NOTICES OF NEW BOOKS. 355 



chromosomes at the time of maturation supplies exactly the mechanism 

 that Mendel's law of segregation calls for. 



Mendel's second law is the independent assortment of the genes. 

 If at the maturation (whether of egg or sperm) the genes "tall" and 

 " colour " go to one cell, then the genes " short " and " white " go to the 

 other, or "short" and "colour" go to one cell, "tall" and "white" to 

 the other. Four classes of germ-cells will be expected in the F^ genera- 

 tion — viz. tall colour, tall white, short colour, and short white. Each 

 pair of chromosomes, just before the reduction division, consists of a 

 maternal and a paternal member ; the evidence points to random or 

 free assortment of some maternal chromosomes to one pole and some to 

 the other, and similarly for the paternal chromosomes. This will 

 account for the independent assortment of genes which Mendel's second 

 law postulates. 



But further investigation is disclosing an increasing number of cases 

 in which free assortment does not occur. Many characters have been 

 found to keep together in successive generations instead of assorting 

 freely. This is called linkage, and it may be complete or occasional. 

 The correlative aspect of linkage is crossing over, and inasmuch as it 

 involves a change in the mechanism that gives linkage, it is entitled to 

 rank as one of the fundamental principles of heredity. It means that 

 there is an interchange of blocks of genes between homologous pairs of 

 chromosomes. Pairs of characters may be spoken of as loosely linked, 

 meaning that crossing over of genes frequently takes place, or as strongly 

 linked, meaning that crossing over is very infrequent. It is probable 

 that there is a limiting value for crossing over, and if this can be 

 established it may lead to the discovery of the lower limit of size of the 

 gene (in terms of chromosome length). The crossing over, which may 

 occur in germ-cells of the male and not in those of the female, is not 

 effected earlier than the time of the conjugation of chromosomes, but it 

 can be effected at the time when the conjugation is known to occur. 

 In regard to all this, however, there is still considerable uncertainty. 



The data in regard to the Hnkage of characters and the correlative 

 phenomenon of crossing over lead to the conclusion that the genes are 

 arranged in linear order, standing at definite levels in the chromosomes 

 and definitely spaced. Ingenious arguments lead to the conclusion that 

 the size of the blocks that interchange in a crossing over depends on the 

 location of the breaking point, and that a break in one region interferes 

 with a break in another region. A correspondence between the number 

 of linkage groups and the number of chromosome pairs has been proved 

 in Drosophila melanoijaster, and no case is known where the number of 

 linkage groups exceeds the number of chromosome pairs. It may be 

 that a limitation of the linkage groups to the number of chromosomes 

 pairs is a fundamental principle of heredity. An interesting fact is the 

 variability of the amount of crossing over in certain cases ; the amount 

 differs at different temperatures in Drosophila, and it has also been 

 shown that there are genes carried by the chromosomes themselves that 

 affect the amount of crossing over. 



One species may have twice as many chromosomes as a closely 

 related one. So frequent is this that it can hardly be due to chance. 



2 A '> 



