CONTENTS. XVll 



evolution difficult to account for by any working of natural selection — 

 What is evolved? — How does the evolution proceed? 269. — Intrinsic 

 and extrinsic development, and intrinsic and extrinsic characters, 270 

 — Example of an intrinsic character — Example of an extrinsic char- 

 acter, 271. — Characters early and rapidly evolved were chiefly intrinsic 

 characters — Application of the terms intrinsic and extrinsic to the 

 elaboration of machinery — Summary and conclusions, 272. 



CHAPTER XVI. 



THE MOD I PICA TION OF GENERIC CHARACTERS, OR GENERIC LIFE- 

 HISTORY. 



Statistics of thejife-history of the spire-bearing Brachiopods (Helicopeg- 

 mata) — The rapid appearance of the different modifications of the bra- 

 chidium, 277. — Three families of the Helicopegmata, 279. — Geological 

 range of the families — Description of the structure of the brachidium, 

 2S0. — Significance of the facts — The loop of the Ancylobrachia and the 

 brachidium of the Helicopegmata, 282. — Relation of the jugum to the 

 primary lamellae, 283. — Relation of the primary lamellae to the crurse, 

 285. — The number of volutions of the spiral, 286. — Direction of the axes 

 of the spiral cones, 2S7. — The form of the loop, 288. — Characters of the 

 brachidium found to be good distinctive characters of genera^Plastic- 

 ity a characteristic of their early initial stage — Evolution of the char- 

 acters of the brachidium relatively rapid, 289. — Rate of initiation of 

 the genera of Helicopegmata — Table expressing the rate of expansion 

 of the family, subfamily, and generic characters of the Helicopeg- 

 mata, 290. — General law of rate of initiation of generic characters — 

 The life-periods of genera and the initiation of a new genus, 291 — 

 During the life-period of the genus its characters constant, 292. — A 

 culminating point or acme in the life-period of a genus — Summary of 

 the geological characters of a genus, 293. 



CHAPTER XVII. 



THE PLASTICITY AND THE PERMANENCY OF CHARACTERS IN THE 

 HIS TOR Y OF ORG A NISMS. 



Races in Paleontology — Phylogeny of the race — Mutability and Phylogeny, 

 294. — The phylogenetic theory of evolution, 295. — Mutability the 

 fundamental law of organisms ; the acquirement of permanency sec- 

 ondary, 296. — Early plasticity succeeded by permanency expressed in 

 geological history — Pritchard's definition in which the constancy of 

 transmission of some peculiarity is made the criterion of species, 297. 

 — Permanency of characters in living forms coordinate with limitation 

 in distribution and breeding — Specific variability restricted with each 

 successive generation in fossil forms — Illustrations of the acquirement 

 of permanency of characters, 299. — The history of the Spirifers — The 

 permanent characters of generic or higher rank, 300. — Characters which 

 are plastic at the first or initial stage of the genus — The fixation of 

 plastic characters in a generic series — Spiral appendages — General 

 proportions of the shells — Delthyrium and deltidium — Hinge area — 

 Surface markings — Plication of surface and median fold and sinus — 



