AN ANALYSIS OF THE EFFECT OF SELECTION. 41 
the constancy or inconstancy of these objective differences that I am dis- 
cussing. If these are quantitatively changeable from generation to genera- 
tion, then change in the variability of the zygote composing a generation 
might arise without factorial recombinations.”! (Castle, 1914a.) 
“The head, the hand, the stomach, stomach-digestion, these are not unit- 
characters so far as any one knows. But if a race without hands were to 
arise and this should Mendelize in crosses with normal races, then we should 
speak of a unit-character or unit-factor for ‘hands,’ loss of which or variation 
in which had produced the abnormal race. But in so doing we should refer 
not to the hand as an anatomical part of the body nor to the thousand and 
one factors concerned in its production, but merely to one hypothetical factor 
to which we assign the failure of the hand to develop in a particular case. 
It is immaterial whether we call this a wnit-character or unit-factor or use both 
terms interchangeably . . . . .”’ (Castle, 19160, p. 100.) 
1. PotypactyL GuInea-Pics. 
The most extensive data on this case are apparently in the paper 
(Castle, 1906) cited in the quotation already given. The extra-toe 
character was at first irregular in appearance, but was improved by 
selection. In five generations, without very close inbreeding, a practi- 
cally uniform race was obtained. When crosses to normal were made, 
the F, results varied from nearly all normal to nearly all polydactylous. 
F, contained both normal and extra-toed individuals. It is pointed 
out by Castle in this paper that the results are very similar to those 
obtained by Bateson from polydactylous fowls. Bateson’s comment 
on that case is given below. 
In the absence of any definite data regarding F, counts, the case 
as reported is entirely explicable on the multiple-factor view. Castle 
himself said of it, five years after the publication of the above paper: 
“An alternative explanation is possible, viz., that the development of the 
fourth toe depends upon the inheritance of several independent factors, and 
that the more of these there are present, the better will the structure be 
developed. The correctness of such an interpretation must be tested by 
further investigation.” (Castle, 1911, p. 101, footnote.) 
So far as I have discovered, such further investigations have not 
yet been reported, although five years later this case is listed as No. 1 
among those that demonstrate contamination of allelomorphs. 
2. LONG-HAIRED GUINEA-PIGs. 
The reference given for this case (Castle and Forbes, 1906) seems 
to contain the most recent and complete data regarding it. 
Angora guinea-pigs appeared in a short-haired stock, apparently 
as segregated recessives. On crossing to short and extracting, there 
were produced some animals of intermediate hair-length, and some 
unusual ratios. But similar intermediates appeared in another strain 
of shorts, apparently uncrossed with angoras, thus making it highly 
probable that we are dealing here with a factor already present in the 
1[talics mine. 
