440 THE AMERICAN NATURALIST [Vol. LII 



(2) the surface pigment of the terminal portion of the 

 hairs is nearly or quite lacking; (3) in the basal zone, the 

 black pigment bodies are represented by small flocculent 

 dark masses. Thus, we are not, as in the next " mutant" 

 to be described, merely dealing with changed proportions 

 of perfectly normal types of hair. These red-eyed mice 

 possess types which I have not found in any of the rest 

 of my stock. 



At the time of my earlier description of these pale 

 sports, no young had been obtained, but their pedigree 

 suggested that they were simple Mendelian recessives. 

 This conjecture has thus far been sustained. The two 

 " mutants," bred to one another, have given six pale 

 young, like themselves, and no others. When bred to 

 dark mates, of the same stock as themselves (smoriensis- 

 rubidits hybrids), the pallid animals gave only dark 

 young, except in a single instance where the dark parent 

 was known to be heterozygous. In this case, one pallid 

 mouse was the outcome. Of the dark progeny, three 

 broods, aggregating eleven individuals, have thus far 

 been born. 



This clear-cut and typical example of Mendelian seg- 

 regation, in respect to these mutant characters, is in 

 striking contrast to the complete lack of segregation — 

 so far as is obvious — in respect to the subspecific charac- 

 ters which have entered into the germinal constitution of 

 these same individuals. 



As I have previously stated, these parent "mutants" 

 were the offspring of Fj sonoriensis-rubidus hybrids. In 

 a recent article (1917) the Hagedoorns have described a 

 number of strongly aberrant types of rats (including 

 some waltzers !) which appeared in a mongrel strain re- 

 sulting from the crossing of Mas alexandrines, M. tecto- 

 rum and M. rattus. The authors recognize in these aber- 

 rant derivations some entirely new products, though they 

 do not attribute their origin to real mutation. In the 

 opinion of the Hagedoorns, as I understand it, these ap- 

 parently "mutant" characters have resulted, in each 

 case, from the chance coming together of two recessive 

 factors (or two "absences," according to the prevailing 



