No. 490] 



ORTHOGENETIC VARIATION 



617 



The manner of apj^earance of tlie spirals must be regarded as a 

 oonsecjuence of the rate of growth of the mantle. Considering 

 that the first two spirals are the result of the first two folds into 

 which the mantle border is thrown on i-etraction, and assuming 

 that the position of these folds is a ju'iiiiancnt one as ai)pears 

 indeed to be unc|uesti()nal)le from tlic continiinx and regularity of 

 the spirals, we perceive that if tlic mantle border grows more 

 rapidly above and below the original folds than between them, the 

 new spirals will appear respectively al)ove and l)elovv the first two. 

 If on the other hand the mantle grows fastest between tlie two 

 original folds, the corresponding sjnrals will diverge and new ones 

 appear between them. This is the case in the foniiatiou of the 

 secondary spirals in the Fusidae and other foinis. 'Vhv (iiiestion 

 may well be asked if these tuo mode^ of ap|KMran<<' ..f the earh 

 spirals do not represent two lines of developnuMit oriiiiiiating in- 

 dependently in various genetic series and pnxhu iiiu- eii(l-nienii)ers, 

 which, within the same seri(>s, are in.Hstinuui>liable in their adult 

 cliaia<-ter^ If. on the other hand, nnc re^anl intercalation a. the 

 method of appea-anee nf .eeondan and hiter spirals, then Gonio- 

 l)a>is and the Melaiiias and other types which sliow intercalation 

 after the appearance of tlie hrst two spirals, nnist be considered as 



