The next important step in the develop ment of the science of genetics was made in 1900 in the rediscovery of Menders law of heredity. Gregor Mendel, while a teacher of natural science in a monastic school at Briinn, Austria (between 1858 and 1865) carried out experiments in the hybridization of different varieties of garden peas which led him to the discovery of a fundamental law of heredity, perhaps the fundamental law of heredity, known after him as Mendel's law. The discovery was later lost sight of, its fundamental character not being recognized until 1900 when it was rediscovered by Hugo DeVries. This law was promptly verified by numerous other plant hy bridizers and was also shown to apply to ani mals as well as to plants. In accordance with this law it is found that when parents differ, as regards hereditary characters, in several particulars, their descendants may show com binations of those particular characters which were found in neither parent, though other descendants may have the same combinations of characters as the parents had. This leads to the conclusion that separate characters are independently inherited. If we adopt Weis mann's suggestion that the bearers of heredity are in the chromosomes, we may suppose that characters which are independently inherited have their determiners in different chromo somes, a conclusion supported by a large amount of evidence derived from a study of the struc ture of the germ-cells. The greatest probabil ity that such a relationship exists is also afforded by breeding experiments with the fly, Drosophila, carried out by T. H. Morgan and his pupils. In these experiments parents have been crossed which differ in many heritable characters and it has been found that certain of these characters tend to be inherited in groups similar to the grouping of Characters in the respective parents. But not all the charac ters of one parent form a single group. There are in reality as many different groups of char acters as there are chromosomes in the germ cell of Drosophila, viz., four. The conclusion is drawn that each group of characters has its determiners located in a single chromosome, and that for this reason the corresponding characters usually go together in heredity. There is, however, found a considerable varia tion in the tenacity with which characters of the same group tend to hang together. Some characters break away from the common group ing more readily than others. It has been suggested that those characters which most commonly go together have their determiners close together in the same chromosome, while those which are more readily separated lie farther apart in the same chromosome. Many lines of experimental evidence support this view, which therefore marks a further im portant advance in our knowledge of the genetics of both animals and plants, for it has been found that the same phenomenon of €couple& or glinkedp character groups occurs both in animals and in plants.
The science of genetics is still very new, a development of scarce 60 years, but the prin ciples already established are finding important applications in animal and particularly in plant breeding. Hybridization allows of the produc tion of new combinations of the heritable characters found in existing races, some of which new combinations may have distinct economic advantages. Thus our plant breeders seek to combine hardiness and disease resist ance found in certain varieties of plants with the greater productiveness of related varieties. Animal breeders often wish to combine the size of one breed with the color, hornlessness or fecundity of another. Mendel's law shows
that such combinations are obtainable in a defi nite number of generations by cross-breeding and indicates the precise procedure necessary to obtain the desired result. But experience shows that the procedure of combining in one race different heritable characters found in separate races is one that requires considerable ingenuity and skill on the part of the breeder. The chief obstacle encountered is the instability in hybridization of the most prized characters of pure-bred races. Characters which dearly follow Mendel's law in heredity, emerging from a cross in new combinations, often emerge in altered form, the desired character of high standard which the parent race possessed emerging with standard lowered. Thus when a parent race possesses an intense black or a clear yellow color which the breeder wishes to transfer to another breed, it is usually found that the intense black emerges from the cross much dulled in character or that the clear yel low has become muddy. Also color patterns, such as those of Dutch-marked cattle, Dutch marked rabbits and hooded rats, usually emerge from crosses in modified form, which only slow and tedious selection can restore to the original condition. (See STOCK BREEDING). Further, some characters, such as size, seem to blend rather than to segregate in crosses, so extensive is the modification which regularly occurs. It is even an open question whether the inheritance of such characters is not better described as blending than in terms of Mendel's law. This is one of the questions which a further study of genetics must clear up.
The great principles so far established in this subject, or found fundamental to it, are: (1) Organic evolution, origin of new races from previously existing ones by descent with modification and survival of the fittest; (2) non-inheritance of acquired characters; (3) great stability of organic form under vegetative or asexual methods of reproduction, as in graft ing, reproduction by stolons, cuttings, etc.; (4) wide prevalence of reproduction the zygote method (single cells uniting in pairs) among all rapidly evolving organisms such as the flow ering plants and higher animals. Since this method allows of crossing in every generation, it favors variability through the production of new character combinations or through charac ter modification; (5) any departure from cross fertilization in reproduction by zygotes, such as the occurrence of self-fertilization among certain plants and among some animal para sites, restores stability of organic form like that found in vegetative or asexual reproduction. Hence the constancy of self fertilizing varieties of cereals and legumes; (6) Mendel's law of heredity shows how in reproduction by zygotes crossing may lead to the production of new character combinations and thus favor progres sive evolution; (7) selection acts in two ways upon the variable material furnished by zygote reproduction attended by crossing. (a) It ,so lates desirable new character combinations. (b) It isolates desirable modifications of char acters in combinations already secured. These modifications may arise in consequence of cross ing or irrespective of it, as they are known to do in asexual reproduction or among self fer tilizing organisms.
Bibliography.— Bateson, W. 'Mendel's Principles of Heredity' ; Castle, W. E., (Genet ics and Eugenics) ; Darwin, Charles, (Animals and Plants under Domestication); Morgan, T. H., et at,