COMPARISON OF SPOROGONIA 287 



superficial position in the Ptecjdophyt.es 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. 



