174 Degeneration in the Ostrich 



birds which possess the break. Even if the break factor first appears 

 germinally as a duplex dominant^ the mating would in all probability 

 be with a nulliplex which would give simplex dominants. Until there- 

 fore the br6ak appears in a much larger proportion of the race the 

 individuals showing it in a mixed assemblage are likely to be simplex. 



The different stages of formation of the break may be as distinctively 

 germinal and transmissible as the completed break itself, and approxi- 

 mately follow the proportions for heterozygotes. This is clearly estab- 

 lished in the series below (Table XV). The parent hen, No. 167, bred 

 as shown in Table XIII from parents both without any break, displays 

 an incomplete single break, only a few vestigial scales serving to connect 

 the tarsal and digital series. Among the eleven progeny obtained, five 

 display scutellar continuity like the parent cock, five have an imperfect 

 break like the parent hen, but varying somewhat in degree, while one 

 has a clearly defined break, representing a further stage of retrogression 

 than any of the other chicks or either of the parents. 



TABLE XV. 



Breeders : 



South African cock, No. 226 No break 



Cross-bred hen, No. 167 ... Incomplete break 



Chicks : 



Five— Nop. 1, 2, 5, 8, 11 ... No break 



Five— Nos. 4, 6, 7, 9, 10 ... Incomplete break, variable 



One— No. 3 Complete, well-defined break 



In this series there can be no question that we are witnessing 

 the loss of scales in actual progress at the tarso-digital joint. While the 

 factors involved in their presence or absence are for the most part stable, 

 we also find fluctuations possibly due to the incoming of new " break " 

 factors, and even a complete loss may appear without any indication 

 of such in either parent. 



The fluctuating nature of the factors is still better exemplified in 

 the next series (Table XVI), as also a marked accentuation. In the cock 

 only a slight narrowing is displayed over the first joint, but in the hen an 



1 As regards the homozygous or heterozygous condition in which new characters first 

 appear Prof, de Vries remarks as follows in a short article {Science, Vol. xlvii. May 10, 

 1918) on "Mass Mutation in Zea Mays " : "It is now generally conceded that mutations 

 take place ordinarily in the production of the sexual cells, some time before fecundation, 

 probably at the time of synapsis. From this conception the conclusion directly follows 

 that the copulation of two similarly mutated gametes must be rather rare. Far more 

 frequent must be the instances in which a mutated sexual cell combines with a normal 

 one." 



