172 Second Report on Cros.t-bretI Mice 



2. The Second Generation. 



It will be sccii by iiispection of the tables rcconiing tho offspring of hybriils, 

 that when a hybrid is paired with a hybrid the üffspring form a miich luore 

 heterogeneous collection than does the offspring froni a hybrid and a white in 

 which the yotnig are more sharply segregated into white and wild-coloured. And 

 this difference is coincident with, if not causally connected with a differcnce in 

 the ancestry of the two sets of young. For the heterogeneous offspriug of two 

 hybrids have a more complex ancestry than do the less variable offspring of 

 a hybrid and an albino ; and it may bo a fact of similar meaning that the hybrids 

 produced by crossing a waltziiig mouse with a cross-bred albiuo are more hetero- 

 geneous than those produeed by a similar cross in which the albiuo, however, was 

 pure-bred (see Table II.) — in both cases the more heterogeueous collection of 

 offspring comes froui pareuts of which the ancestry is more complex ; that is 

 to say the young of the cross-bred albinos are more heterogeneous than those 

 of the pure-bred ; and the young produeed by pairing two hybrids are more 

 heterogeueous than those produeed by crossing h3'brids with albinos. 



No theory of Compound allelomorphs such as that put forward by Mendel will 

 account for this striking differcuce between the variability of the two groups 

 of offspring. 



Romenibering that Gc or 6d indicates a mouse indistiiiguishable from the 

 common Mus musculus, the curious t'act will be uoticed that only one such 

 appeared among 154 hybrids of the first generation, whereas out of the 88 offspring 

 of a cross between a hybrid and an albiuo there were 10. This rosult may be 

 parallel with that obtained by Darwin* when he crossed a white fantail with 

 a black barb and also a barb with a spot and then crossed the mongrel bard fantail 

 with the mongrel barb spot and got "a bird of as beautiful a blue colour, with the 

 white loins, double black wing-bar, and barrcd and white-edgcd tail-fcathers, 

 as any wild rock-pigeon ! " ; i.e. reversiou, in both cases, did not occur until the 

 third generation. 



The facts so far observed which are in possible accordance with some form 

 of Mendelian hypothesis are (1) the apparently regulär appearance of albiuos 

 when hybrids are crossed with albinos, although the evidence at present available 

 does not suffice to shew whether these occur in Mendelian proportions (50 % 

 albino and 50 7o hybrid) or not, (2) the wcll-kuown fact that albinos of any 

 ancestry when paired together produce albino young, exceptions to this rule being 

 at least very rare, and (3) the appearance of waltzing and albino mice in the 

 second hybrid generation. 



The first of these results although not inconsistent with the truth of Mendels 

 hypothesis canuot be taken as proof that this h^-pothcsis applies; for a similar 

 result is observed in such cases as that of human eye-colour where Mendel's Laws 

 have been shewn not to hold -f. 



* The Oriijin of Species, pp. 17, 18. + Karl Pcarson : see below, pp. 213 et leq. 



