PIEBALD RATS AND SELECTION. 



INTRODUCTION. 



The fundamental importance of Mendel's law of heredity is generally 

 recognized among biologists. It is a working hypothesis whose utility 

 is fully substantiated by abundant results daily increasing in amount. 

 But biologists are not in agreement as to how much this law includes. 

 All perhaps would agree that it implies the existence in the germ- cell of 

 specific determiners essential for the production of particular character- 

 istics in the offspring. Further, no one probably will object to the 

 statement that it implies a dual or duplex condition of the zygote as 

 regards determiners and a simple or simplex condition of the gamete. 

 Thirdly, the fact will be admitted by all that most mendelizing char- 

 acters are wholly independent of each other in heredity, for which 

 reason we are forced to suppose that their determiners are distinct 

 within the germ-cell. 



But beyond these few generalizations great diversity of opinion 

 exists. As regards the very nature and function of the determiners, 

 some consider them unvarying, and explain the observed variation of 

 mendelizing characters in organisms as due to a modifying action of 

 other determiners. At one time even a modifying action of other 

 determiners was denied, and the theory was advanced that the gametes 

 extracted from a mendelian cross are 'pure as regards the single char- 

 acters which may have been concerned in that cross. Investigations 

 carried out by Castle have done something to dispel this idea. In 

 particular it was shown (Castle, 1905, 1906; Castle and Forbes, 1906) 

 that in guinea-pigs, polydactylism, long-hair, and rough coat are men- 

 delizing characters which are affected in the degree of their develop- 

 ment by crosses — that is, when these characters are "extracted" from 

 crosses the characters are not exactly the same as before; hence the 

 gametes are not "pure." 



The experimental result is not denied, but in order to save the sub- 

 stance of the theory its advocates now suppose that the determiners 

 have not changed, but in consequence of the cross certain modifiers 

 have become associated with them which change their appearance in 

 the organism. The real unchanging thing is now called the "geno- 

 type," its appearance the "phenotype. " 



In this genotype theory we are dealing only with a new and more 

 refined aspect of the "theory of pure gametes." It is not a necessary 

 part of mendelism, not even an original part ; but it is very important 

 for us to know whether it is true or not. For if it is true, selection 

 unattended by hybridization is largely a waste of time, as De Vries and 

 Johannsen have maintained, and Jennings and Pearl have reiterated. 



