THE HEREDITARY BASIS 17 



trasted characters. What is now known of the germ cells enables 

 us to point with great probability to the cellular mechanism by 

 which this purity of the gametes or mature germ cells is main- 

 tained. The same mechanism also affords an explanation of the 

 phenomenon of linkage or the tendency of diverse characters to 

 maintain a certain association in inheritance. The mechanism 

 consists of the chromosomes of the nucleus which there are strong 

 reasons for believing maintain their individuality, as they do their 

 number, not only through numerous cell generations in the life of 

 the individual, but through an indefinite number of life cycles of 

 individual organisms. The behavior of these chromosomes in 

 maturation and the process of synapsis immediately preceding 

 maturation is precisely such as would explain the distribution of 

 characters according to Mendel's law if we grant that individual 

 chromosomes contain factors for the production of particular 

 characters. We cannot give an idea of the remarkable success 

 that has been attained in connecting the phenomena of inheri- 

 tance with peculiarities of chromosome behavior, and must refer 

 the reader to special works and papers dealing with this topic. I 

 can scarcely do more than indicate in a short chapter the various 

 appHcations of Mendel's law in interpreting many enigmatical 

 phenomena of inheritance. The phenomena of reversion, the 

 results of inbreeding, the heredity of sex and the peculiar phe- 

 nomena of sex-linked inheritance are seen in a new light since 

 the discovery of Mendel's law. 



Since Mendel's law has been found so widely applicable in 

 plants and animals, we should expect to find it expressed also in 

 the inheritance of man. Already numerous human traits are 

 known which give strong evidence of being transmitted in accord- 

 ance with this law. Since it is not feasible to treat human beings 

 as we do plants and animals it is difficult to ascertain in many 

 cases whether inheritance is in fact strictly Mendelian. A list, 

 though incomplete, of traits which are probably transmitted 

 according to Mendel's law is given in the following table: 



