382 



NA TURE 



[August i6, 1900 



According to the Darwinian theory of evolution the members 

 of a community less fitted to their environment are removed by 

 death. But this process of natural selection could not perma- 

 nenily modify a race if the members thus removed were able 

 before death to propagate their species in average numbers. It 

 then becomes an important question to ascertain how far 

 duration of life is related to fertility. In the case of many 

 insects death can interfere only with their single chance of off- 

 spring ; they live or not for their one breeding season only. 

 A similar statement holds good with regard to annual and 

 biennial plants. In such cases there nught still be a correlation 

 between duration of life and fertility, but it would be of the 

 indirect character, which we actually find in a case ot men and 

 women living beyond sixty years of age — a long life means 

 better physique, and better physique increased fertility. On the 

 other hand, there is a direct correlation of fertility and duration 

 of life in the case of those animals which generally survive a 

 number of breeding seasons, and it is this correlation which we 

 had at first in view when investigating the influence of duration 

 of life on fertility in man. The discovery of the indirect factor 

 in the correlation referred to above was therefore a point of 

 much interest. For it seems to show that the physique fittest 

 to survive is really the physique which is in itself (and inde- 

 pendently of the duration of life) most fecund. 



The data dealt with in this paper consists of four series, the 

 first three collected and reduced by Miss M. Beeton, and the 

 fourth series by Mr. G. U. Yule. 



Aloihers. Length of Life and Size of Family. 



Series I. — Taken from the "Whitney Family of Connecticut," 

 a well-known history of an American Quaker family. 



Series II. — Taken from purely English Quaker records. The 

 data for this series were drawn from a great variety of histories 

 and records of the Society of Friends. 



Fathers. Length of Life and Size oj Family. 



Series III. — The great bulk of the data was extracted from 



the American Whitney Family. 



Series IV. — Extracted from Burke's " Landed Gentry." 

 The following are some of the chief results obtained from the 



reduction of these series : — 



Table of General Results. 



It is shown that the peculiar physique in both men and women 

 which leads to longevity is also associated with greater fecundity. 

 Of two women who both live beyond fifty jears, the longer 

 lived is likely to have had before fifty the larger family. The 

 association is, however, much greater for American than English 

 parents, although the American parents dealt with are, in the 

 great majority of cases, of Anglo-Saxon race. Climate, mode 

 of life, in general selection and environment, seem to be dif- 

 ferentiating in this respect the English and the Anglo-American. 

 The English Friends, we should suppose, would be as a class very 

 comparable with the American Friends ; yet their average life 

 is longer, their fertility greater, and there is less association be- 

 tween longevity and fecundity. In both cases our algebraical 

 formulae show that American men and women are more alike, 

 and English men and women are more alike than the women 

 to the women or the men to the men of the two races. This 

 is the more remarkable, as the English Friends as a class are 

 by no means identical with the Landed Gentry. 

 . In order to represent the continuous change in the regression, 

 which cannot be done by two straight lines, which only enable 

 NO. 1607, VOL. 62] 



us to distinguish the fecund and non-fecund periods of life, the 

 statistics were fitted with cubical parabolas. The regression line 

 at any age in life may then be looked upon as the tangent to the 

 cubical parabola at that age. An inspection of the diagram 

 given below for American mothers shows what an excellent 

 expression such parabolas are for these statistics 



For American mothers and fathers we see dyjdx consistently 

 positive throughout life, and we have a most excellent graphical 

 demonstration of the physical characters which tend to longevity 

 being also associated with fecundity, 



Weismann has suggested that it may be an advantage to a 

 species that its duration of life should be shortened. This is not, 

 a priori, confirmed by the case of a man in the American 

 series : the longer the parents live, the greater the number of 

 their offspring. But if we can lay any stress on a bend-in for 

 the English mothers, and on a similar, but less marked, ten- 

 dency for the English fathers, we might argue that reproductive 

 selection was possibly in England working against extreme 

 longevity, although favouring parents living till sixty-five or 

 seventy. Indeed, those who* rush rapidly to brilliant, but not 



4 5 6 7 



Size of FamiLy. 



over-stable, conclusions might emphasise Weismann's views by 

 showing how in an old community, with much greater pressure 

 on the material resources, there is a tendency to reduce the 

 fertility of the long-lived parents ; while in a new community, 

 with plenty of food and occupation for all, the longest-lived 

 parents are the mostfertile ! However, all that we can safely 

 say is that there is a marked difference between English and 

 American parents, and that this distinguishing characteristic is 

 almost equally visible if we take opposite sexes of such diverse 

 classes as English Friends and English country gentlemen. We 

 would leave to further investigations its true interpretation. 



Admitting a substantial correlation between length of life and 

 fertility, it is of great interest to investigate what effect, other 

 things being equal, reproductive selection would have in modifying 

 the duration of life. 



