ORIGIN OF A POLYDACTYLOUS RACE OF GUINEA-PIGS. 23 



The attempt has twice been made to increase by selection the asymmetry 

 of the two sides of the body with reference to the extra-toe, but without 

 success. The slightly superior development of the extra-toe on the left 

 side of the body apparently remains unaffected. The cause of the asymmetry 

 is unknown. The case doubtless belongs in the same category as the 

 unequally developed right and left sex-glands of certain birds and mammals. 



9. RESULT OF MATING NORMAL WITH POLYDACTYLOUS INDIVIDUALS. 



Matings between normal females and polydactylous males have repeat- 

 edly been made, as will appear from tables 2 to 14. Crosses reciprocal to 

 these have yielded the results indicated in table 17. The results, it will be 

 seen, are not uniform. The offspring have, in some cases, a greatly weak- 

 ened condition of the extra-toe, in other cases no extra-toe at all ; in still other 

 cases, the extra-toe may be present in a fairly well-developed condition. 

 The inheritance is evidently neither sharply alternative (/. e., Mendelian) 

 nor completely blending. The result of a cross involving the extra-toe 

 character is influenced by the individual potency of the respective parents. 

 Fewer polydactylous young are produced if the normal parent comes of a 

 stock in which the polydactylous character does not occur. 



10. INHERITANCE NEITHER ALTERNATIVE NOR BLENDING. 



It is very evident that in the inheritance of the extra-toe we are not 

 dealing with a case of simple Mendelian dominance. An occasional case, 

 like the matings of ^628 with normal females (see table 4), would indicate 

 that the extra-toe is a recessive character, but most polydactylous parents, 

 in matings with normal individuals, give a mixture of normal with poly- 

 dactylous offspring. Further, in these mixed lots of offspring, the polydac- 

 tylous and normal individuals are rarely equal to each other in number, as we 

 should expect if one parent were a Mendelian heterozygote, the other pure. 



On the other hand, when two of the offspring produced by a cross 

 between polydactylous and normal parents are mated together, we do get 

 some evidence of Mendelian segregation. The offspring are highly variable 

 as regards the character, extra-toe. Some are normal, some have poorly 

 developed toes, and some have very well developed toes. The experiments 

 are still incomplete as regards this matter, but so far do not indicate the for- 

 mation of sharply separated Mendelian classes. 



On the whole, it seems probable that the extra-toe is inherited in a 

 manner intermediate between blending and alternative inheritance. The 

 gametes only partially blend in the zygote, producing a variable result, most 



