354 



NOTICES OF NEW BOOKS. 



Modern Study of Heredity. ByT. H. Morgan. {The Phijsical Basis of 

 Heredity, Philadelphia and London, 1919, 1-305, 117 figs.) 



The two fundamental principles of heredity discovered by Mendel 

 were the law of segregation and the law of independent assortment of 

 the genes. Sutton, in 1902, was the first to point out clearly how the 

 chromosomal mechanism, then known, supplied the necessary mechanism 

 to account for Mendel's two laws. The acceptance of this mechanism at 

 once leads to the logical conclusion that Mendel's discovery of segrega- 

 tion applies not only to hybrids, but also to normal processes that are 

 taking place at all times in all animals and plants, whether hybrids or 

 not. Since 1900 four other principles have been added. These are 

 known as linkage, the linear order of the genes, interference, and the 

 limitation of the linkage groups. 



Mendelism rests on the theory of a clean separation of the members 

 of each pair of factors (genes). In every heterozygote the factor for 

 the dominant and that for the recessive are supposed to come into 

 relation to each other and then to separate at the ripening of the germ- 

 cells. The point is the clean separation of the genes without contamina- 

 tion (unless as an exceptional phenomenon). Mendelian characters are 

 not confined to the surface. A common class of characters showing 

 perfect Mendelian behaviour are so-called lethals that destroy the 

 individual when in homozygous condition. In recent years an entirely 

 unexpected and important discovery in regard to segregating pairs of 

 genes (allelomorphs) has been made. In an ever-increasing number of 

 cases it has been found that there may be more than two distinct 

 characters that act as allelomorphs to each other. For example, in mice, 

 yellow, sable, black, white-bellied grey, and grey-bellied grey (wild type) 

 are allelomorphs — i.e. any two may be present (as a pair) in an indi- 

 vidual, but never more than two. In all probability, apart from liybrids 

 altogether, the germ-plasm is at first made up of pairs of elements, but 

 at the ripening of the germ-cells these elements (genes) separate, one 

 member of each pair going to one daughter-cell, the other member to 

 the other cell. 



The sperm and the (igg pass through essentially the same stages 

 during maturation, the essential feature of which is the conjugation of 

 homologous (paternal and maternal) chromosomes followed by their 

 subsequent segregation. Each egg and each sperm is left with half the 

 original nnmber of chromosonies — one of each kind, i.e. only a paternal 

 or a maternal member of each chromosome pair. It is obvious that if 

 one member of any pair contains material that produces an effect on 

 some character as one of the end results of its activity, and the other 

 member of the pair contains a different material, the behaviour of the 



