264 GENETICS: W. E. CASTLE ^ 
We come now to the alternative question whether evidence is forth- 
coming of a few genetic factors or of single genetic factors affecting 
size, showing themselves in multimodal variation or in aberrant indi- 
viduals. There is in Tables 1-3 no evidence of multimodal variation. 
As regards aberrant individuals we have two and only two notable cases, 
which are found in F2 of the Cutleri X race B cross, as already stated. 
These are (1) a female with very short skull and femur closely resembling 
pure Cutleri females in these measurements, but not in skull-width, 
and (2) a male with very long skull and femur, closely resembling race 
B animals in these respects but not in skull-width. It seems quite 
possible that some special factor affecting duration of growth is con- 
cerned in the production of these extreme individuals. Thus castration 
of cattle is known to permit abnomally prolonged growth of the skeleton 
in length. Early sexual maturity is probably correlated with an opposite 
change. Whether physiological factors affecting the sexual system are 
concerned in the production of these unusual individuals, or whether 
genetic agencies are concerned is at present uncertain. It is regrettable 
that the exceptional character of these two individuals was not recognized 
while they were still alive and so capable of genetic tests. Possibly further 
investigations now in progress may throw light on these questions. 
If the foregoing reasoning is sound, we have from these crosses no 
evidence showing either the existence of numerous multiple Mendelian 
factors affecting size, or the existence of a few Mendelian factors affecting 
size, or the existence of a single Mendelian factor affecting size. We 
have however clear evidence of a physiological factor causing a marked 
increase of size in Fi and probably persisting to a small extent in F2 
and so increasing slightly the variability of F2. This factor, though its 
existence has long been known, has been largely neglected in those recent 
studies of size inheritance which seek to give a Mendelian interpretation 
of the phenomena observed in size crosses, and it is quite possible that 
this neglect has caused erroneous conclusions to be reached. 
Castle, W. E., and H. E. Walter, R. C. Mullenix, and S. Cobb, Studies of inheritance in 
rabbits, Carnegie Inst, of Wash., Puhl. No. 114 (1909). 
Lang, A., Die Erblichkeitsverhaltnisse der Ohrenlange der Kaninchen nach Castle and das 
Problem der intermediaren Vererbung and Bildung konstanter Bastardrassen. Zs. indukt. 
Ahst. Vererhungs-lehre, 4, 1-23. (1910.) 
Nilsson-Ehle, H., Kreuzungsuntersuchungen an Hafer und Weizen, I. Lunds Univer- 
sitats Arsskrift,^. F.,7 (1909). 
Phillips, J. C, Inheritance of size in ducks. /. Exp.ZooL, 12, 193 (1911); A further 
study of size inheritance in ducks with observations on the sex ratio of hybrid birds. Ibid., 
16, 131-148 (1914). 
Punnett, R. C, and P. G. Bailey, On inheritance of weight in poultry. /. Genetics, 4, 
23-39, (1914) PI. IV. 
