

HEREDITY 523 



the general principle that the binomial coefficients represent pop- 

 ulations in general. If there is no blend, then the coefficients 

 represent the proportions of distinct character elements. If a 

 blend has occurred, then the coefficients still represent the pro- 

 portions within the blend. There is and can be no uncertainty 

 upon this feature of the case. The dominance of some characters 

 over others is a matter of common observation and may be 

 accepted as a matter of common sense. 



The feature of Mendelism on which further light is needed is 

 the matter of gametic purity, a biological element of the prob- 

 lem whose universal truth is not yet established, but which seems 

 necessary to a rational explanation of the law as to separation 

 into distinct forms ; indeed, Mendelism in its present form seems 

 to mean substantially that hybrid individuals do not produce 

 hybrid germs, but rather that they produce successively the 

 pure germs of both lines of parentage. 



It maybe considered that Mendel's law is well established for 

 certain characters ; that is, for those that do not blend. This may 

 be, however, only another way of admitting its truth for those 

 species which maintain gametic purity those in whose germs 

 the different characters do not mix (blend), or in which, if they 

 do mix, the process is very slow. This in turn is another way 

 of saying that Mendel's law is a demonstrated fact, the only 

 unanswered question being, To what species and characters 

 does it apply ? and the answer to this depends upon the extent 

 of gametic purity. In the opinion of the writer this is the 

 great unanswered question, and here is the object of inquiry 

 toward which all investigation of Mendel's law should be 

 directed, namely, to discover to what species and to what char- 

 acters its applications are limited. 



Upon this point it is worth while to note that crossed forms 

 fall into one of three classes, so far as appearances are concerned : 

 (i) the hybrid may resemble one of its " pure " parents so 

 closely as to be indistinguishable from it ; (2) it may be a kind 

 of intermediate between the two (different) parents ; (3) it may 

 be quite distinct from either parent. 



Of these three classes the first is clearly Mendelian, while the 

 second and third are doubtful, the second exceedingly so. 



