16 PHYSICAL BASIS OF HEEEDITY 



law of independent assortment of the genes. They rest 

 on numerical data, and are therefore quantitative and can 

 be turned into mathematical form wherever it seems desir- 

 able. But though the statements were exact, they were 

 left without any suggestion as to how the processes 

 involved take place in the living organism. Even a purely 

 mathematical formulation of the principles of segregation 

 and of free assortment would hardly satisfy the botanist 

 and zoologist for long. Inevitably search would be made 

 for the place, the time, and the means by which segre- 

 gation and assortment take place, and attempts would 

 sooner or later be made to correlate these processes with 

 the remarkable and unique changes that take place in the 

 germ-cells. 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 knowledge to which Sutton appealed, had been 

 accumulating between the years 1865, when MendePs 

 work was published, and 1900, when its importance became 

 generally known. An account of the chromosomal 

 mechanism may be deferred, but I have spoken of it here 

 in order to call attention to a point rarely appreciated, 

 namely, that the acceptance of this mechanism at once 

 leads to the logical conclusion that MendePs discovery 

 of segregation 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. In conse- 

 quence we find that we are dealing with a principle that 

 concerns the actual composition of the material that car- 

 ries one generation over to the next. 



Segregation and independent assortment were the two 

 fundamental principles of heredity discovered by Mendel. 

 Since 1900, four other principles have been added. These 

 are known as linkage, the linear order of the genes, inter- 

 ference, and the limitation of the linkage groups. In the 

 same sense in which in the physical sciences it is custo- 



