HEREDITY. 405 



limbs with their specialized segments are fully devel- 

 oped, so that the individual is mature as it issues from 

 its prison. This illustration of inheritance derives its 

 point in the present connection from the fact that it 

 presents an example of the inheritance of characters 

 which were plainly acquired by mechanical stimuli 

 during post-embryonic life of the primitive ancestors 

 of the Arthropoda. 



3. EVIDENCE FROM PALEONTOLOGY. 



a. The Impressed Zone of the Nautiloids. 



I have already quoted Professor Hyatt on the par- 

 allelism which is characteristic of the various series of 

 nautiloid Cephalopoda, as discovered by paleontologic 

 research. (P. 182.) The impressed zone is a character 

 which has been produced by mechanical causes (pres- 

 sure), and Prof. Hyatt has observed cases where this 

 acquired peculiarity has been inherited in instances 

 where the mechanical cause which produced it no longer 

 existed. He describes these instances as follows : x 



"The characteristic dealt with in the paper of 

 which this is an abstract, is of essential importance 

 among nautiloids and ammonoids or all of the Cepha- 

 lopoda having chambered shells and living within their 

 shells. It consists mainly of an impression made on 

 the inner side or dorsum of each outer whorl during 

 the coiling up, as the whorl grows and is moulded over 

 the venter or outer side of the next inner whorl. 



"This matter will be better understood, if a short 

 description is given of the following figures. Fig. 115 

 shows an almost complete fossil cast of a full grown 



\American Naturalist, 1893. October, p. 865. Professor Hyatt has person- 

 ally looked over and corrected these quotations. 



