COMPARISON OF SPOROGONIA 287 
superficial position in the Pteridophytes is intelligible on biological 
grounds: it is closely related to the simultaneous development of the 
spores in Bryophytes, as against the successive spore-production in the 
Pteridophytes. Still in the former some degree of decentralisation, as we 
have seen, brings advantages of nutrition, and its structural expression 
is the sterile columella; but decentralisation does not become a peremptory 
condition .of success of the Bryophyte-type, as it appears to have been in 
the Pteridophytes. There is thus a biological reason for the nearer 
relation which all Bryophytes show to that condition which comparison 
indicates as primitive, where the fertile tissue is deeply seated, or even 
occupies a central position in the simpler types. Such considerations lend 
a biological probability to the theory of progressive sterilisation applied in 
the above pages to the sporogonia of the Bryophyta. 
Reviewing the Musci as a whole, the evidence of progressive sterilisation 
in them is less cogent than it is in the Hepaticae. They probably represent 
a more or less distinct phyletic sequence from the latter; but still analogies 
may be drawn between the two; such analogies strengthen the weaker 
evidence in the Musci; and, as there appear to be no facts which preclude 
such a view, while many give a reasonable measure of support, it may be 
held that progressive sterilisation has been effective here in essentially the 
same way as it is more clearly demonstrated in the Hepaticae. 
