442 THE AMERICAN NATURALIST [ VOL. XL II 



very important bearing upon the interpretation of what 

 are known as mid-races, i. e., races which regularly pro- 

 duce two forms in about equal proportions, for, as has 

 been seen, the mottled beans of all the hybrid families 

 which did not have a mottled bean as one of its original 

 pure-bred ancestors, constitutes a mid-race. This fact 

 was recognized by Tschermak (loc. cit., p. 47), though he 

 attributed it to an unexplained effect of cross-fertiliza- 

 tion, and not to the characteristic behavior of a definite 

 Mendelian allelomorph. Other mid-races may likewise 

 represent instances of latency due to combination. Wher- 

 ever there is a double series of characters occurring in 

 about equal numbers in the progeny of a self-fertilized 

 individual, this type of latency should be looked for. 



Purple punctation and brown flecking, which occur as 

 novelties in the seed-coats of hybrid peas, were found by 

 Tschermak to behave in a manner quite analogous to the 

 mottling in beans, the first generation showing dominance 

 of the novelty and subsequent generations always split- 

 ting into the punctate and non-punctate or the flecked and 

 unflecked, respectively, and these no doubt are also cases 

 of latency due to combination. Lock 8 has shown, on the 

 other hand, that when certain mottled and spotted peas 

 are crossed with self-colored peas, the mottling and spot- 

 ting act as typical Mendelian dominants capable of ex- 

 traction as characteristics of pure-breeding races, which 

 ought to be expected, since the homozygous parental 

 strains possessed these characters. The apparent dis- 

 crepancy between his results and those of Tschermak will 

 be fully explained if we assume that there are two types 

 of these color-pattern characters in peas, as there are in 

 beans. 



In all of these cases of latency due to combination, the 

 two units involved are of the same kind, so that the 

 latency occurs only in the homozygous individuals, thus 

 resulting in a striking contrast between homozygotes and 



8 Lock, E. H. On the Inheritance of Certain Invisible Characters in 

 Peas. Proc. Roy. Soc., B, 79, pp. 28-34, 1907. 



