THEORIES CONTINUED—WEISMANN. DE VRIES, 
CONTENTS 
PAGE 
and neglect, 372. Changes from botany to zodlogy 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 1800, 375. His Philosophie Zoologique 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. Salient 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. Variation, 380. Inheritance, 382. 
Those variations will 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 consequences, 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 
sélection, 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 XVIII 
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. II- 
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- 
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