122 PROF. BOWER : 
parallel development, and even convergence of characters are common 
phenomena. They may be traced in respect of almost all of the characters, 
as having been carried out in sequences that must have been phyletically 
distinct. It is possible, and with some degree of probability, to correlate 
certain of these changes with the external cireumstances. For instance, in 
the Paleozoic Period forest shade was either absent or imperfect. The 
robust constitution of the Eusporangiate Ferns suited such conditions well 
enough. The great outburst of the more delicate Leptosporangiate Ferns 
took place in the Mesozoic Period, and doubtless the advent then of broad- 
leaved trees, with their more effective shade, permitted plants like these, of 
less robust habit, to flourish. At the same time many of them, through their 
self-protection by dermal scales and indusial growths, had become more 
independent. A curious biological feature becomes, however, evident-in their 
further advance. For along many separate but parallel lines their indusial 
coverings have become aborted in the species of the present day, and the 
sori fully exposed again. It is as though the organisms had themselves 
become in some way more resistant to exposure. For such present-day plants 
often occupy exposed stations. This is the case in our own Polypodium 
alpestre, properly called by Newman Pseudathyrium ; also in the Oak Fern 
and Beech Fern. All of these may be found on hill stations, though they 
have exposed sori. This condition is due to the abortion of indusia, in the 
first case of the type of Athyrium, in the latter of Dryopteris. 
It is not only parallelism of development, however, but even convergence 
which is frequently seen. A good example is provided by the genera 
Cystopteris and Acrophorus, which have been notoriously difficult to place. 
They are ranked sometimes with the Davallioid, sometimes with the 
Nephrodioid Ferns, two quite distinct phyla. So far as their venation 
or their sori go, they might belong to either. But their anatomy, and 
the presence of the characteristic chaffy scales, proves them to be of 
Nephrodioid affinity. Another case is that of Doryopteris, long ranked 
with Pteris, with which its fusion-sorus is in close agreement. But it 
was separated by Prantl on more general grounds, and ranked, probably 
correctly, with Pellea and the Cheilanthinz. Other examples are the 
Davallioid genera Nephrolepis and Oleandra, which curiously mimic the 
Nephrodioid sorus, though of quite a different origin from it. Thus con- 
vergence of characters is a recurrent feature in the Filicales. There is, 
indeed, no group of highly organized plants which shows more frequently, 
and indeed consistently, evidence of parallel or convergent progressions in 
distinct phyletic lines ; while the parallelisms and convergences involve 
a great variety of characters, both vegetative and propagative. I commend 
such facts to evolutionary theorists as a pressing problem for them. 
The question will be, what eauses have been at work to produce such 
results ? They are usually set down to the selection of favourable diver- 
gences from type out of inheritable variations, or mutations, produced 
