302 bulletin: museum of comparative zoology. 



character, D, or tlie normal recessive character, i?, and that equal num- 

 bers of D's and i?'s will be produced. Offspring of the crossbreds will 

 therefore show these characters in the following ratios : — \ D -.2 DR : 1 R. 

 But the character D being dominant, not only the 1 Z)'« but the 2 DR'^ 

 will be polydactylous and therefore oidy one-fourth of the chicks will 

 have normal toes. Bateson's experiments show that this is really the 

 case. 



To us the significance of Mendel's law lies in the fact that a certain 

 character may be transmitted pure from generation to generation of 

 germ-cells in a latent condition ; that is, the character may not appear 

 in the structure of the animal, tiiough present in its germ-cells. 



The occurrence in a latent condition of characters which when active 

 are dominant may thus explain the constant outcrojjping of these 

 characters, such, for example, as the continual a})pearance of "rogues," 

 in apparently pure races of plants and in animals which have been 

 selectively bred for generations. The appearance of reversionary poly- 

 dactylism may be explained in this way. 



Although we know that in the horse, ruminants, swine, and the pes 

 of carnivores the extra digits may be of vestigial origin, yet Gegenbaur 

 has objected that there is no other evidence of reversion, either in the 

 polydactyle extremity or in the general appearance of polydactyle animals. 



We have shown that in polydactyle swine the abnormality is con- 

 fined to the manus, and that in most, if not all, cases the extra digits 

 represent the development of the normally vestigial pollex. In a third 

 of the cases a well-formed digit of two or three phalanges is found, and 

 when these conditions are compared Avith those of the manus of the 

 earliest fossil swine, it appears that the two are similar ; for a pollex is 

 found in the manus of the fossil pig, while in the pes the hallux is 

 entirely wanting. In addition to the development of the pollex, other 

 modifications were found in the structure of the polydactyle manus, 

 which seemed to reproduce a primitive, ancestral condition. We have 

 also seen that in most cases of polydactylism in the ox and horse the 

 extra digits represent the development of digital parts normally rudi- 

 mentary, — a development winch might bo regarded as duo to rever- 

 sion, for other parts of the polydactyle member show correlated variations, 

 and related fossil ancestors also have the same digits normally developed 

 and functional. Moreover, according to recent discoveries in hereditj'^, 

 single segregated characters may be inherited, without general modilica- 

 tion of the germ-plasm. This has been proved by Bateson and Saunders 

 (:02), Castle (;03, :03") and others in agreement with Mendel's law. 



