22 Natality Table of K6rfsi. [Jan. 18, 



law ceases to hold true. The connexion between it and the straight- 

 ness of the isobar is easily understood from the equation to a straight 

 line of x+y = constant, for if x represent the age of the father, /.and 

 if y represent that of the mother, TO, then/-|-7n = constant. That 

 this is a fact is conspicuously evident from the columns headed B + C 

 in Table II. This is the first curious law. 



Again, through a coincidence between the increasing age of either 

 parent and the decrease of fertility, it happens that the sum of the 

 three elements of (1) father's age, (2) mother's age, (3) percental 

 birth-rate in a year has a value that is itself approximately constant, 

 as is seen in the column headed A + B + C. Its lowest limit is 90 

 and its highest up to the isogen of 10 per cent, is 96, but it has in- 

 creased to 98 at the isogen of 5 per cent. If we accept for it a con- 

 stant value of 93 or 94 we shall never be far wrong in the larger 

 part of the chart. 



From this follows the second curious law that if we wish to cal- 

 culate the percental birth-rate per annum for a married couple within 

 the limits of the chart where the isogens run straight and parallel, 

 we have only to add the ages of the father and mother and subtract 

 the total from 93 or 94, in order to obtain it with considerable pre 

 cision. The approximate limits within which this law obtains are : 

 (1) the wife is not to be older than her husband; (2) she is not to 

 be less than twenty-three years of age, nor (3) more than forty. 



Example. In any large number of husbands and wives living 

 under like conditions to the inhabitants of Budapest, whose re- 

 spective ages at their nearest birthdays, to 21st June, 18'J2, were : 

 that of the father, thirty-five, that of the mother, twenty-seven ; then 

 the number of children born to them during the year 1892 would be 

 at the rate of 93 (35 + 27) per cent. = 31 per cent. ; the isogen makes 

 it about 32 per cent.* 



I shall not now enter into the other salient peculiarities of the 

 isogens further than to allude to the curious change in their course 

 which occurs when the wife is older than the husband. When she 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 with one who is younger. I should hesitate to ascribe this to 

 physiological causes without corroborative evidence derived from 

 breeders of stock. It is very possible that indifference on the part of 

 young husbands to ageing wives may have something to do with it. 



It is almost needless to say that if it be desired to obtain the ob- 

 served birth-rates for a mother of any specified age and for fathers of 



A rough mechanical arrangement was exhibited bj which isogens may be 

 drawn. It consists of three sliding pieces connected by a string. A coloured patch 

 is pasted on the back board to show the limits within which the isogens drawn by 

 it are trustworthy. 



