HEEEDITY 



duces no visible structures. The occasional 

 occurrence of an imperfectly horned animal as 

 a sport within a hornless race need not, then, 

 occasion surprise. It would be a variation of 

 the same sort as the extra toe in guinea-pigs 

 (see Fig. 38), which, from a single sport, was 

 built up by selection into a well-established race 

 within a very few generations. This character, 

 seemingly lost from the germ-plasm for an in- 

 definite period, had perhaps merely fallen so low 

 in potency that it no longer produced the fourth 

 toe on the hind foot, though this was still pres- 

 ent on the front foot. In the variant observed, 

 the first polydactylus guinea-pig of my stock, 

 the toe was imperfectly developed on one hind 

 foot, doubtless as the result of an unusually 

 potent condition of the character in one of the 

 gametes which produced the individual. This 

 manifestation of the character, though feeble, 

 was sufficient to afford a guide for selection of 

 those individuals which formed the most potent 

 gametes, and so a polydactylous race was 

 formed by selection and inbreeding. 



Great as has been the contribution of Men- 

 delian principles to our knowledge of heredity, 



104 



