306 
IOWA ACADEMY OF SCIENCE Vol. XXIY, 1M.7 
cause. There is a possibility that one cause for atrophy might 
be inherent in the sperm. Third, it is possible that there may 
be an influence of the sperm on the egg similar to that which 
causes duplicate twins. This is difficult to prove. 
Recognizing that there are three such classes of possibilities, 
all of which were impossible to test with data the writer had at 
hand, an examination of a large amount of new material was 
made with the view of discovering whether such conditions ex- 
isted inherently in all cases. Southdown sheep and Chester 
White hogs were chosen for this purpose, as well as a few records 
on Collie dogs. 
In Southdown pedigrees begun from single births the average 
progenies were of course decidedly, lower than in those begun 
from multiple births, since there were only seven possible matings 
on each pedigree blank (the great-grandparental generation be- 
ing the last). Furthermore, in many cases the sire, grand- 
parents, or great-grandparents, were imported, with few data 
obtainable as to birth rank. This gave considerable statistical 
weight to. the mating from which the pedigree started. ITence 
the results from Southdown pedigrees begun with multiple 
births are presented separately. The data for each are presented 
in Tables I and II. 
TABLE I. 
BREEDING PERFORMANCE OF SOUTHDOWN MALES IN PEDI- 
GREES STARTED FROM SINGLE BIRTHS. 
Birth Rank of Sire 
Number Cases 
Birth Rank of Progeny 
1 
2811 
1.2864±.0059 
886 
1.2776±.0103 
3 
18 
1.3888±.0775 
TABLE II. 
BREEDING PERFORMANCE OF SOUTHDOWN MALES IN PEDI- 
GREES STARTED FROM TWIN BIRTHS. 
Birth Rank of Sire 
Number Cases 
Birth Rank of Progeny 
1 
4120 
1.5296±.0054 
2 
1165 
1.5682±.0072 
3 
26 
1.6154±.0644 
