214 DEGENERATION 



that in the whelk shell it is out of the reach of such blows. 

 When, therefore, the cuticular armour atrophies exactly in 

 proportion as the body is covered by the protecting mollusc- 

 shell, this can only be explained by the consideration that 

 on the parts of the body covered by the whelk shell the 

 armour is superfluous and of no consequence; and that 

 natural selection, therefore, could no longer be concerned in 

 its maintenance." 



In this instance I can only make the objection which I feel 

 obliged to make in all the rest : By selection alone nothing 

 whatever can be produced, and the cessation of selection is 

 not the only cause of the disappearance of anything which 

 has been evolved. 



It is scarcely necessary to mention what hinders the 

 perfect development of the cuticular armour (by which is 

 meant that of the posterior end of the hermit crab ensconced 

 in its protecting gasteropod shell). The armour, according to 

 my view, was produced in consequence of external stimuli 

 acting continually on the epidermis of the animal : the 

 cuticular structure was excreted as the effect of these stimuli. 

 The stimuli were the first cause of the commencement of its 

 formation. It served the animal as a protection. Selection 

 thereupon picked out those animals which were most 

 efficiently armoured. After the crabs had concealed them- 

 selves in gasteropod shells, the external stimuli could no 

 longer act upon their skin, and at the same time the selection 

 ceased, and so ensued the degeneration of the hard cuticle. 



By the co-operation of the degeneration caused by disuse, 

 and not by pammixis alone, as Weismann insists, according to 

 my view, all cases are explained in which the mouth parts 

 have atrophied or even the intestine degenerated in animals 

 which have altogether ceased to feed. The latter applies to 

 the males of the Eotifera, the former to many nocturnal 

 Lepidoptera, and to the Ephemera. Such animals either have 



