322 THE TREND OF THE RACE 



two years, and over twice as great when the interval was over 

 two years. There was also a slightly greater death rate between 

 the first and fifth years when the intervals between births were 

 short, but the differences were slight. Ewart has adduced data to 

 show that frequent births handicap offspring both physically and 

 intellectually even at six years of age. The initial inferiority 

 of children resulting from too frequent births is probably due 

 in large part to the reduced vitality of the mother. The rela- 

 tively poorer intellectual development which has been noted 

 (and our data on this score are hardly sufficient to warrant a 

 general conclusion) may be due largely to the selection of stocks. 

 The people who exercise no control over the rapidity of their 

 multiplication are not apt to produce children who excel in tests 

 of intellectual development. 



It is uncertain that any of the agencies considered in the present 

 rather unsatisfactory chapter cause any changes that may prop- 

 erly be called hereditary. They may influence offspring, possibly 

 throughout life, but it is probable that their effects are mostly 

 purely somatic. It is possible that parental age, for instance, 

 might influence selective fertilization, or the selective elimination 

 of embryos. Since an old body affords an environment for the 

 germ plasm different in many ways from that afforded by a young 

 body, it is not improbable that this circumstance might be re- 

 flected in the trend of germinal variability. It might be con- 

 jectured that whatever causes the vitality of our bodies to run 

 down with advancing years might also affect the germ plasm in 

 a deleterious manner. But there is little use at present in indulg- 

 ing in mere conjectures. Experiments on animals may throw 

 light on some of these matters about which we are now in com- 

 plete ignorance. 



REFERENCES 



Ansell, C. Statistics of Families, London, 1874. 



Auerbach, E. Kurzsichtigkeit und Erstgeburt. Arch. Rass. Ges. Biol. 9, 762-763, 



1912. 

 Bell, A. G. The Duration of Life and the Conditions associated with Longevity. 



Washington, D. C., 1918. 



