HEREDITY AND VARIATION 223 



cases are not as simple as Mendel found them in garden peas. 

 Some factors arc "linked" together and do not display the inde- 

 pendence of segregation which we have noted. Others depend 

 for their expression not upon one but upon a whole scries of 

 independent factors. Others are influenced in appearance and 

 inheritance by sex. Size characters in general (those of quantity 

 as opposed to quality) rarely show simple Mendelian segregation 

 at all but blend more or less completely and require for their 

 investigation the use of measurements and statistical methods. 



Fig. 128. — "Bud" sport or mutation, arising in a portion of the plant and not 

 from seed. At extreme left a loaf of the original Boston fern, and, at right, leaves 

 of three mutants which have arisen from it. (Courtesy Brooklyn Botanic Garden). 



All of these cases, however, can be understood or explained by 

 amplifying and interpreting Mendel's original law without at 

 all destroying its fundamental principles, and it remains toda}^ 

 as one of the most profound generalizations of biological science. 

 Mutation. — We have already spoken of the variations produced 

 directly by differences in the environment to which the plant 

 is subjected, variations which apparently are never inherited. 

 It is now clear from our consideration of Mendelism that another 

 and doubtless a very common cause of variation is the recom- 

 bination of characters which follows the crossing of two different 

 types or races; and these variations, being due to differences in 

 the inherent genetic factors themselves, are clearly inheritable 



