20 Mr. F. Gallon. Results derived from the Natality [Jan. 18, 



more of the four squares from which they were derived was blank. 

 They are, of course, less trustworthy than the rest ; moreover, they 

 may depend on less than 100 families. 



The ages of married couples are distributed over only about one-half 

 of the squares of Table I, as there are too few examples of other ages to 

 be statistically available. This partial distribution is well seen in the 

 diagram of isogens, where a dotted outline encloses all the material 

 that can be used with safety. The broken lino AB corresponds to 

 the instances in which both parents are of the same age. The chart 

 is practically limited to marriages in which the wife is less than 

 five years older, and less than seventeen years younger, than her 

 husband. 



Isogens. 

 Father's Age. 



It will be noticed that the isogens run in nearly straight, diagonal, 

 and equidistant lines across the greater part of the chart. If we 

 omit six squares in the upper left-hand corner where there is no room 

 for an isogen, we shall find these diagonal lines to cross 89 of the 

 total number of 118 entries, or between eight and nine tenths of 

 them. This indicates the existence of a very curious and unexpected 

 law of natality, which is well brought out by Table II, which shows 

 the values measured from the dots marked on the isogens. They have 

 been taken at convenient places to serve as examples, one at the 

 beginning, one at the end of the straight portion of each, and at some 

 other intervening places. 



In Table II are given the ages of the father and mother that corre- 

 spond to each of these dots. 



As a consequence of the straightness of the isogens, the sums of the 



