HEREDITY. 415 



whorl which took place in consequence of the mechan- 

 ical effects produced by close coiling. This charac- 

 teristic is slight when the coiling is slight and is de- 

 veloped in precise proportion to the increase of coiling 

 or involution of the whorls, and, on the other hand, 

 when through degeneration due to age, or to other 

 causes, the whorls cease growing in contact, the im- 

 pressed zone gradually disappears. 



"Thus it generally appears preceded and accompa- 

 nied by an observable tendency in the mode of growth 

 toward closer coiling and that this tendency is quite 

 capable of producing the impressed zone can hardly 

 be denied with any show of reason, since the charac- 

 teristic tends to disappear in proportion as the pressure 

 is relieved through the degeneration of the powers of 

 growth-force to continue the normal rate of progres- 

 sive increase of bulk in old or young or prematurely 

 degenerate shells and in uncoiled whorls of all kinds 

 and all ages. 



"The shells of Devonian series of nautiloids have 

 also been extensively examined, especially in the more 

 involute nautilian forms of the genus Nephriticeras, 

 and so far not one has been found with the slightest 

 indication of the presence of an impressed zone in 

 the cyrtoceran or gyroceran stages of development. 

 In several examples also, the disappearance of this 

 characteristic has been observed in the last stages of 

 old whorls. There is, therefore, every reason for re- 

 garding the impressed zone as a ctetic characteristic 

 acquired in the later stages of growth and not heredi- 

 tary so far as is known in any shells of the earlier Pa- 

 leozoic periods.^ 



1 Certain exceptions have been found since this was written, but their evi- 

 dence is purely negative, it is impossible to say of them at present whether 



