igo PRIMARY FACTORS OF ORGANIC EVOLUTION. 



series of the orthoceran, cyrtoceran, gyroceran shells 

 with which each distinct complete genetic series of 

 nautiloids arose in time. 



"While the nautiloids coil up in their progressive 

 evolution and the Ammonitinae increase this coiling up 

 tendency in the primitive and progressive forms of 

 each genetic series, the latter in becoming retrogres- 

 sive reverse the processes of progressive evolution. 

 They become more and more uncoiled, each complete 

 retrogressive series ending with a'straight cone. All 

 other characters correlate with this uncoiling and in a 

 general way may be said to degenerate in greater or 

 less proportion to the amount of the uncoiling. To 

 make this extraordinary picture complete it is only 

 necessary to add that these retrogressive series followed 

 out to their ultimate development are distinctly parallel 

 with changes or stages of modification taking place in 

 the senile stages of individuals of the same genetic 

 group. 



"In old age the highly ornamented shell gradually 

 parts with its spines and other ornaments, the whorls 

 slowly diminish, the involution decreases and even- 

 tually in extreme age it becomes separated from the 

 spiral and completely rounded and smooth. The aper- 

 ture becomes correlatively modified, and also the su- 

 tures. If an old ammonite could have its life pro- 

 longed, it would become Baculites, and the full-grown 

 part of the shell would, in some forms of Lytoceratinae 

 be very similar to the minute nepionic shell of the Ba- 

 culites as described and figured by Dr. Brown. If now 

 the coiled adult part of this imaginary shell were broken 

 off and lost, the straight senile fragment would be re- 

 ferred to the old genus Baculites. The morphic char- 

 acters of the gerontic or old-age stage of ontogeny are 



