332 WILLIAM LAWRENCE TOWER, 



The chief variables In these experiments were the conditions sur- 

 rounding and incident upon the germ cells at the time of fertiliza- 

 tion. All of the crosses herein recorded were made between indi- 

 viduals of the same age and always between vigorous individuals 

 of their respective population; that is, the materials for crossing 

 were always homogeneous as far as could possibly be determined, 

 and I never mated an old male with a young female, or vice 

 versa. 



In these experiments I have succeeded in creating a series of 

 behaviors in which the same characters are dominant to the 

 complete exclusion of the others; dominant to a lesser degree, 

 or in which there is a complete blend between the two in the F^ 

 generation, or the appearance of both parental types in F^ and 

 both breed true. As far as it has been possible to determine, 

 the only variable in these experiments is that stated, and I am 

 led to the conclusion that the conditions surrounding and inci- 

 dent upon the germ cell at the time of fertilization may be to a 

 very considerable extent responsible for the determination of the 

 dominant character in the cross and largely responsible for vari- 

 ability of such characters. 



In Exp. No. H 701 B the behavior, where the recessive 

 appears as a pure breeding race in F^, while by no means unique, 

 is difficult of explanation on the factor hypothesis as at present 

 understood. 



It could not for a moment be maintained that external con- 

 ditions are alone responsible for the determination of dominance 

 or recessiveness, because it would be about as far from the truth 

 as it is possible for one to get. External conditions have the 

 same role in organisms that they have in any physico-chemical 

 process, of accelerating, retarding, or changing the directioji 

 of the activity or of the reaction which is going on. The funda- 

 mental reaction in any of these hybrid crosses is that which goes 

 on between the two physico-chemical complexes comprised in the 

 combining germ cells. What that action actually is, is at the 

 present time absolutely unknown. That it is due to combina- 

 tions of the pangenes or biophores is a preposterous idea, and 

 that a given ferment, a chromogen, of itself, or represented by 

 any specific substance, is present as an entity in the germ cell is 



