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.1 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 
1Even in the early days roan race-horses were not at all common. Both roan and gray have 
been selected against. 
