MENDEL'S LAW. 275 



Percentage of each Type in first six Generations of the Dihybrid 



VB X GS. 



Sixth 

 First. Second. Third. Fourth. Fifth. Genera- 



tion. 



Hence, if we sow all the seed of each generation, it is seen that 

 each of the homozygote types approaches 25 per cent, of the whole, 

 while all the heterozygote types approach zero, and the larger the 

 number of latent characters in a type, the more rapidly it decreases 

 in proportion. In trihybrids, we should have eight homozygote types, 

 each increasing toward 12y 2 per cent, of the whole. Hence the 

 generalization, a hybrid of the wth order tends to split up into 2" 

 fixed types, all types not fixed tending to disappear. The effect of 

 such a law, in the case of accidental hybrids between species and 

 varieties in the wild state, can not fail to be important in the evolu- 

 tion of species. I leave the discussion of this interesting phase of 

 the subject till the law is more generally confirmed. In this con- 

 nection it may be well to state that, at the recent international con- 

 ference of plant breeders in New York, Professor Bateson asserted 

 that Mendel's law has been found to hold in every case where it 

 had been thoroughly tested.* The groups in which these tests have 

 been made are so varied, representing both plants and animals, that 

 the presumption in favor of the generality of the law is strong enough 

 to warrant breeders in searching for it everywhere. 



It has been urged by certain breeders that, even if the law is gen- 

 eral, it can not be put to practical use by breeders; for it nearly 

 always happens that the varieties crossed differ in an indefinite num- 

 ber of respects, and we should therefore get so vast a number of re- 

 sulting types that no two individuals could be classed together. This 

 objection is not altogether valid. In the first place, if we take any 

 established variety and examine the individuals closely, we find no 

 two of them alike. Hence, even if the variety we are trying to pro- 

 duce must consist of an indefinite number of types differing only in 

 minor details, we are no more than duplicating the actual conditions 

 existing amongst present useful varieties. In the second place, a 

 very common problem of the breeder is to unite two characters found 



* This does not agree with Correns ( I. c. ) . I think, however, that the cases- 

 to which Correns refers may be explained by means of Mendel's law. 



