2S6 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



tremes corresponding to the canal and the posterior 

 commissure of the body and outer lip. Inside of this 

 we have a thin, loose epithelial cone, the mantle, of 

 which the external surface, especially toward the mar- 

 gin, is shell-secreting ; lastly, inside of the mantle- 

 cone we have a more or less solid third cone, consist- 

 ing of the foot and other external parts of the body of 

 the animal, which can be extended beyond the mantle- 

 cone outwardly, as the mantle-cone can be beyond the 

 shell- cone. The body-cone and the mantle-cone are 

 attached at one of the angles of the shell-cone some 

 distance within the opening of the spiral of the latter. 

 The two outer cones constitute a loose, flexible funnel 

 within a rigid, inflexible funnel, while the body-cone 

 forms a solid, elastic stopper inside of all. 



"What will happen according to mechanical prin- 

 ciples (which can be tested by anybody with the sim- 

 plest apparatus) when the mantle-cone is withdrawn 

 into a part of the shell-cone too small for the natural 

 diameter of the contracted mantle-cone ? It must 

 wrinkle longitudinally. Where will the wrinkles come? 

 They will come at the angles of the shell-cone first ; 

 they will be most numerous toward the aperture, since 

 toward the aperture the mantle-cone enlarges dispro- 

 portionately to the caliber of the shell, owing to its 

 processes, the natural fold of the canal, etc., etc.; the 

 deepest and strongest wrinkles will be on the pillar, 

 owing to the fact that the attachment of the adductor 

 prevents perfect freedom in wrinkling and the groove 

 of the canal will mechanically induce the first fold in 

 that vicinity. The most numerous small wrinkles will 

 be near the aperture opposite the pillar, because of 

 the mantle-edge this is the most expanded part, and 

 there will be a tendency to a ridge near the angle of 



