Species crosses in Rats. IQl 



far has produced oulj^ three varieties, which are sometimo.s bred as 

 domestic species, sometimes produced apparently spontaneously from 

 hybrid stock. These are the .lapanned, the white and the spotted. No 

 trace of the enormous vai'iability in feathering, shape, size, disposition 

 of the domestic fowl is seen here. In the guineafowl again, variability 

 is almost lacking. There is a white, and a spotted guineafowl and one 

 without specks, but this list exhausts the range of variability. 



If we contiast the domestic duck and the fowl to tjjese two species, 

 we see a stupendous variability, hosts of novel characters, new not only 

 in the sense of not being seen in the pai-entspecies, but in the sense 

 of not existing in the genus. In the duck we see extremes in size 

 which go far beyond the extremes in size in the genus Anas, such as 

 that of the miniature calling duck and the Pekin. We see a novel 

 coloui- of the eggs, black, we find topknots, albinism, partial albinism 

 and black colour, and we meet with a shape and stature not seen in 

 any wild duck. In the duck we know that specieshybrids are quite 

 fertile and have entered largely into the stock of domestic ducks. Turning 

 to the fowl, we meet with a variability in almost every point. Novel 

 shapes of the comb, all kinds of variations in feathers, such as recur- 

 vation, absence of barbules, fastigiation, naked (luills, hoiny tips, com- 

 plete absence of feathers, we see polydactylism, absence of tail, of wings, 

 of comb. In size w^e have such extremes as the Fighting bantam and 

 the Laugshan. In colours, the animals range from black with even the 

 comb and wattles pigmented, through birds with yellow and blue combs 

 down to albinos and partial albinos. Here again we know that hybrids 

 with at least two wild species, G. furcatus and Temminckii are fertile. In 

 the domestic rodents, which have been used most extensively for genetical 

 investigations, the evidence is not quite as complete up to date. All 

 the colours of the domestic rabbit, of the cavy, of the laboratory rat 

 and the domestic mouse are commonly thought to have originated by 

 loss mutations. Now, in the first place we know, that the variability 

 in these animals is much larger and more diverse than in the peacock 

 and the guineafowl, where lossmutations probably furnished all the 

 variability. But in the second place, a bettei- examination of the animals 

 furnishes some clue as to the origin of these variations. A lossmutatiou 

 which lesults in, let us say albinism, does not generally change the 

 animal in anything but in this particulai- point. In fact, the white 

 peacock or the spotted guineafowl aie in no sense but colour different 

 from the wildcoloured bii'ds. They are in no wise bettei' domestic afli- 



