OLD TYPES AND NEW 173 



gray, as in No. 1, to a gray from which sixteen pos- 

 sible types of young may be expected as in No. 32. 

 Up to the time when Castle's paper upon the factor 

 hypothesis ^ was published in 1909, nine genotypic 

 kinds of gray rabbits had been obtained in his ex- 

 periments, whose genotypic formulae correspond to 

 the following numbers in the list : 1, 3, 6, 10, 13, 20, 

 22, 28, 29. 



8. Conclusion 



That a relatively small number of factors may pro- 

 duce an extensive array of combinations is evident 

 from this data. 



The analysis of germplasm by the factor hypothesis 

 is now being generally applied by geneticists to the 

 particular organisms with which they are concerned. 

 It has been carried out notably in detail by both 

 Bateson and Davenport for poultry and byBaur for 

 the snapdragon. Antirrhinum. 



Finally, the elucidation of the factor hypothesis 

 makes any further explanation of reversion super- 

 fluous. It is now easy to see how a particular char- 

 acter may remain latent for generations and at last 

 come to expression only when the missing factor 

 necessary to its activity is supplied by some cross. 



It is also clear how hybridization, in w^hich many 

 characters are concerned, is bound to furnish far more 

 new combinations than would, at first thought, be 

 expected. 



^ "Studies of Inheritance in Rabbits." Carnegie Institution Publica- 

 tions, No. 114, 1909. W. E. Castle in collaboration \vith Walter, Mul- 

 lenix and Cobb. 



