CONTENTS 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. 12'/. 



Study 18. Fern development, p. 140. 



Study ig. 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. 1^4. 



Study 2j. Observations on the structure of the hydra, p. 162. 



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



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



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



Study 2J. The structures of the body wall in an amphibian, p. 207. 



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



Study 2g. The early development of an amphibian, p. 2og. 



Study JO. Determination of homologies in three series of closely 

 allied insects, p. 22g. 



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

 in Malacostraca, p. 2jj. 



Study J2. An attempt at interpreting a possible phylogeny, p. 2j8. 



Study jj. A comparison of convergent species, p. 24J. 



Study J4. The ontogeny of organs in the frog or salamander, p. 261, 



Study j^. Fluctuating numerical variations, p. 272. 



Study j6. 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. 



