OBSERVATIONS AND EXPERIMENTS 223 



and more accentuated until it forms a spiral, whose 

 whorls are at first very loose (gyroceran shell) and 

 then coiled closely (nautilian shell). Later, those 

 shells undergo the opposite evolution. Owing to a 

 kind of degeneration, the whorls uncoil, and when the 

 snails are on the point of becoming extinct, the shells 

 revert to the curved or even to the straight shape. 

 During the upward evolution, however, as the whorls 

 become tighter and tighter, each whorl imprints itself 

 upon the next and smaller whorl, producing a char- 

 acteristic furrow or "impressed zone." Hyatt demon- 

 strated through a series of experiments that the im- 

 pressed zone had a purely mechanical origin. This 

 impressed zone, which in the adult is due to mechanical 

 causes, is also apparent in the larval shell at a stage of 

 growth in which no effective pressure has yet been 

 exerted, and the earlier the geological strata are, the 

 earlier the impressed zone appears in the larvae. 8 

 This is a perfect example of a purely local influence 

 affecting the offspring, an example which the many 

 and accurate observations taken by Hyatt made in- 

 dubitable. 



Le Dantec, who takes Hyatt's experiments as evi- 

 dence of the heredity of acquired characters, states 

 that the impressed zone produced by the pressure of 

 whorl upon whorl persists in the degenerated and 



8 A. Hyatt. Phylogeny of an Acquired Characteristic. {Proceed. 

 Amer. Philos. Soc, Vol. XXXII, 1893, pp. 349-616.) 



