Evolution

(ev-o-lu'-shun). 1. The development or temporary growth and organic change that take place in the uterus during pregnancy. 2. In biology, the doctrine of incasement or preformation of the early physiologists, which supposed that all parts of the fully-formed animal or plant were present in a minute form in the germ. Cf. Epigenesis . The doctrine of descent often spoken of as Darwin ism, based upon the capacity of every plant and animal to produce other individuals of a like kind, but varying to a greater or le degree according to the differentiation induced by special environment and natural selection. Cf. Pangenesis , Blastogenesis, Epigenesis, Ontogeny, Phytogeny, Natural Selection. E., Factors of, the recognized factors of evolution are at least five, viz.: Pre ure of an environment affecting function and function affecting structure, and the changed structure and function inherited and integrated through succe ive generations. Use and disuse of organs reacting on growth-force and producing change in form, structure, and size of parts, and such change inherited and integrated through succe ive generations. Natural selection among individuals of those most in accord with an ever-changing environment or, as it has been otherwise called, " survival of the fittest. " Sexual selection: the selection by the female, among varying male individuals, of the strongest or the most attractive. Among mammals, the selection is mainly of the strongest, as decided by battle; among birds, of the most attractive, as determined by splendor of color or beauty of song. Physiologic selection, or selection of those varieties the individuals of which are fertile among themselves, but sterile or le fertile with other varieties and with the parent stock. This has also been called " segregate fecundity" by Gulick and homogamy by Romanes. These five factors are not universally recognized. The first two are Lamarckian, the second two Darwin i an factors. In the Lamarckian factors the changes occur during individual life, and the offspring is supposed to inherit them unchanged. In the Darwin i an factors, on the contrary, the changes are in the offspring, and the individuals during life are supposed to remain substantially unchanged. The fifth factor has been brought forward by Romanes and Gulick and is not yet universally recognized.