CONTENTS. XV11 



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 MODIFICATION OF GENERIC CHARACTERS, OR GENERIC LIFE- 

 HISTORY. 



Statistics of the^life-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, 

 280. Significance of the facts The loop of the Ancylobrachia and the 

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

 primary lamellse, 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, 287. 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 

 HISTORY OF ORGANISMS. 



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 



