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 vnthout 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 unit-character or unit-factor or use both 

 terms interchangeably " (Castle, 19166, p. 100.) 



1. POLYDACTYL GuiNEA-PlGS. 



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 Fi results varied from nearly all normal to nearly all polydactylous. 

 F2 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 F2 counts, the case 

 as reported is entkely 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 



'Italics mine. 



