Physical Basis of Heredity and Evolution 33 



female chromosome, in which case the new individual would 

 be a mixture, resembling both parents. Expressing the 

 chances in the form of a ratio, they could be represented as 

 1:2:1. This is a simple expression of Mendel's law, defined 

 in the preceding chapter. The formulation of Mendel's 

 law was based upon the observed facts of heredity; and this 

 chromosome situation supplies for it a cytological basis. 



It must be understood clearly that so simple an illus- 

 tration does not represent the actual facts, for chromosomes 

 are usually more numerous, and eggs and sperms already 

 contain a mixture of paternal and maternal chromosomes 

 derived from the preceding generations. It does illustrate, 

 however, the mixture of hereditary qualities by the sexual 

 fusion, the domination of certain of these qualities, and the 

 chances of resemblances in the progeny. It should be 

 understood also that the structural resemblance to the 

 maternal form or to the paternal form does not include sex 

 determination. For example, the new individual which 

 resembles the maternal form may prove to be either male 

 or female, and vice versa. 



It is evidently impossible that the chromosomes con- 

 tinue to be doubled at each generation, without any reverse 

 process of reduction. The two cardinal points in every life- 

 history, therefore, are fertilization, by means of which the 

 chromosomes are doubled, and reduction, by means of 

 which the doubled number is halved. The reduction pro- 

 cess occurs at different stages in the life-history in different 

 organisms. Among animals, it occurs in connection with 

 the formation of the eggs and sperms ; and therefore reduc- 

 tion and fertilization are practically consecutive events. 

 In most plants, however, the two processes are farther 

 apart, separated from one another by two distinct indi- 



