S. Ikkno 47 



fact, bociiiiso I gt»t thmugh this ffrtiliMation O'-tyiH* ivi well ns M-ly\n' 

 plants in this giMioration (p. 42). 



Thus the hypothosis which rr^unls cuic ol tin- two s«'x<*,s as iK-ing 

 hfti»n»zygou8 <l«>es not acronl with the \m:\^ actually ohsrrvrd and is 

 untenable. 



The thin! hyjMjthesis is t'oun<le(l on what is variously called " inijMr- 

 fection of dominance/' "reversal of d«»nnnanc('," m- " fluctiiatiun in 

 dominanee," because our c^usc sii'ins. nl lea-st, ap|»ai«iit,|y very much 

 ivh\te<l U} that plienonienon. We have many examphrs of the latter 

 in pi>ultry acconiing to Hateson and Puiniett', tis well as l)av<'nport'. 

 To cit4.» only one example from the latter author, extra-toed individuals 

 of poultry mated with normal ^ive extra toe only in 7.*^ °/ of th«' 

 ot^spring, the remaining 27 "j having the normal number (jf toes^ yet 

 that both kinds of the offspring are heterozygotes was proven ])y the 

 fact that each of them mated inter se ha>! exhibited segregation in FJ. 

 Acconling t^> the author just named extra toe is dominant to normal, 

 but in 27 ° '. of the offspring this dominant character wjis not i)owerful 

 enough to exhibit itself; we have here to deal with the phenomenon 

 which is called " imperfection of dominance," etc. 



The appearance of 11 G-type and 3 3/- type plants in /', of our 

 SdliX'Cross would, according to this hypothesis, be due to the latter 

 phenomenon, and the (7-type character which is generally dominant 

 to the other should be regarded as having failed, in the present case, to 



Q ^ 100 

 be so in — ^= 21 % ^^ the offspring. The fact that the Fi plants, 



1 1 -|- o 



whether G^-type or J/-type, are heterozygotes, and undergo segrega- 

 tion in F<,, has also been proven. Thus, ' according to the present 

 hypothesis, the production of the two types of plants in F^ is not to be 

 regarded as a process of segregation. 



Let us now examine whether this hypothesis explains the facts 

 actually observed. First of all, it must be marked that what some 

 authors regarded as the "reversal of dominance" or a phenomenon 

 similar to it was found sometimes on further inquiry to be explained 

 in quite another way. Thus, for instance, Coutagne ' and Kellogg*^ 



> Reports to the Evolution Committee of the Jioijul Society, Report II, 11)05, pp. 1 14—1 16. 



■« Carncifie Inst. Washington Ptibl. No. 121, 11)00; Amer. Xat. Vol. xliv. I'JIO, 

 pp. 129—135 ; Amer. Breeders' Assoc. Vol. vi. 1911, pp. 29—32, etc. 



•'» Carnetjie Inst. Washington Ptibl. No. 121, p. 19, Table 10. 



* L.r. pp. 20, 21, Tables 11 aud 12. 



^ Recherches e.rperimentahs sitr V Ilerf:dile ehez is I'ers ii Soie. These. Faculte d. 

 Sciences, Lille, 1902. '• Leland Stanford Junior i'nic. VubL, University Series 1, 190h. 



