6 DAVIS— THE TEST OF A t April 23. 



gation of characters such as has been demonstrated by the Men- 

 delian research of recent years ? To these questions it must frankly 

 be answered that only here and there are glimpses of situations 

 which may possibly be interpreted in terms of Mendelian analysis. 

 For example the characters of the " mutants " are frequently clearly 

 retrogressive which indicates that gametes are formed lacking cer- 

 tain factors and suggests phenomena characteristic of segregation 

 from heterozygous stock and very common in Mendelian behavior. 

 Again, the repetition of the same " mutants " in a series of genera- 

 tions suggests a mechanism of precision such as we have come to 

 associate with Mendelian inheritance. It is not, however, my pur- 

 pose to argue at present this phase of the discussion for the experi- 

 mental data before us is not in such shape that it can be handled to 

 the best advantage. We admit that the " mutants " themselves do 

 not establish their parents as in their nature hybrids. If they did 

 there would of course be no discussion. 



Under two conditions and apparently two only can a hetero- 

 zygous species be conceived as breeding true. 



First, if of the varied possible types of gametes only such unite 

 and produce fertile zygotes as will perpetuate the same germinal 

 constitution as the parent, then from such zygotes a heterozygous 

 line might continue indefinitely as an impure or hybrid species. 

 Under such conditions gametes which might in varied combinations 

 give a series of dififerent forms (segregates) are either not matured 

 or if matured fail to function. Some degree of pollen and ovule 

 sterility must be expected as the result of such conditions. 



Second, if of a varied assortment of zygotes formed by the 

 union of dififerent types of gametes, only those develop which have 

 the germinal constitution of the parent then again a heterozygous 

 line might continue indefinitely and constitute a species, although 

 impure or hybrid in its nature. Since all of the zygotes which re- 

 sult from other combinations of gametes either die or fail to develop 

 beyond some early stage in the life history this condition would 

 result in some degree of seed sterility or in the production of weak 

 plants that must soon perish. 



Now the Oenotheras as a group exhibit a very remarkable amount 

 of pollen sterility and also a high degree of ovule abortion, and 



