SIMULTANEOUS OR SUCCESSIVE 117 
earliest, the sporangia in near juxtaposition show a simultaneous origin; 
or some degree of succession may be seen from those earlier formed 
near to the base of the shoot or leaf, and leading to the apical region, 
where they appear later. Such simultaneity, or such acropetal succession, 
may be regarded as a primary condition, and it is seen in the Lycopodiales, 
Equisetales, and Sphenophyllales, as well as in those Ferns which are 
designated below the Simplices (see Part IJ.). But in certain Ferns, which 
the Palaeontological record, as well as comparison, would mark out as 
-secondary, the sporangia in near juxtaposition do not arise simultaneously: 
sometimes, as in those which will be styled the Gradatae, there is a regular 
basipetal succession within the sorus, those lowest on the receptacle 
appearing latest. In others, again, there is no such regularity, and sporangia 
of different ages are found promiscuously intermixed: these Ferns are 
styled the Mixtae, and the Palaeontological record indicates that these 
were the latest to appear. Such facts, which will be stated at length 
below (Part II.), may be summed up into the following general statement. 
In'the most primitive forms the sporangia in near proximity to one another 
develop simultaneously, though an acropetal succession may often be 
traced on the shoot or strobilus as a whole. Those successions, whether 
in regular order or irregular, which appear in various forms upon the 
leaves, may be held to be later derived, and secondary. 
It will be readily gathered from the contemplation of those plants 
in which sporangia are numerous that accurate comparison of individual 
sporangia as identical bodies in parent and offspring, or in different, less 
closely related specimens, is not possible in plants at large. For the most 
part sporangia are merely examples of ‘essential correspondence” rather 
than of “individual repetition.” The actual sporangia of the offspring are 
not coincident, as a rule, either in exact position or in number with those 
of the parent. This is a consequence of that continued embryology which 
is a leading feature in all vascular sporophytes. As a consequence the 
individual sporangia of any one individual plant or species cannot be 
held to be the exact ontogenetic correlatives of those seen on another 
individual or species. The possibility of such a recognition is most nearly 
approached in the Lycopods, where the sporangia are borne singly in 
definite relation to the axis and leaf. It is departed from furthest in 
the large-leaved Ferns: especially is this so in the Polypodiaceae, where 
the mixed character of the sorus is the rule: but most of all in such a 
case as that of Aspidium anomalum, Hk. and Arm, a Fern found on 
the uplands of Ceylon, and sometimes regarded as a mere variety of 
A. aculeatum, Sw.: its peculiarity consists in the appearance of sori upon 
the upper surface of the leaf, where normally they do not occur. As 
there is no question of mere, inversion of the leaf, it can only be assumed 
that there has been a transfer of the stimulus, whatever it be, to soral 
development from the lower to the upper surface of the leaf. Clearly 
the sori which result cannot. be the ontogenetic correlatives of any 
