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Statistical Investigations
Isogens or Lines of Equal Fertility.

Father's Age

409

4G

50

 

20

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C

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0   ∎∎∎∎∎∎∎∎a∎a=v

oil

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WE so '

on   ME   INAN

50

Illustrations of the accuracy of the Isogens.

Wife

Husband   I Calculated

Actual

 
 
   

23

27

43'/.

42 °/°

 

23

37

33-/.

30'/ °

 

27

33

33'/.

31

 

31

35

27-/.

26

 

35

41

17-/.

16-/.

 

39

49

5./.

6'/.

 

Probably a little adjustment would give a better fitting plane to the portion of the surface under consideration, but Galton's rule gives at any rate a first approximation of a quite serviceable kind.

Our author notes the curious change in the direction of the isogens when the wife is older than the husband. This seems to me to indicate that when the wife is older than the husband, the age of the latter is of minor importance. Galton interprets it as follows

"When she [the wife] is from thirty to thirty-eight she certainly seems to be appreciably more fertile with a husband of her own age or somewhat older than she is [than] with one who is younger'. I should hesitate to ascribe this to physiological causes without corroborative

' The following results drawn from his data do not seem to confirm Galton's views

 

Age of Wife   ...   ...

29

31

33

35

37

39

41   43

45

Percentage Children

Husband of same age ...

35

22

22

19

17

15

10

6

3

Mean for husbands younger

34

25

26.5

21.3

17.3

16.3

11.7

6.3

3.3

PoII

s?