RUMPLESSNESS. 41 



ence of the undiluted interrupter. Actually in matings of the latter sort 

 (table 27) 3 families show no trace of the tail-interrupter, but in 7 there is 

 evidence of a disturbance, as shown by the small size of the uropygium and 

 the bent back. In these famihes there are 13 cases of small uropygium to 

 53 of large, being about 20 per cent of the affected uropygium where 25 

 per cent was to be looked for not a wide departure, considering the Ua- 

 bility of not recognizing the reduced uropygium as such. This failure even 

 of the extracted dominants completely to stop the development of the tail 

 gives a measure of the weakness of the inhibitor in this case. Also, in 

 table 28, matings are varied. Some are probably matings of two heter- 

 ozygotes, others of two recessives, and others still of a recessive with a 

 heterozygote. On our hypothesis we should expect some of the families of 

 the mated hybrids to show evidence of the inhibiting factor and others 

 to show no such evidence. In those families in which small tail appears 

 it is found in about 19 per cent of the cases. On account of this weakness 

 of the inliibitor in the germ-plasm of No. 117 that inhibitor is rarely fully 

 activated. Only in one case out of the 250 or more in which that germ- 

 plasm is used is the development of the tail completely stopped. In this 

 case a hybrid cock derived from pen 626 (series 2, table 26) was crossed with 

 various birds of tailed races (probable RR's), and produced in addition 

 to 20 tailed offspring 1 devoid of uropygium and oil-gland. In this case 

 we may conceive that an unusually potent condition of the inhibitor wholly 

 stopped the development of the tail. 



The behavior of No. 116 is that of a pure dominant. Mated to DR 

 (and some RR?) females he produces pure dominants and heterozygotes. 

 His inhibiting factor is potent enough to be active in the DD offspring at 

 least; as a matter of fact 47 per cent of his get have their tails inhibited. 

 Even in the DR's the inhibitor may sometimes work itself out. Thus No. 

 116 crossed on No. 508, without tailless ancestry, had 56 per cent of the 

 progeny without tail. Since tailless birds may be either pure dominants or 

 DR's, we may expect famihes of two sorts when two such are bred together 

 those containing only tailless offspring and those containing only 75 per 

 cent or less of such. Both sorts of famihes are to be expected in a table 

 with the composition of table 30, and both appear there. 



The case of the rumpless fowl that arose de novo will be explained, 

 then, as follows: Even in normal RR matings the inhibiting factor may 

 arise by mutation. But even when two of these inhibiting factors are 

 paired they show themselves so weak as not to appear in 25 per cent, much 

 less the typical 75 per cent of cases, but, as in our case, merely 4 per cent. 

 The strain takes on, indeed, the essential features of the " eversporting varie- 

 ties" of De Vries (1905). It seems probable, therefore, that even in ever- 

 sporting varieties inheritance may be Mendehan, modified by variations in 

 "potency" as shown by irregularities in dominance. 



