20 ORIGIN OF A POLYDACTYLOUS RACE OF GUINEA-PIGS. 



Method of Grading the Progeny. 



Of course the female mates of J* 193 were in many cases superior in 

 toe development to the mates of his father, ,^ 23, though in many cases they 

 were identical. Accordingly it is necessary, in estimating the respective 

 potency of the two animals, to separate the mothers into groups of similar 

 toe development, or of similar pedigree when no extra-toes were present. 

 Consequently, five groups of mothers have been made (see tables 2-14), 

 viz, G, F, P, N and A''. Group G includes only mothers having well-devel- 

 oped extra-toes on both feet (GG) ; F includes mothers having fairly well- 

 developed toes, or with one good toe only (FF, FG, or GF) ; P includes 

 mothers with at least one toe poorly developed (P) ; the other toe may be fair 

 or poor, or the foot may be normal. A' includes normal (A'.Y) mothers 

 descended from (^ 41.1^ or nearly related to him ; N' includes normal mothers 

 not descended from (^4i.i-'', but belonging to families in which, when 

 inbred, polydactylism does not occur. 



When the mothers are grouped in this way, we find, first, that the pro- 

 portion of polydactylous young produced by a male decreases in the suc- 

 cessive classes from G to A''; and secondly, that the degree of development 

 of the toes produced on those polydactylous young diminishes in the same 

 order. 



It would manifestly be unfair in estimating the potency of transmission 

 in a given case to omit either of these considerations — the proportion of 

 polydactylous young, or the degree of development of their extra-toes. So 

 an attempt has been made to combine the two into a numerical grade for 

 the young by each group of mothers. This grade is given in the last 

 column of tables 2 to 14. In making up the grade each G toe counts 

 100, each F toe 80, and each P toe 50, while A' counts o. The total thus 

 obtained for a group of young is divided by twice the total number of 

 young in that group, that is, by the whole number of feet which might bear 

 the extra-toe. Measured by this standard, a group of young, all of which 

 had the extra-toe well developed on both hind feet, would be graded 100; 

 a group of young with no extra-toes would be graded o, while mixed groups 

 would come in between o and 100. A group of young averaging P would 

 be graded 50, and a group averaging F would be graded 80. 



Comparing the young of ^ 193 (table 3) with those of his father, c? 23 

 (table 2), we see a marked increase in the potency of transmission of the 

 extra-toe within the same group of mothers. The grades of the young 

 produced by the last four groups of mothers (F to A'') in the case of (^ 23 

 are 29, 19, 17, and 1.5, respectively, while in the case of ^ 193 they are 

 58, 57.5, 22 and 18.5. 



