LIMITS OF HEREDITARY CONTROL 905 
the striking identity of the four quadruplets with respect to the 
peculiarity on question. This is the only undoubted case of the 
direct transmission of a definite seute peculiarity from the mother 
to the offspring which we have encountered. In this case it 
seems clear that hereditary control and hereditary transmission 
are equally precise, and it appears probable that most of the cases 
we have described are inherited either from the unknown fathers 
or from some recent ancestor. 
PAIRING; AN INTRA-FRATERNAL CORRELATION; AND ITS BEARING 
ON THE PROBLEMS OF HEREDITARY CONTROL 
The most difficult question that confronts us in attempting 
to gain an insight into the operation of the predetermining mech- 
anism concerns not so much the resemblances between the quad- 
ruplets as their differences. Why are they not all exactly alike 
and why are there closer resemblances between some individuals 
of a set than between others? 
It has been shown that, with respect to their uterine connections, 
the four foetuses are definitely paired, in that two of them are 
attached to each of the lateral placental discs of the chorionic 
vesicle. Embryological investigations have revealed indications 
of a much earlier and more intimate pairing. With very few 
exceptions the resemblances between the foetuses are strictly in 
accord with their placental pairing, the closest resemblances 
occurring between the two individuals attached to the same 
placental disc. This is not a mere matter of arbitrary judgment, 
but is based on a comparison of the inter-pair and intra-pair 
correlation. By the use of the difference method previously 
employed the two constants have been determined as follows: 
Inter-pair coefficient of correlation = 0.9257 
Intra-pair coefficient of correlation = 0.9517 
It will be noted that the correlation between the individuals 
of opposite pairs (inter-pair correlation) is lower than that deter- 
mined for the whole set (.9348), while the correlation between 
the individuals of the same pairs is decidedly higher; in fact this 
is probably the highest correlation constant determined for any 
