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. 



