420 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



of importance only in so far as they show that the ex- 

 isting Nautilus does not represent the acme of pro- 

 gress of its order, but is a descendant of shells with 

 less complicated structures than many of the genera of 

 the Carboniferous, Jura, and Cretaceous." 



In these cases it seems that the mechanically ac- 

 quired impressed zone is inherited from the greater 

 part of the soma where it existed to a part of the soma 

 of the young where it could not be produced by me- 

 chanical causes, by reason of the non-contact of the 

 parts. This acquisition appears in a few Carboniferous 

 species, and then it is present in the cyrtoceran or 

 mesozoid stage of all the Jurassic, Cretaceous, and 

 Cenozoic species. Professor Hyatt, in his "Phylogeny 

 of an Acquired Characteristic," thus summarizes his 

 conclusions : 



"The facts and arguments brought forward seem 

 to justify the following conclusions : 



"I. The impressed zone is primitively a contact 

 furrow, an acquired characteristic of the dorsum of the 

 whorls of nautilian shells having large umbilical per- 

 forations, which appeared either in the ananeanic or 

 metaneanic (maturing) substages, and rarely later in 

 their ontogeny. There is abundant positive evidence 

 that in these primitive forms this furrow is a purely 

 mechanical result of the nautilian mode of growth, not 

 appearing in the ontogeny before contact, and either 

 partially or entirely disappearing on the free gerontic 

 (senile) volution. 



"2. The impressed zone does occur independently 

 of contact on the free dorsum of the paranepionic (ado- 

 lescent) substage as a dorsal furrow in some close- 

 coiled, highly tachygenic (accelerated) nautilian shells 

 in the Quebec group and in the Devonian. 



