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Proceedings of the Royal Society of Edinburgh. [Sess- 
days between the births. Fig. 9 shows the total number of progeny in 
each of the A. I generations. The maxima occur in the third, fifth, and 
eighth generations, and are succeeded by a gradual decline as autumn 
approaches. The curve formed by the total length of life of the parent 
Fig. 9. — Curves to show the total number of young produced in each 
of the A. I generation series, and the longevity of the mother. 
= Total number of young of each $ . 
Length of life of $ . 
The abscissae = the number of the generations. 
is superimposed, and shows that the diminution in the birth-curve is a 
real decrease in fertility, and not a shortening of the life of the mother. 
This agrees with the observations of Davis on Macrosiphum pisi and 
Aphis maidi-radicis, and of Webster on Toxoptera graminum. 
Fig. 9 represents as nearly as possible the total number of young bom 
