THE MECHANISM OF HEEEDITY 69 



tors, the distribution of the latter cannot be followed with 

 the same ease as one would follow the distribution of 

 cotyledon colors in the garden pea. This is true because 

 the visible effects of certain factors is sure to be very 

 small, and because varying external conditions obscure 

 the effects of inheritance. For example, a plant which 

 through its inheritance should become 6 feet tall under 

 average conditions may become only 4 feet tall if planted 

 in a sterile soil, but a plant which under average condi- 

 tions would become only 4 feet tall might become 5 feet 

 tall if grown in a very fertile soil. 



Nevertheless, in spite of these drawbacks, one can 

 select size characters for study which are influenced but 

 slightly by external conditions, and by studying large 

 numbers through several generations, and by applying 

 mathematical tests to determine the uniformity or the 

 variability of the resulting populations, he can find out 

 whether quantitative characters satisfy the six require- 

 ments seen to be fulfilled by qualitative characters. This 

 has been done in numerous cases, and the results firmly 

 convince all unprejudiced investigators that the inheri- 

 tance of all types of characters is the same. 



Table I, from crosses between two varieties of Nico- 

 tiana longiflora 54 differing in the size of their flowers, 

 illustrates the point. One does not need any refined 

 mathematical methods to see that when the small variety 

 having flowers about 40 mm. in length is crossed with the 

 large variety having flowers about 94 mm. in length ; the 

 result is a uniform F l population having flowers about 64 

 mm. in length. The two F 2 populations which it produced 

 are much more variable ; and one can easily calculate that 

 if several thousand plants had been grown instead of 



