522 TRANSMISSION 



the law of chance. This means that each parent produces succes- 

 sively germ cells of both characters (B and R}, so that hybrid 

 forms produce pollen, spermatozoa, ova, etc., of both original 

 kinds, which thereafter combine by the law of chance ; that is, 

 B of one parent unites with either B or R of the other, produc- 

 ing either pure ^'s or BR's ; and also, in a large number of 

 instances, some 7?' s unite withe's, producing hybrids, and others 

 with ^'s, producing pure R's from hybrid parents. This is the 

 theory of gametic * purity, an assumption necessarily involved as 

 a fundamental conception in Mendel's law, which, in the opinion 

 of Mendel himself, applies only to characters that do not blend. 



Proving or disproving Mendel's law. A good many investi- 

 gators are trying, often with numbers inadequately small, to 

 prove or disprove Mendel's law as a general principle of hered- 

 ity. It may not be out of place to call the attention of the 

 student to the uselessness of this attempt, and to direct his 

 attention to the problems connected with this question which 

 really call for further and much-extended study. 



First of all, Mendel's law as a general proposition needs no 

 further proof. The experiments on which it was founded have 

 been carefully repeated by De Vries, by Correns, and by Tscher- 

 mak, who agree as to the correctness of his results. They have 

 been repeated upon many other species, with uniform results in 

 most cases, and no new evidence is worth noting that is not 

 founded on new species and that does not involve relatively 

 large numbers. 



Besides all this, the fundamental conception of Mendel's law 

 rests, like Galton's law of ancestral heredity, upon the inevitable 

 mathematical relations in reproduction as outlined in the previous 

 section. This " law "arises, therefore, as a special case out of 



1 The term " gamete " is coming to be used, as synonymous with "germ cell," 

 to mean the [unfertilized germ, without regard to sex. When it is fertilized it is 

 spoken of as a zygote. 



Thus, in the language of these terms, we should say that each hybrid parent 

 produces two kinds of gametes, B and R, or dominant and recessive, or whatever 

 characters have been combined. Now some gametes of the B kind will unite with 

 gametes /?, producing zygotes BB (pure) ; others will unite with R gametes, pro- 

 ducing zygotes BR (crossed) ; and still other ^gametes will unite with R gametes, 

 producing zygotes RR (also pure). Of mathematical necessity, for an entire popu- 

 lation these proportions will be as i BB : 2 BR : I RR. 





