Duplirate genes for capsule form in Bursa hiirsa-pastoris. \27 



in more than yeiy slight degree to the fact that the speriii-iiucleus was 

 introduced into a new cytopLisiiiic environment, the permanence of that 

 cytophismic environment and its freedom from modification by the nuclear 

 elements which are so intimately associated with it, are more complete 

 than is generally believed. 



This stimulation or increased metalioUc activity due to the dif- 

 ferences in uniting gametes, is a purely physiological phenomenon and 

 is of very widespread occurrence, but need not be universally mani- 

 fest because, when differences between the gametes become too 

 numerous or too great, sterility or even complete iucompatability may 

 result, and in any particular case the expected stimulation may be more 

 than counterbalanced by the specific action of some depressing or in- 

 hibiting factor or combination of factors. To avoid the implication that 

 all the genotypic differences which stimulate cell-division, gi-owth, and 

 other i)hysiological activities of an organism, are Mendelian in their 

 inheritance, and also to gain in brevity of expression, I suggest that 

 instead of the phrases, "stimulus of heterozygosis", "heterozygotic 

 stimulation"', "the stimulating effects of hybridity", "stimulation due to 

 differences in uniting gametes", etc. which have been used by myself 

 and others, the word '•heterosis" be adopted'). The corresponding ad- 

 jective "heterotie" may also be useful in such expressions as "heterotic 

 effects", or "heterotic stimulation", — the latter expression being 

 synonymous with "heterosis". 



Returning now to the bearing of this phenomenon on the increased 

 variability of the F2 as compared with pure types and their Fi-hybrids: 

 In the pure types there is no "heterosis" or stimulating effect of hetero- 

 zj'gosis and in tlie Fi where this stimulation is at its maximum, it 

 is the same in all the individuals and consequently can have only an 

 indirect effect (if any) upon variability-'). In the F> on the other hand, 



') The word "heterozygosis" has been occasionally used for tliis "stimulus of 

 hybridity", but it should not be considered available tor such restricted usage even by 

 those who hold that all inheritance is essentially Mendelian, for the reason that, as used 

 by its originator. Dr. Spillman, and also as rather generally used in genetic literature, 

 the word "heterozygosis" signifies only the state of being heterozygous. 



') It is conceivable that a vigorous plant may be less susceptible than a weak 

 one to certain variations in the environment, and if this be true in any ease, the varia- 

 bility (fluctuation) will be indirectly decreased in that case by heterosis. On the 

 other hand, there is at least one simple waj' in which variability can be indirectly 

 increased by heterosis, especially in regarded to repeated organs. A highly heterotic 

 plant, for instance, because of its unusual vigor may develop branches from buds which 

 in a weaker plant would remain dormant. The foliar or floral organs borne by such 



