44 THE AMEBIC AN NATURALIST [Vol. LI 



cum reaction system. These contaminations affect the 

 whole plant, not alone any particular character complex. 

 All plants except those which were of the pure sylvestris 

 type were sterile. Until this year we have not been able 

 to secure seed from back crosses with the Tahacum par- 

 ent, but from open pollinated seed a variety of forms is 

 produced, practically all of which are of the Tahacum 

 type in general appearance. This result is evidently due 

 to pollination of the flowers with pollen from the wide 

 series of Tahacum forms which, in predominating num- 

 bers, have always been grown in the cultures. Some of 

 these plants resulting from uncontrolled pollination were 

 likewise fertile. They have been grown for several gen- 

 erations and although displaying segregation, this segre- 

 gation has never involved the production of sylvestris 

 characters, but has been of a type normally found within 

 varietal hybrids of Tahacum. The sterile forms in this 

 series largely resembled the hybrids of Tahacum and 

 sylvestris, and the occurrence of a few aberrant and syl- 

 vestris forms which were obtained from the sowing of the 

 open pollinated seed are what would be expected, if pol- 

 lination was sometimes effected with sylvestris pollen. 



It appears, therefore, that for these species hybrids the 

 conception of the factors as making up for each species 

 a reaction system in which the elements have a specitic 

 relation to one another harmonizes the results obtained 

 with the more recent Mendelian developments. The ob- 

 jection which might be made that interchanges of factors 

 which behave normally in one system should not logically 

 be followed by such profound disturbances as to com- 

 pletely prevent the formation of a functional reaction 

 system is met by several counter considerations. In the 

 discussion of lethal factors it has been pointed out that 

 Morgan (1914, 1. c.) has demonstrated that changes in 

 many loci of the Drosophila system have been followed by 

 failure of the resulting individual to develop. It is en- 

 tirely conceivable that, if a certain factor A in one system 



