575. 3 21 



591.16 



V. 



Reproductive Selection . 1 



(i) In a recent memoir (" Contributions to the Mathematical 

 Theory of Evolution, III. Regression, Heredity, and Panmixia," 

 now in type for the Philosophical Transactions) I have found it necessary 

 to note the difference in mean and variation of a population when (a) 

 the individuals of a sex are taken into account once as mates, (b) when 

 the individuals of a sex are treated as parents or weighted with their 

 fertility. The mean and variation of the population are supposed to 

 be taken with regard to any organ whatever. If such a difference is 

 found to exist between the variation curves for mates and for parents, 

 then there is a correlation between fertility and the organ (or charac- 

 teristic) measured. Under the action of heredity there will accord- 

 ingly be a progressive evolution in this organ, unless this evolution be 

 checked by some other factor of progressive change, e.g., natural 

 selection. In my memoir I term this factor of progressive evolution 

 Reproductive Selection. 2 Without wishing at present to publish my 

 complete work on this subject, I should like to put on record the 

 following conclusions already reached : — 



(2) Let any organ in individuals of one sex be selected, and let_y 

 be the fertility of an individual, whose organ differs x from the mean 

 organ of all mated individuals. Let M m be the mean organ for all 

 mates, 3 M^ be the mean organ for all parents, i.e., a mate reckoned 



1 A Contribution to the Mathematical Theory of Evolution, read before the 

 Royal Society on March 5. 



2 The influence of variation in fertility has been considered by Mr. Romanes 

 under the title of ' Physiological Selection,' but the idea he expresses by this term 

 appears to me very different from that of reproductive selection. In mathematical 

 language, Mr. Romanes supposes the fertility curve and the correlation surfaces, 

 owing to some cause or other, to become double-humped ; they may accordingly be 

 resolved into two components, each corresponding to a distinct species. Physio- 

 logical selection thus aims at an explanation of the origin of species. Reproductive 

 selection supposes the fertility curve and correlation surfaces to embrace only 

 homogeneous material, and it can accordingly never give rise to a new species ; it is 

 purely a source of progressive change in the same species. The only approach to a 

 double hump which occurs in the curves of human fertility that I have dealt with 

 is a secondary maximum at absolute infertility, due in all probability to artificial 

 restraint on fertility. As those couples who fall into this component leave no 

 offspring, they cannot give rise to a new species. 



3 If there be preferential mating, M will not be the mean organ for all indi- 

 viduals. I have adopted the mate mean in order to free the investigations from the 

 influence of this portion of sexual selection. 



2 A 



