CONTEXTS xi 



III. THE PROCESSES OF EVOLUTION; ATTEMPTED EXPLANATIONS, 

 p. 266. Natural selection, p. 266. Variation, p. 267. 

 Mutation, p. 273. The struggle for existence, p. 276. Arti- 

 ficial selection, p. 279. Orthogenesis, p. 281. Segregation, 

 p. 283. The interaction of external and internal forces, p. 

 286. 



PRACTICAL EXERCISES. 



Study 17. An examination of bryophyte characters, p. 127. 



Study 18. Fern development, p. 140. 



Study i p. The general structure of the fern sporophyte, p. 141. 



Study 20. A comparison of developmental features of other pterido- 

 phytes, p. 141. 



Study 21. Spermatophyte structure, p. 154. 



Study 22. Spermatophyte development, p. 154. 



Study 23. Observations on the structure of the hydra, p. 162. 



Study 24. The general structure of the earth worm, p. 178. 



Study 25. The cellular structure of the earth worm, p. 179. 



Study 26. The internal organs of an amphibian, p. 206. 



Study 27. The structures of the body wall in an amphibian, p. 207. 



Study 28. The cellular structure of an amphibian, pp. 208. 



Study 29. The early development of an amphibian, p. 209. 



Study jo. Determination of homologies in three series of closely 

 allied insects, p. 229. 



Study 37. Observations on plasticity of form and persistence of type 

 in Malacostraca, p. 233. 



Study 32. An attempt at interpreting a possible phylogeny, p. 238. 



Study 33. A comparison of convergent species, p. 243. 



Study 34. The ontogeny of organs in the frog or salamander, p. 261. 



Study 35. Fluctuating numerical variations, p. 272. 



Study 36. The struggle for existence among seedlings, p. 278. 



CHAPTER IV. 

 INHERITANCE. 



I. THE VISIBLE MECHANISM OF HEREDITY, p. 289. The history 

 of the germ cells, p. 296. Fertilization and maturation, p. 

 299. Parthenogenesis, p. 204. 



II. THE OBSERVABLE RESULTS OF INHERITANCE. Types of in- 

 heritance, p. 308. Alternative inheritance, p. 310. 

 III. NATURE AND NURTURE, p. 318. Inheritance of acquired 

 characters, p. 318. The meaning of nurture, p. 321. 



