i7o POPULAR SCIENCE MONTHLY. 



discussion it is to be understood, unless otherwise stated, that the 

 parent forms are all well-established varieties or species (i. e., they 

 come true to type from seed), and that the hybrids are close fertilized 

 (either self-fertilized or fertilized by others like them). 



Mendel's law is, briefly stated, as follows: In the second and 

 later generations of a hybrid, every possible combination of the parent 

 characters occurs, and each combination appears in a definite pro- 

 portion of the individuals. 



Mendel did not leave his work unfinished. He proposed a theory, 

 or rather deduced one by a simple course of reasoning that renders 

 the theory almost an established fact as far as results thus far secured 

 are concerned, that explains the facts in the case in a brilliant 

 manner.* As I have shown elsewhere, this theory explains most of 

 the so-called exceptions to the law pointed out by several investigators. 

 It explains so many apparent anomalies in such a simple manner 

 that there is danger of applying it too extensively. This point will 

 be brought up again below. 



The theory may be illustrated as follows: Suppose the two parent 

 varieties differ with respect to a single character. For instance, a 

 variety of bearded wheat is crossed with one that is smooth (not 

 bearded). When the hybrid thus produced forms its germ cells 

 (pollen and ovules), the characters of the two parents separate, the 

 beard-producing character passing into part of the pollen grains 

 and part of the ovules, the smooth-head character of the other parent 

 passing into the remaining germ cells. The character passing into 

 any single germ cell being governed by chance, on the average half 

 the pollen and half the ovules will receive the character of one parent, 

 the other half that of the other. The results of this separation of 

 characters may be illustrated by the following diagram, in which B 

 stands for the beard-producing character and 8 for its opposite. The 

 reason for the use of a small b in designating the hybrid will appear 

 later. 



B 



8 



Since the ovules of each kind are offered both kinds of pollen, half 

 of each, on the average, will be fertilized by one and half by the other 

 kind of pollen, giving the four fertilizations shown at the right. 

 (Abundant experience has shown that Sy(B=BX.S.) From this 

 it appears that one fourth of the progeny of the hybrid Sb will be like 

 the parent B, one fourth like 8, and one half like the hybrid itself. 



* It seems that both Correns and De Vries had arrived at the same theory 

 independently. 



