HORTICULTURAL SOCIETY OF NEW YORK. 



beardless. You take out the bearded, and again there will be a beardless crop 

 with a certain number of bearded. To get your crop pure in a few generations 

 you should make your selection from individual plants. Then you will begin 

 to find that some plants carry only the bearded character and some will carry 

 both. It is the coming out of these recessive characters, owing to the for- 

 tuitous union of recessive germs, which shows itself in the offspring you 

 desire to get rid of. 



Whenever, then, it is desired, in a crossbred strain, to fix a dominant 

 character selections must always be made of single families containing no 

 recessive members. 



We reach, therefore, a fact of immediate interest to the practical breeder. 

 We have lost forever, I think, the conception that fixity of character is solely 

 or chiefly a function of the number of generations during which that char- 

 acter has been manifested, or of the number of successive selections of that 

 particular variety which have been made. Purity of strain or fixity of char- 

 acter is, on the contrary, due primarily to the union of similar gametes in 

 fertilisation. Such purity may therefore occur among the immediate offspring 

 of crossbred organisms. 



Another question of considerable practical significance is that of the nature 

 and causation of dominance, involving the further question whether the breeder 

 has any means at his disposal by which dominance may be created, modified 

 or controlled. Upon this point experimental results are still to seek, and 

 though there are a few cases* where we know that the dominance of one 

 character over another varies in intensity, we have no clear indication as to the 

 causes governing these differences of intensity. We may naturally be disposed 

 to consider whether continued pure-breeding, or, perhaps, in-breeding, may 

 not be concerned in the creation of dominance, but the facts at present ascer- 

 tained give no clear light on this question. We have, however, abundant 

 evidence that pure breeding is not essential to the constitution of dominance ; 

 for in any simple Mendelian case the pure dominants, offspring of one cross- 

 bred and one dominant parent, or of two crossbred parents, may, and com- 

 monly do, show unimpaired dominance over recessives of pure lineage. 



But there is another class of facts which, to my thinking, is far more 

 interesting than that, and is of more significance to the practical breeder, and 

 that is this: I spoke in the case of the green and the yellow pea of the 

 offspring resembling the dominant, the yellow. But in a great number of 

 cases we find a phenomenon not nearly so simple as that. When similar 

 germs meet they produce a pure bred organism, which in my terminology is 

 railed a homozygote, a yoking together of like germs. When the germs are 

 dissimilar they make a new form, a hybrid form, which in this terminology 

 we may call a hcterozygote, the yoking together of two dissimilar germs in 

 the zygote form. Until we have seen the heterozygote, its form is not predica- 

 ble in any specific case. You cannot say until you have made the specific cross 

 what the character of that heterozygote will be. It may be that, through dom- 

 inance, one character only prevails to the exclusion of the other, or the hetero- 

 zygote may have some form totally distinct from that of either of the parents. 



*The extra toe of the Dorking fowl, for . instance, is uniformly dominant in some 

 strains, but not in others. 



