20 



The Journal of Heredity 



basis of some strikinjj; superficial char- 

 acter. Attention was directed to the 

 fact that it is probable in the main that 

 this character is transmitted alone and 

 independently of other characters, yet 

 such need not by any means be the case, 

 since the phenomenon of linkage and 

 coupling assures us of a mechanism 

 whereby totally unrelated characters 

 physiologically may be part of one 

 hereditary comi^lex. Thus T. H. Mor- 

 gan and his associates have demon- 

 strated in the jiomace-fiy, Drosoj^hila, 

 the tendency of miniature wings, yellow 

 body, and white eyes to be grouped 

 together, in segregating out of crosses 

 with the normal fly that has the gray 

 body, red eyes, and long wings. Alany 

 other combinations of similarly linked 

 characters have also appeared in their 

 work, some linked to sex as the char- 

 acters just mentioned, and some unin- 

 fluenced by the sex determining factor. 

 They have interpreted these results 

 on the basis of the factors in the germ 

 cells that produce these characters 

 somatically, having practically a com- 

 mon locus (]jrobably a chromosome). 

 If such be the case in the domestic 

 mammals, and there is no more reason 



for doubting it with them than with 

 Dr()S()])hila, there is really a genetic 

 foundation for the belief of the breeder 

 that the sujjerficial characters on which 

 the degree of ]jreiJOtenc\' is determined 

 also indicate the transmission of other 

 characters, at least characters that find 

 their origin at the same germ locus 

 (chromosome). 



As a result of this, one can see very 

 clearly that the distinction between the 

 "breeder of breeders" and the "breeder 

 of performers" is almost entirely a 

 question of character linkage, the 

 "breeder of performers" ending his 

 function when he has contributed a 

 number of characters to his progeny, 

 so distributed among different heredi- 

 tary com]:)lexes that the segregations in 

 germ cell formation separate the neces- 

 sary interacting factors. The "breeder 

 of breeders" must bear his factors that 

 interact to produce performers in one 

 hereditary complex only. The "breeder 

 of performers" possibly has similar 

 factors to the "breeder of breeders," 

 but they are located in two or more 

 com]jIcxes, thus permitting segregation 

 and se])aration. 



LITERATURE CITED. 



Darbishire, a. D. — "Note on the Results of Crossing Japanese Waltzing Mice with European 

 Albino Races." Biometrika II, pp. 101, 165 and 282; III, p. 1. 



Davenport, E. — "Principles of Breeding." Ginn & Co., 1907. 



Morgan, T. H. — "Mechanism of Heredity as Indicated by the Inheritance of Linked Charac- 

 ters." Pop. Sci. Monthly, Jan., 1914. Pp. 1-16. 



Wentworth, E. N. — "Color Inheritance in the Horse." Zeit. f. ind. Abstammungs- und Verer- 

 Ijungslehrc, 1913. Bd. 11, Heft 1 u. 2, p. 14. 



Origin of Shetland Ponies 



"The Making of the Shetland Pony" is discussed l)y J. Cossar Ewart as an 

 appendix to "The Shetland Pony" by Charles and Anne Douglas (Edinburgh and 

 London: W. Blackwood and Sons, 1913). He comes to the conclusion that 

 "Shetland ponies are mainly descended from the 'small and fleet' race yoked to the 

 chariots of the Caledonians at the battle of Mons (jraujjius. This ancient race, 

 again, was probably to begin with a blend of the slender-limbed, Arab-like ponies 

 of the Swiss lake-dwellers, and of a thick-set race of the elei)hant-bed [a fossil deposit 

 near Brighton] type." Their small size he exjjlains as follows: "If Shetland ponies 

 have not sprung from a small wild pigtny race, it may be safely asserted that their 

 small size is mainly due to isolation in small areas where they were forced to shift for 

 themselves under, as a rule, exlremelv unfav()ural)le conditions." 



