46 PROBLEMS OF GENETICS 



of various parts of the same body may occur. Such divisions 

 affect especially those organs which have an axis of bilateral 

 symmetry, such as the thumb, a cotyledon, a median petal, 

 the frond of a fern or the anal fin of a fish. From the little 

 yet known it is clear that the genetic analysis of these conditions 

 must be very difficult, but evidence of any kind regarding them 

 will be valuable. We want especially to know whether these 

 divisions are due to the addition of some factor or power which 

 enables the part to divide, or whether the division results from 

 the absence of something which in the normal body prevents 

 the part from dividing. Breeding experiments, so far as they 

 go, suggest that the less divided state is usually dominant to 

 the more divided. 9 The two-celled Tomato fruit is dominant to 

 the many-celled type. The Manx Cat's tail, with its suppression 

 of caudal segmentation is a partial dominant over the normal 

 tail. The tail of the Fowl in what is called the "Rumpless" 

 condition is at least superficially comparable with that of the 

 Manx Cat, and though the evidence is not wholly consistent, 

 Davenport obtained facts indicating that this suppressed con- 

 dition of the caudal vertebrae is an imperfect dominant. 10 



Some evidence may also be derived from other examples of 

 differences which at first sight appear to be substantive though 

 they are more probably meristic in ultimate nature. The 

 distinction between the normal and the "Angora" hair of the 

 Rabbit is a case in point. We can scarcely doubt that one of 

 the essential differences between these two types is that in the 

 Angora coat the hair-follicles are more finely divided than they 

 are in the normal coat, and we know that the normal, or less- 

 divided condition, is dominant to the Angora, or more finely 

 divided. 



In the case of the solid-hoofed or "mule-footed" swine, the 



9 Polydactylism which is often a dominant, and the web-foot of Pigeons which 

 is recessive, should be remembered as possible exceptions (see p. 49). 



10 Davenport inclined at first to regard rumplessness as a recessive, but in his 

 latest oublication on the subject he definitely concludes that it is an imperfect 

 dominant. This conclusion accords well with evidence quoted by Darwin (An. 

 and Pits., II, ed. 2, p. 4) that rumpless fowls may throw tailed offspring. (Amer. 

 Nat., 1910, XLIV, p. 134.) 



