depend for their development on stimuli always present in 

 the normal environment. Others, depending on a new or 

 occasional stimulus, do not reapppear in the next genera- 

 tion unless the stimulus is present" (Goodrich). 



Range of Application of Mendel's Laws. — The unravel- 

 ling of many intricate cases of crossing during the last few 

 years has brought some supposed instances of Blended In- 

 heritance into the Mendelian list (see pages 74-95). En- 

 thusiastic Mendelists assert that the law holds true for all 

 sorts of characters in both plants and animals, but this re- 

 mains to be seen. It must be remembered, however, that 

 many complications often arise to blur the segregation, such 

 as imperfect dominance, multiplicity of factors, direct effect of 

 'environment, etc. 



Some plant breeders believe that it will be possible soon 

 to predict varieties with some degree of accuracy. Gilbert 

 gives his reasons for doubting this belief as follows : 



" (1) We do not know what plants will Mendelize until 

 we try. 



(2) Even in plants that do not Mendelize, one half of 

 the offspring have stable characters. 



(3) Mendel's laws deal primarily with mere characters 

 not with a variety or with a plant as a whole. 



(4). The breeder usually wants wholly new characters 

 as well as recombinations of old ones, or he wants 

 augumented characters, and these lie outside the 

 true Mendelian categories. 



(5) New and unprofitable characters are likely to arise 

 from the influence of environment or other causes. 



(6) Variability itself may be a unit character and there- 

 fore pass over. 



(7) Many plants with which we must work will not 

 close-fertilize; some are monoecious or dioecious. 



(8) One can never predict just what combination of 

 characters any plant of a cultural variety will have 

 even though it be strictly Mendelian." 



The new biological principles, especially Mendel's laws, 

 are of value to the commercial plant breeders. They are 

 time-savers, for the breeders know that it is in the second 

 hybrid generation that segregation takes place, not in the 

 first. Moreover, it can be now ascertained what characters 

 are heterozygous and therefore unfixable. 



128 



