GENETICS: CASTLE AND HADLEY 
39 
THE ENGLISH RABBIT AND THE QUESTION OF MENDELIAN 
UNIT-CHARACTER CONSTANCY 
By W. E. Castle and Philip B. Hadley 
LABORATORY OF GENETICS OF THE BUSSEY INSTITUTION. HARVARD UNIVERSITY. 
AND OF THE AGRICULTURAL EXPERIMENT STATION OF 
THE RHODE ISLAND STATE COLLEGE 
Presented to the Academy, November 20, 1914 
Whatever the theoretical importance of Mendel's law, its practical 
utility depends largely upon the purity of the gametes. If Mendelian 
unit-characters can through hybridization be recombined in desirable 
ways without essential modification during the process, Mendel's law is 
evidently a distinct acquisition to the practical breeder. Nevertheless 
if crossing is likely to produce considerable changes in the characters 
which it is desired to combine in a new race, it is evident that Mendelian 
crosses must be used judiciously and with caution by the practical 
breeder. 
Considerations such as these have led W. E. Castle for several years 
to concentrate his studies of genetic problems upon the question of 
gametic purity. As a crucial experiment he conceived the plan of deriv- 
ing an entire race of animals, not from a single pair of ancestors, but 
from a single gamete, so far as concerns a particular unit-character. It 
was thought that in a race so derived, if the principle of gametic purity 
holds, there should be no variation whatever in the particular unit- 
character concerned. 
Color patterns of mammals seemed especially well adapted for such 
studies, since they are early differentiated and clearly Mendelize in 
crosses. The so-called 'English' piebald rabbit presents an especially 
fine example of such a color pattern. The figures give a good idea of 
this striking pattern in which white and colored areas are interspersed 
much as in the 'coach-dog.' It would be a distinct gain to breeders 
if they could reduce the variation in details of the English pattern so 
that 'prizewinners' could be bred without the production of so many 
'wasters,' which depart in essential points from the standard pattern 
adopted for the breed. This was an additional reason for undertaking 
work with the English rabbit. 
The first standard-bred English rabbits which Castle had under obser- 
vation, when mated inter se, produced young of three sorts. About 
half the young were fairly good ' standard' English extensively marked 
with colored spots (see fig. 3). About one-fourth were much whiter 
than the standard demands, their spots being fewer and smaller (see 
fig. 1). And the remaining fourth were without spots, that is were 
