48 AN ANALYSIS OF THE EFFECT OF SELECTION. 



ones." We do not know where the rest of the stock came from, and 

 we do not know how the animals used to start the selection experi- 

 ments were derived from these sources. We do not know how many 

 individuals were used to start the selection experiment ; and we do not 

 know anything as to the relationship between the rats in the two series 

 (plus and minus). And, finally, we have only very indefinite data 

 as to what system of breeding was followed during the experiment. 

 All this information is very much needed, if we are to know how to 

 interpret the results. It is conceivable that each series was split up 

 into a number of separate lines, and that these have been crossed 

 from time to time. Such a system would result in bringing together 

 modifying factors more slowly than would a system of very close in- 

 breeding. It is, of course, very improbable that any such system has 

 been followed; and such an assumption is by no means necessary for 

 a multiple-factor interpretation of the results. But definite informa- 

 tion is very desirable, as is indicated by an analogous case. 



In connection with certain work that the writer has been carrying 

 on with Mr. J. W. Gowen, pedigrees of the two famous thorough- 

 bred race-horses, Sysonby and Artful, have been tabulated. These 

 pedigrees are both practically complete for 10 ancestral generations. 

 They constitute a fair random sample of pedigrees in the breed, for 

 Sysonby was of pure English blood, while Artful had many American- 

 bred ancestors. The two pedigrees show no name in common until 

 we reach the fifth ancestral generation. In that generation there are 

 three names that appear in both pedigrees. But by the time we reach 

 the tenth ancestral generation, approximately 90 per cent of the 1,024 

 names in Artful' s pedigree appear also in the first ten generations of 

 Sysonby's pedigree. And the result would certainly be even more 

 striking if the pedigrees were studied for a few more generations, or 

 if two English-bred horses were compared. Here, then, we have a 

 clear case of ''interlocking" pedigrees. Yet in spite of the long in- 

 breeding (12 to 20 or more generations, with scarcely any out-crosses) 

 which the breed has undergone, there are still a large number of bay 

 or brown and of chestnut race-horses, besides a few grays and blacks. 

 Of the four Mendelian factor pairs (see Sturtevant, 1912) for which 

 the race was originally heterozygous, it has become homogeneous only 

 in that the roan factor has been eliminated.^ Clearly, selection for 

 any one of the colors now present would still be effective in eliminating 

 the others. The breed, which we may suppose to be inbred to some- 

 thing like the same degree as Castle's hooded rats, is still very far 

 from a "pure line." 



The new data presented by Castle and not taken up by MacDowell 

 consist of two points: The crosses of extracted hoodeds (from plus 



^Even in the early days roan race-horses were not at all common. Both roan and gray have 

 been selected against. 



