The attempt to indicate the active factors of evolution is the source of the different theories, 368. The theories of Lamarck, Darwin, and Weismann have attracted the widest attention, 369. Lamarck, the man, 368-374. His education, 370. Leaves priestly studies for the army, 370. Great bravery, 371. Physical injury makes it necessary for him to give up military life, 371. Portrait, 373. Important work in botany, 371. Pathetic poverty and neglect, 372. Changes from botany to zoölogy 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_ published 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 characters, 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. Various 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 XVIII Theories Continued--Weismann. De Vries, 392