XX CONTEXTS 



PAGE 



and neglect, 372. Changes from botany to zoolog)- at the age of 

 fifty years, 372. Profound influence of this change in shaping 

 his ideas, 374. His theory of evolution, 374-380. First public 

 announcement in iSoo, 375. His Philosophic Zoologiqiie pub- 

 lished in 1809, 375. His two laws of evolution, 376. The first 

 law embodies the principle of use and disuse of organs, the second 

 that of heredity, 376. A simple exposition of his theory, 377. 

 His employment of the word besoin, 377. Lamarck's view of 

 heredity, 377. His belief in the inheritance of acquired char- 

 acters, 377. His attempt to account for variation, 377. Time 

 and favorable conditions the two principal means employed by 

 nature, 378. SaHent points in Lamarck's theory, 378. His 

 definition of species, 379. Neo-Lamarckism, 380. Darwin. His 

 theory rests on three sets of facts. The central feature of his 

 theory is natural selection. A'ariation, 380. Inheritance, 382. 

 Those variations ^\'ill be inherited that are of advantage to the 

 race, 383. Illustrations of the meaning of natural selection, 383- 

 389. The struggle for existence and its consec^uences, 384. Vari- 

 ous aspects of natural selection, 384. It does not always operate 

 toward increasing the efficiency of an organ — short-winged 

 beetles, 385. Color of animals, 386. Mimicry, 387. Sexual 

 selection, 388. Inadequacy of natural selection, 389. Darwin the 

 first to call attention to the inadequacy of this principle, 389. 

 Confusion between the theories of Lamarck and Darwin, 390. 

 Illustrations, 391. The Origin of Species published in 1859, 391. 

 Other writings of Darwin, 391. 



CHAPTER XVni 



Theories Continued — Weismann. De Vries, . , . '392 



Weismann's views have passed through various stages of remodeling, 

 392. The Evolution Theory published in 1904 is the best ex- 

 position of his views, 392. His theory the field for much contro- 

 versy. Primarily a theory of heredity, 393. Weismann's theory 

 summarized, 393. Continuity of the germ-plasm the central idea 

 in Weismann's theory, 394. Complexity of the germ-plasm. Il- 

 lustrations, 395. The origin of variations, 396. The union of 

 two complex germ-plasms gives rise to variations, 396. His ex- 

 tension of the principle of natural selection — germinal selection, 

 397. The inheritance of acquired characters, 398. Weismann's 

 analysis of the subject the best, 398. Illustrations, 399. The 

 question still open to experimental observation, 399. Weis- 



