The Editor: The Long-Lived First-Born 397 
. | Total No. of | No. lived to be | Per cent lived to 
Order of birth | children aged be aged 
Birstibornt childrenk 106s oes * 22 pees 802 FAG | 2h305 
Second Herechildrena+ otis. we. cc cete > 786 118 15.01 
AManrgeleloforachralaultokge\ akan chet e ehrt an weReMeny Gear 765 104 13.59 
Hourshybommenivdrens: 84.0. ks eae: 705 95 13.47 
Yt EheMonmrCniUnremien 2s -cseet ptnesas a eee cee 6306 82 13.01 
Sixth pornyehuadken s: . ok.. 4 claw hee 542 40 7.38 
Deven bornsehtldrens wee Ji he. ee 450 Se ila PY) 
Bish the pOLischildrenies mtu. wee cera: 369 30 8.13 
INiriahel aly] ofoyane tel abi Kelsi ale pean te oe, Dees ia tae ea 271 22 8.11 
Menmunmornscoic ren] twee. es rains ens 181 20 11.04 
Eleventh, twelfth, thirteenth, fourteenth 
and fifteenth born children........... | 188 21 ala stl7/ 
MOTs Waa sere ae a | 5,689 / 802 14.09 
the first-born, that infant mortality is 
higher among the first-born, that the 
health of the first-born child,. during 
its early years, is below par? 
INFLUENCE OF NATURAL SELECTION 
Natural selection appears to offer a 
satisfactory explanation. At the birth 
of the first child, the maternal mechan- 
ism is less well adapted to its work than 
is the case at subsequent births. Par- 
ticularly in the case of middle-aged 
women, physicians say that the strain 
to which the child is subjected at birth 
is greater at the first than at subsequent 
parturitions. The first-born child is, 
therefore, more stringently selected than 
are his brothers and sisters; a greater 
percentage of the first children die at 
birth. Now if we make the assumption 
that those who die are, on the average, 
inherently weaker than those who 
survive the ordeal, it follows that the 
average of strength, among the first- 
born adults in a population, would be 
higher than among the second or later 
born; not because they were as a rank 
superior physically from the start, but 
simply because a greater proportion of 
the weaklings were eliminated at the start. 
by birth-rank. See JoURNAL oF HEREDITY, Vol. 
If, then, we draw a sample of long- 
lived people from the population, we 
would expect to find more first-born 
among them because the initial in- 
cidence of natural selection left the 
surviving first-born more fit, on the 
average, than the surviving second, 
third, or fourth-born. 
This hypothesis may well be supple- 
mented by the fact which biometricians® 
have found, that the elder children are 
more variable in respect to longevity, 
than are the later-born. Increased 
variability naturally gives all the more 
scope for the action of natural selection; 
and while those who vary in the direc- 
tion of physical inferiority will be elimi- 
nated at birth, the survivors will repre- 
sent children who vary in physical 
superiority to a greater extent than do 
their younger brothers and sisters. This 
variation will naturally result in the 
production of a considerable number of 
long-lived individuals. 
If the facts have been correctly inter- 
preted, then the prevalence of first-born 
in a collection of long-lived individuals 
has a real biological foundation, and 
is not a mere statistical fallacy. This 
conclusion is supported by the fact, 
V, p. 268. Researches cited by H. H. Hibbs, Jr., 
(Infant Mortality, p. 56, N. Y., 1916) show that in many cases the infant mortality is lowest 
when the mother is under 20 and when the birth in question may fairly be supposed to be her 
first, in a large proportion of cases. 
These data may not be so weighty as the larger series quoted 
by Pearson, but should at least be taken into account. 
> Beeton, Mary and Pearson, Karl. 
pp. 50-99, London, 1901. 
Inheritance of the Duration of Life. 
Biometrika, Vol. I, 
