XVI 11 CONTENTS. 



Structure of shell — Surface spines, granulation, etc. — Special develop- 

 ment of the median septum, 301 — Evolution of extrinsic specific char- 

 acters comparatively slow, although their plasticity is greatest at the 

 initial stage — Laws of intrinsic and extrinsic evolution expressed in 

 variability and permanency of characters, 311. — Hall's analysis of the 

 genus Spirifer and classification of its species — Range of species of 

 Spirifer in American formations, 312. — Each type of Spirifer shows a 

 continuous series of species, 313. — Each of the chief types represented 

 at the initial period of the genus — Three epochs of expansion, with 

 slow and gradual change during the rest of the history of the genus, 

 314. — Characteristics of the life-history of Atrypa reticularis, 315. — • 

 Considerable and continuous plasticity of the species — Nature and ex- 

 tent of the variations, 316. — Hall's comment on the variability of the 

 species, 317. — In the closing part of the life-period of the race the ex- 

 tremes of acceleration and retardation expressed— Summary, 319. — 

 Conclusions suggested by the study of Atrypa reticularis, 320. — The 

 initiation of the species of Ptychopteria, 322. — The law of progressive 

 evolution of Mammals, formulated by Osborne, 323. 



CHAPTER XVIII. 



THE RA TE OF MORPHOLOGICAL DIFFERENTIA TION IN A GENETIC SERIES, 

 ILL US TR A TED BY A STUDY OF CEPHALOPODS. 



The evidence furnished by the Cephalopods — Lankester's schematic Mol- 

 lusk, 325. — Supposed characteristics of the primitive mollusk — Differ- 

 entiation of the foot organ in mollusks, 327. — The structure of the 

 Cephalopods, 329. — Numerical rate of differentiation expressed in terms 

 of the initiation of new genera, 336. — Rate of differentiation of the sub- 

 order Nautiloidea, 337. — Mode of curvature of the Nautiloid shell — 

 Rate of initiation of the Orthoceratidae — of the Cyrtoceratidae, 339 — of 

 the Nautilidae — History of Trochoceras by species — General law of 

 evolution of shell curvature in the Nautiloidea — Rate of evolution of 

 new species in the American region, 340. — Hyatt's formulation of the 

 law of rapid expansion of differentiation at the point of origin of a 

 new type of organism, 341. — Summary, 342. 



CHAPTER XIX. 



PROGRESSIVE MODIFICATION OF AN EXTRINSIC CHARACTER ; 



ILLUSTRA TED BY THE EVOLUTION OF THE 



SUTURE-LINES OF AMMONOIDS. 



The Ammonoids illustrate the law of acquirement of differences by grad- 

 ual modification, 344. — Description of the characters of the Ammo- 

 noids, 345. — Two divisions of the Retrosiphonatae : Goniatites and 

 Clymenias, 348. — Quick evolution of the Clymeniidae — Classification of 

 the Goniatites, 349. — Differences in the sutures of the Ammonoidea 

 explained as various degrees of crimping of the edge of the diaphragms 

 — Classification of the types of sutures — (A) the Nautilian type of 

 suture — (B) the Goniatite type of suture — (C) the Ceratitic, Helictitic, 

 and the Medlicottian types of suture — (D) the Ammonitic type of sut- 



