170 Mi- M. r."-ton, -Mi. ('.. I". Vulf. :ni.i l'i..f. K. IVurson. 



question. Generally, we may eon. -hide, that the data are not very 

 suitable for real purposes of comparison, but that there is nothing in 

 them opposed to the suggestion that a >nisi)le part of the increased 

 duration of life of this century may le due to the inheritance of 

 longevity and the correlation of longevity with fertility. Further 

 determination of the inheritance of duration of life in the case of 

 minors may help to throw additional light on the matter. 



7. The following method of illustrating the influence of longevity on 

 fertility may serve to impress the matter on the reader: 



In Series I the longer-lived moiety of the mothers produce 64'0 per 



cent, of the children, and the shorter-lived moiety 36'0 per cent. 

 In Series III the longer-lived moiety of the fathers produce 61 - 1 per 



cent, of the children, and the shorter-lived moiety 38'9 per cent. 

 In Series II the longer-lived moiety of the mothers produce 55'2 per 



cent, of the children, and the shorter-lived moiety 44'8 per cent. 

 In Series IV the longer-lived moiety of the fathers produce 53 - 5 per 



cent, of the children, and the shorter-lived moiety 46'5 per cent. 



Thus, while the results are all very sensible, those for the American 

 parents are markedly so. In both American and English statistics the 

 influence of longevity on the fertility of the mother is greater than its 

 influence on the father. 



8. (\nii-lnJimj l!<-)iwrkf. A somewhat widespread view of evolution 

 stops at the survival of the fitter without discussing the mode whereby 

 the less fit leave no, or fewer, offspring than the fit. Of course, if the 

 unfit are exterminated before adult life, there is no chance of their 

 reproducing themselves. It has been shown in the second paper of this 

 series that a selective death-rate does exist for adults, so that the whole 

 work of selection does not take place before the reproductive stage is 

 reached. But Miss Beeton's data for the correlation of duration of life 

 in the case of brethren dying as minors seem to show that the selective 

 death-rate for children is rather less than greater than its value for 

 adults."* Hence, for the reduction or extermination of stock unsuited 

 to its environment, we should have to look largely to selection in the 

 adult state. In the present paper we have made what we believe to be 

 the first quantitative determination of how a selective mortality reduces 

 the numbers of the offspring of the less fit relatively to the fitter. In 

 the case of life under wild conditions, the correlation l>etween fertility 

 and power of surviving would probably be far greater. But for such 

 life it is almost impossible to get statistics of this nature ; we are thrown 



* The matter is still under investigation, so that this conclusion is stated subject 

 to modification. Of course, the selective death-rate among children may largely 

 remove those not weak from inherited constitution, but by physical or physiolo- 

 gical accident. These our method of investigation would throw into the non- 

 selective death-rate. 



