Eeactions and Products in Inteespecific Crosses 165 



original form and arrangement, so that the reaction, as far as the adult con- 

 ditions are concerned, presents only the array of a simple monohybrid type as 

 far as the crossing of these two biotypic lines are concerned. 



In the larvae the arrays presented are complicated by the unstability of the 

 larval body-colors when in heterozygous combination, so that F, always presents 

 two classes, reds and yellows, in variable numbers. 



This is entirely the product of the potentiality possessed by the heterozygous 

 members of the fraternity for range in the characters presented, and is in the 

 main, if not entirely, a product of the conditions of the medium or other con- 

 ditions of life and of purely transient nature. When these fraternities are 

 reared under constant conditions the Fo array of larvae shows yellows, yellow- 

 reds, and reds, in close approximations of 1 : 2 : 1. 



In nature and imder the usual conditions of operation, the F, array in its 

 heterozygous members is able to range widely in both directions, so that the 

 result is to produce the curious ratio of an apparent 1 : 1. The homozygous 

 yellow is always more easy of separation than the red, which even in the pure 

 stock is variable in its intensity, depending upon conditions of nutrition and 

 surroundings, so that separation of F, fraternities is by no means easy and is 

 complicated by the action of heterozygous individuals, which tend to swing to 

 one or the other extreme of the possible series, giving either reds or yellows, 

 and few uncertain intermediates under ordinary conditions. The Fj hetero- 

 zygotes in all of these experiments are more sensitive to the action of the 

 external conditions, as shown by their ability to fluctuate between the possible 

 extremes. This fluctuation of the heterozygous individuals is by no means 

 common in described crossings, largely due to the absence of observations upon 

 the juvenile stages of organisms in crosses. Xewman has recorded variations 

 in the aspect of the embryos of Fundulus, and I have seen in many of the 

 materials that have passed through my hands this same fluctuation in the 

 juvenile characters in ontogeny, and have experimentally altered them one way 

 or the other by change of conditions or food. These changes are thus far in my 

 experience entirely without genetic significance, although they show in the 

 heterozygotes susceptibility to external conditions not shown in the homozygous 

 types, and further these are always far more strongly manifested in the juvenile 

 stages than in any of the adult characters where fluctuations of this sort are not 

 common. 



The evidence from this cross of the mean biotypic lines of these two species 

 shows that the two gametic systems act in crossing as unit systems coming out 

 of the cross in totality, without interchange of the characteristic of either parent, 

 so that the reaction in the cross is of the monohybrid type. This result is true 

 only of the condition in the stocks used and under the conditions of experiment 

 given. Further, in experience with the cross the regularity of the reaction is 

 increased with the approach to constancy in the conditions of the medium, the 

 sharpest reactions being observed in controlled series, where the conditions 

 were constant and at the mean of the environmental complex present in the 

 breeding-quarters antecedent to the time the stocks were crossed. 



These two modal biotypes represent the species complex devoid of any 

 accessory or complicating agents, and the reaction shown between them in this 

 cross is, no doubt, that which is entirely proper to them when brought into 

 combination. Any such reaction is in all probability entirely a laboratory event. 



