334 WILLIAM LAWRENCE TOWER. 



may well be modified and lead to endless confusion by this very 

 fact which I have established, of the determining and influ- 

 encing of dominance by external conditions. Thus, for example, 

 in Exp. No. H 410, external conditions determined the whole 

 future history of that culture. By the conditions surrounding 

 the germ cells at the initial cross the total character of the race 

 was determined for as many generations as I cared to continue 

 the experiment. If this is generally true, and I see no reason 

 why it may not be, then the determination of dominance, which 

 determines also the resulting products, is a most vital factor in 

 evolution. 



Again, the variability in products which one finds, as, for ex- 

 ample, the differences which MacDougal (1905) found between 

 crosses of (Enotheras made by him in New York and those made 

 by DeVries in Holland, when they were using, as far as could be 

 determined, identical material, has a very direct bearing upon 

 this point. MacDougal says: ". . . , the very differences be- 

 tween the results of the hybridizations, as carried out in Amster- 

 dam and New York, suggest that the manner in which the 

 various qualities in the two parents are grouped in the progeny 

 might be capable of a wide range of variation. Many indica- 

 tions lead to the suggestion that the dominancy and preva- 

 lency, latency and recessivity of any character may be more or 

 less influenced by the conditions attendant upon the hybridiza- 

 tion ; the operative factors might include individual qualities as 

 well as external conditions." 



This at once suggests differences in results, and difficulties 

 that will arise through the carrying out of like experiments 

 under unlike conditions. This phase of the situation is set forth 

 to some extent in the second part of this paper in the experiments 

 in synthesis. These experiments, while not susceptible of analy- 

 sis along certain lines, indicate very clearly that the operation of 

 the principles of alternative inheritance will be productive of 

 diversity of results under diverse conditions when using homo- 

 geneous materials. This gives a clue which may be of para- 

 mount importance in the further investigation of the problem 

 of the origin of species in nature. 



In many organisms there exists a physiological isolation, which, 



