Plants in Relation to Man 511 



complex, combining many characters in all sorts of ways. The 

 analysis of the physical and mental characters of man is so 

 difficult, and human generations are so long, that comparatively 

 little positive knowledge has been acquired, and what has been 

 won is far from being complete. It is therefore impossible at 

 present to apply with any considerable degree of certainty the 

 Mendelian law, however vital this may be, to human heredity 

 except in respect to a few relatively simple characters. 



247. The Mendelian principle. According to the Men- 

 delian belief there is no blending of the unit characters to form 

 a new variety in the first group of descendants of crossed parent 

 plants, if both are of pure strain. All the offspring exhibit the 

 dominant character, which is usually greatly accentuated. The 

 recessive character does not appear in this generation, but will 

 be repeated in the offspring of the hybrids. 



In the next generation produced from hybrids there are 

 usually new varieties, from which selection of desirable char- 

 acters may be made for propagation under control. But on 

 the whole the hybrids' offspring fall roughly into two groups 

 which express the ratio of three to one ; three out of four re- 

 peating the dominant character, and the fourth the recessive 

 character of the original parents. 



Practical value of the Mendelian principle. The important 

 fact is that hybrids are especially valuable in revealing new 

 variations which, when selected and properly fertilized, may 

 be productive of individual plants embodying desirable char- 

 acters. Usually, however, it is only one in thousands that 

 combines the desired characters. This one must be carefully 

 preserved and propagated by bud and by seed. Unfortunately 

 it requires long and patient attention to fix or make permanent 

 a new variety so that it will reproduce true to seed. It can be 

 done only by continuous selection and controlled pollination 

 through many generations. 



It is also possible, through the intelligent application of 

 Mendel's law, to eliminate undesirable characters, such as 



