318 THE TREND OF THE RACE 
would be a regular average increase of age according to the order 
of their birth. Let us consider families of four children the 
fathers marrying at the ages of 20, 25, 30, and 35. Suppose these 
fathers, by virtue of differences in inherited vitality, live to the 
ages of 40, 45, 50, and 55 years, respectively. Suppose also that 
at intervals of five years each father has a son who lived to be 
several years older than himself. We may represent the ages of 
the four fathers A, B, C, and D at the time of the birth of their 
sons in the upper horizontal column and the ages of the sons 
begotten at these respective ages immediately below. 
A sh Satan tdicot SS 20 25 30 35 
Age of son 40 39 38 ue 
Big cc epee eae 25 30 35 4o 
Age of son 45 44 43 42 
Coe We eee te Mew istesmeadicn ta 30 35 4o 45 
Age of son 50 49 48 47 
DDS Sei ea err ty a ee ne redo) 35 40 45 50 
Age of son 55 54- 8e «Be 
Averages of sons...... 4o 42 44 46 48 5° 52 
In the cases of these four families thus arbitrarily chosen the 
sons in each family have a diminished duration of life as the age 
of their fathers increases, but their average ages give an entirely 
misleading indication of the relation of parental age to longevity 
of offspring. In our table the older fathers produce the older sons, 
but the influence of age per se is to reduce the son’s expectation of 
life. Of course, the supposition we have made is very artificial 
and arbitrary, but it will make it clear, I think, that the data 
which Redfield presents do not necessarily prove his case, or 
obviate the objection which he admits might plausibly be urged 
against his conclusions. The arbitrary assumption may be not 
far from the truth, however, since stocks which marry early, 
such as unskilled laborers, do not have as great longevity as 
stocks which, like the professional classes, marry late in life.1 
The chief thesis of Redfield’s book on The Control of Heredity 
1 And it must not be forgotten that the decline in the general rate of mortality 
tends to give the later born members of a family a greater expectation of life. 
