BIOTYPES AND HYBRIDS. 53 



behavior can only rest upon the presence of an internal factor, of whatever 

 nature, capable of independent movement at the time of the reduction- 

 division in the formation of the germ-cells. It may be assumed, perhaps, 

 that this factor or determiner is incapable of division at the time of 

 the reduction-division ; but however it is explained, the result is the pro- 

 duction of certain individuals (homozygotes) which produce germ-cells all 

 of one kind with respect to a particular character and which must there- 

 fore breed true with respect to that character, and other individuals (het- 

 erozygotes) which produce germ-cells of two kinds with respect to the same 

 character and which consequently can not breed true. Individuals which 

 possess the same characteristics and which are homozygous with respect 

 to all of these characteristics, collectively form a biotype ; hence the state- 

 ment that the Mendelian inheritance of the several forms of Bursa demon- 

 strate beyond a possible question that they are distinct biotypes. I will 

 not be understood, however, to imply that biotypes must Mendelize on 

 being crossed, for many are known which do not, as, for example, most of 

 the various biotypes of Oenothera. 



The importance of Mendel's discoveries for our conception of the signifi- 

 cance of different kinds of variations in the evolution of plants and animals 

 is now generally recognized. It is seen that in the presence of Mendelian 

 inheritance the ' ' swamping " of a new character by crossing with the 

 parent form does not take place, and that the very kind of variation which 

 was supposed to be swamped by crossing is just the kind which is most 

 certain to be preserved. The new form may completely disappear when 

 crossed, but it is only hidden momentarily, not destroyed. Thus when 

 Bursa heegeri crosses with its parent Bursa bursa-pastoris , all of the offspring 

 are indistinguishable from the latter, but in the next generation a portion 

 of the offspring are just as typical B. heegeri as the original specimen, and 

 no transitional forms occur. If the dominance of the triangular capsules 

 were incomplete there would be some transitional stages, but this would 

 not modify the situation in any essential manner, as there would still be 

 just as many typical homozygotes as before which would breed true when- 

 ever they chanced to be fertilized by their own kind, and if fertilized again 

 by the parent form they would form heterozygotes which would produce 

 as large proportion of typical B. heegeri as was produced by the heterozy- 

 gotes of the previous generations. 



I have shown elsewhere (Shull, 1907 a) that this capacity of a mutant 

 to disappear upon crossing with its parent may be a great advantage in the 

 struggle for existence under certain conditions, and if we grant that evolu- 

 tion is in the main retrogressive (Shull, 1907^), the new forms will be 

 generally recessive to the parent and will thus be in position to profit by 

 whatever advantage recessiveness gives. From the data I have as to the 

 geographic distribution of the several biotypes of B. bursa-pastoris it 



