222 PROCEEDINGS OF THE AMERICAN ACADEMY. 



the optic chiasma, it is an example of alternative dominance, for 

 neither condition is uniformly dominant over the other. The results 

 shown in Table III, where the crossing of the nerves was different in the 

 two parents, correspond closely with those to be expected if dominance 

 is alternative. If such is the case, it is necessary to assume either that 

 the parents were a "pure" right and a "pure" left, respectively, or 

 that both were heterozygotes. On the former assumption, the results 

 to be expected from the Mendelian formula would be equal numbers of 

 rights and lefts, all heterozygotes, thus : 



R + R = gametes of first parent. 

 L + L == " " second " . 



2 R (L) + 2 L (R) = offspring. 



If both parents were heterozygotes, the apparent result would be the 

 same, but half each of the right and of the left offspring would be 

 " pure," the other half heterozygous, thus : 



R + L = gametes of first parent. 

 R + L = " " second " . 



R + R (L) + L (R) + L = offspring. 



If one parent were a pure right and the other a heterozygote, the 

 proportion of rights to be expected would be 75 per cent, thus : 



R + R = gametes of first parent. 

 R + L = " " second " . 



R + R + R (L) + L (R) =: offspring. 



The last assumption evidently is untenable, for in none of the lots 

 studied is this proportion approximated. 



This principle of alternative dominance could be applied to the results 

 in Table II only by assuming that both parents were heteroz)^gous. On 

 this assumption alone could the observed almost equal proportions of 

 right and of left offspring be explained. 



The first generation furnishes no direct evidence which would allow 

 a positive assumption that the facts are to be explained by Mendel's 

 Law. 



Further, the results of the first generation show conclusively that 

 prepotency is not found in any individual. In Lots 1, 2, 4, and 5, 

 1902, the eggs of two trout were divided each into two lots, making 

 four in all. These were fertilized by two males so as to get as many 



