2 \ 



Shorthorn cattle were made from the Anglo-Saxon reds 

 —Class 1 of the above table No. VIII; the Flecking— 

 Class 6; the rJomano-hiritish— Class 3, and probably some 

 other primitive types. Evidently none of the breeds of 

 domestic cattle lias yet readied stage 7. /. <>.. solid domi- 

 nant white not capable of reversion. The Shorthorns of 

 today present all the possible combinations of Classes 1, 

 2, 3 and 6. 



In reference to the fact that the race of duplex yellow 

 mice has never been produced and in view of what ( 'astle 2 " 

 says, viz., that the union between germ cells carrying 

 only yellow pigment is doubtless affected, still all such 

 germ cells from some cause are doomed to destruction, 

 may it not be that in so delicately adjusted a mechanism 

 two of these specific determiners present a lethal dose? 

 May this not be one of the causes of the limits of hybridi- 

 zation and of the sterility of hybrids? The germ cells are 

 doubtless distinguished by both a specific architectural 

 and a specific chemical organization of the greatest nicety 

 of adjustment and balance. The closest approach in the 

 chemical world to their behavior is that of the enzymes, 

 which, though not entering into reactions, may bring them 



plasm' gives rise to cells of its own kind, supplying them 

 with bodies behaving in an enzyme-like manner sufficient 

 for their own continuity and for a long series of onto 

 genetic processes. 



It is obvious that a disturbance of some consequence 

 would follow the advent of a foreign body or of unusual 



""Modified Mendelian Ratio amors Yellow Mice," Science, December 

 16, 1910, Vol. XXXII, p. 868. 



