ZOOLOGY — CASTLE AND MARK. 22$ 



ZOOLOGY. 



Castle, W. E., and Mark, E. L., Harvard University, Cambridge, Massa- 

 chusetts. Grant No. 418. Continuation of experimental studies in 

 heredity. (For previous reports see Year Book No. 3, p. 136; Year 

 Book No. 4, p. 276, and Year Book No. 5, pp. 243, 244.) $500. 



Dr. Castle reports that since the last report was made by him an adequate 

 explanation has been obtained of the curious phenomenon of atavism or re- 

 version, so far as concerns the pigmented coat of guinea-pigs, rabbits, and 

 other rodents. The phenomenon is this : Upon crossing two simple color 

 varieties, such as black and yellow, young are obtained which are identical 

 in appearance with closely related wild rodents, and doubtless also with 

 the actual ancestral species from which the domesticated varieties have 

 descended. The resemblance to a wild ancestor has been supposed to ex- 

 tend, in such cases, to all characters of the species, including temperament, 

 but this is certainly not true in the cases which have come under his observa- 

 tion. If reversion in mental traits occurs as a result of crossing (a thing 

 in itself entirely possible), it is not because of a necessary association of 

 mental traits with any particular type of pigmented coat. 



Reversion in mental constitution might occur (we have no certain evidence 

 that it ever has occurred) along with reversion in coat-pigmentation; but, 

 if so, it would be due to a simultaneous inheritance of factors independent of 

 those which produce the pigmented coat. The explanation obtained for re- 

 version is this : The wild or ancestral type of coat is a composite character, 

 resulting from the simultaneous presence of at least three independent factors, 

 one or another of which was wanting in each parental variety, but all of 

 which were brought together by a combination of the parental varieties. This 

 explanation constitutes an extension merely of the basic Mendelian principle 

 of inheritance, the principle of unit-characters independent of each other in 

 their inheritance. 



In two brief communications explanation has been made of the factors 

 which are involved in the reversion in coat-pigmentation in guinea-pigs and 

 in rabbits, respectively. A fuller account is deferred to a later publication. 



Whether unit-characters are mutable or immutable in nature remains as yet 

 the central problem for experimental study in the field of heredity. Towards 

 its solution the experiments in progress are mainly directed. They include 

 (i) selection experiments for the modification of color-patterns among 

 spotted rodents, or for the fixation of patterns arbitrarily chosen; (2) ex- 

 periments in repeated crossing of contrasted types of pigmentation, to see 

 whether modification of those types can thus be brought about; (3) experi- 

 ments to ascertain what are the physiological diflferences between those types. 



Experiments in hybridizing Cazna aperea (the Brazilian cavy) with the 

 guinea-pig have been attended with considerable success. While in previous 



IS— YB 



