356 NOTICES or new books. 



The implication is that the number of tlie original chromosomes has 

 either become doubled or halved. If the number is simply doubled, 

 there would be at first four of each kind of chromosome from the point 

 of view of genetic contents. There is some direct evidence that this 

 tetraploidy may occur. There may be also doubling in one pair of 

 chromosomes, and there are other modes of variation in the number of 

 chromosomes. 



The discovery that the female in certain species has two X-chromo- 

 somes, and the male only one X-chromosome, either with a Y-chromosomo 

 in addition (Stevens) or without the Y (Wilson), established a view first 

 suo-o-ested by McClung that the difference between the sexes is connected 



'OO 



with the distribution of particular chromosomes. It may be that the 

 presence of two chromosomes (XX), in connection with the rest of the 

 cell complex, causes a female to develop ; while only one sex chromosome 

 (X), in connection Avith the rest of the cell, causes a male to develop. 

 Or it may be that XX and X are merely indices of sex — i.e. that the sex- 

 chromosomes follow sex and do not determine sex. According to 

 Morgan, the evidence is now conclusive that sex follows the chromosomes. 

 He also shows how the chromosome theory of sex may apply to 

 " intersexes," gynandromorphs, and allied phenomena. 



In so far as parthenogenetic reproduction takes place without 

 redaction in the number of the chromosomes, the expectation of any 

 character is that it will have the same frequency distribution in 

 successive generations, because the chromosome group is identical in 

 eacli generation. The same will apply to a species propagating 

 vegetatively, or to cases of sexual reproduction in a homozygous group 

 of individuals (as in Johanusen's pure lines). 



Almost the whole interpretation outlined above rests on the postulate 

 that the chromosomes are the bearers of the hereditary factors or genes. 

 There is cytological and eml)ryological evidence supporting this view, 

 but it is the genetic evidence that is convincing. That there may be 

 substances in the cytoplasm that propagate themselves there and that 

 are outside the influence of the nucleus must be conceded as possible ; 

 but, aside from certain plastids, all the Mendelian evidence fails to show 

 that there are such characters. It is difficult to determine whether a 

 peculiarity of the ovum-cytoplasm, such as colour, is due to inherited 

 plastids or to the influence of the ovum-nucleus before fertihzation. 



A gene is to be tliought of as a certain amount of material in the 

 cln'omosome that may separate from the chromosome in wliicli it lies, 

 and be replaced by a corresponding part (and none other) of the 

 homologous chromosome. It is of fundtimental significance in this 

 connexion to recognize that the genes of tlie pair do not jump out of 

 one chromosome into the other, so to speak, but are changed l)y the 

 thread breaking as a piece in front of or else behind them, but not in 

 both places at once, as would be the case if only a single pair of 

 allelomorphs were involved each time. 



A number of general propositions may be stated : — 1. A gene is 

 associated with manifold effects. AVhatever it is in the germ-plasm 

 that produces white eyes produces other peculiarities as well. 2. The 

 variability of ;i character is not necessarily due to variability in the gene ;. 



