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GENETICS: W. E. CASTLE 
PIEBALD RATS AND THE THEORY OF GENES 
By W. E. Castle. 
BussEY Institution, Forest Hills, Boston, Mass. 
Communicated February 26, 1919 
The study of heredity as an exact science dates from the rediscovery of Men* 
del's law in 1900. After the validity of the law had been estabHshed by abun- 
dant and conclusive evidence, the question arose, are the gametes pure. Is a 
character which disappears in crosses, and then reappears a generation later 
in 25% of the offspring, subject to contamination or modification during the 
process? The idea of gametic purity was at first looked on with favor. Bate- 
son^ although he never gave unqualified adhesion to this view, formulated it 
very clearly, thus. "The pure [homozygous] dominant and the pure reces- 
cesive members of each generation are not merely like, but identical with the 
pure parents, and their descendants obtained by self-fertilization are simi- 
larly pure. If they are pure, surely the male and female elements of which 
they were composed must also be pure.'" 
My own experimental studies of heredity, begun in 1902, early led me to 
observe characters which were unmistakably changed by crosses and so I have 
for many years advocated the view that the gametes are not pure in the sense 
expressed by Bateson'. Moreover it was observed that characters which men- 
delize in crosses may, even when uncrossed, show fluctuating or graded varia- 
ation in consequence of which systematic selection is able to produce very 
diverse races as regards a single mendelizing character, the ordinary allelo- 
morph of which is wholly excluded from the experiment. This observation 
shows that characters may vary otherwise than by contamination and I was 
in consequence led to adopt the hypothesis that unit-characters are ''incon- 
stant" in varying degrees, but probably never perfectly constant. 
This view has been repeatedly challenged, either by those who questioned 
the evidence cited in support of it, or by those who first substituted a different 
concept, 'gene,' for that of 'unit-character' and then denied that a 'gene' can 
vary. Dissent to the evidence for character variability has gradually dis- 
appeared as others have independently undertaken to study the visible char- 
acters of organisms as affected by crossing or systematic selection. The find- 
ings are commonly such as I have described in the case of the hooded pattern j 
of piebald rats, which I have been studying for several years. This pattern 
is a simple recessive in crosses with the self pattern of wild rats, but it usually 
emerges from such crosses in a modified form, the amount of white in the pat- 
tern being either increased or diminished according to what stock is selected 
for experimental study. Even when uncrossed and bred as pure as possible, 
I have always found a certain amount of genetic variability to persist in a 
hooded race, so that selection, plus or minus is effective in changing it. The 
