HERITABLE DIFFERENCES 51 



though several may be "combinations" or must be 

 placed even in the non-inheritable class of "freaks." 



The chief reason why definite examples of mutation 

 are so infrequently noted and recorded is because 

 the attention of the investigator has generally been 

 directed, not to them, but to gradual fluctuating varia- 

 tions which, according to Darwin's conception, furnish 

 the material for the operation of natural selection. 

 Mutations are doubtless much more common than has 

 been generally supposed, and it is likely that they will 

 receive more attention in the future than they have in 

 the past. 



No stock when bred on a large scale breeds abso- 

 lutely true for all specific characters. Gerould re- 

 ports that in his butterflies (Colias), he found blue- 

 green instead of yellow-green eyes, uncoiled instead of 

 coiled tongue, the absence of orthodox wing spots, 

 one proleg less in the caterpillar, etc. Drosophila 

 is a famous example of many deviations from type 

 which have been revealed upon persistent and careful 

 scrutiny. 



6. KINDS OF MUTATION 



Multiple or aggregate mutations are those germinal 

 upsets that affect many parts of an organism instead 

 of a single part. This type is of frequent occurrence 

 and is in contrast to a single gene mutation which in- 

 volves only an hereditary unit that determines a single 

 somatic feature. For example, Babcock describes a 

 new walnut, Juglans quercina, which appeared inde- 



