the indusium. In this respect the indusium and the 
embryonic haustoria are probably alike. It is important to recognise 
such cases as these, for they go far to break down the dogma, that the 
prior existent parts in the development of the individual were necessarily 
prior existent in the evolution of the race. 
The views of Prantl have already been quoted, which involve a 
comprehensive shifting of sori from the margin to the surface. This has 
brought about great modifications of the indusium. Starting with a 
strictly basipetal sorus, with cup-like indusium, slightly two-lipped it may 
be, as in Loxsoma (Fig. 320), we have seen that the type in Denxnstaedtia 
aptifolia (Fig. 332) is similar in position and structure. The indusial lips, 
which are equal in the above plants, become unequal in Jficrolepia 
speluncae (Fig. 332), the superior lip (s) being both longer and thicker than 
the inferior (7), and taking on itself the structural characters and appearance 
of a continuation of the leaf; this is repeated in Davallia Gwriffithiana 
