February 24, 1899.] 



SCIENCE. 



283 



MEPBODUCTIVE OR GENETIC SELECTION. * 



1. The object of this memoir is twofold : 

 first, to develop the theorj' of reproductive 

 or geuetic selection f on the assumption 

 that fertility and fecundity may be heritable 

 characters ; and, secondly, to demonstrate 

 from two concrete examples that fertility 

 and fecundity actually are inherited. 



The problem of whether fertility is or is 

 not inherited is one of very far reaching 

 consequences. It stands on an entirely dif- 

 ferent footing to the question of inheritance 

 of other characters. That any other organ 

 or character is inherited, provided that in- 

 heritance is not stronger for one value of 

 the organ or character than another, is per- 

 fectly consistent with the organic stability 

 of a community of individuals. That fer- 

 tility should be inherited is not consistent 

 with the stability of such a community, un- 

 less there is a differential death-rate, more 

 intense for the offspring of the fertile, i. e., 

 unless natural selection or other factor of 

 evolution holds reproductive selection in 

 check. The inheritance of fertility and the 

 correlation of fertility with other characters 

 are principles momentous in their results 

 for our conceptions of evolution ; they 

 mark a continual tendency in a race to pro- 

 gress in a definite direction, unless equili- 

 brium be maintained by any other equi- 



* Mathematical Contributions to the Theory of 

 Evolution. Part I, Theoretical : By Karl Pearson. 

 Part II, On the Inheritance of Fertility in Man : By 

 Karl Pearson and Alice Lee. Part III, On the In- 

 heritance of Fecundity in Thoroughbred Eace-horses : 

 By Karl Pearson, with the assistance of Leslie Bram- 

 ley-Moore. Abstracts read before the Eoyal Society, 

 December 8, 1898. 



1 1 have retained the term ' reproductive ' selection 

 here, although objection has been raised to it, be- 

 cause it has been used in the earlier memoirs of this 

 series. Mr. Galton has kindly provided me with 

 ' genetic ' and ' prefertal ' selection. The term is 

 used to describe the selection of predominant types 

 owing to the different grades of reproductivity being 

 inherited, and without the influence of a differential 

 death-rate. 



pollent factors, exhibited in the form of a 

 differential death-rate on the most fertile. 

 Such a differential death-rate probably ex- 

 ists in wild life, at any rate until the en- 

 vironment changes and the equilibrium be- 

 tween natural and reproductive selection is 

 upset. How far it exists in civilized com- 

 munities of mankind is another and more 

 difiicult problem, which I have partially 

 dealt with elsewhere. * At any rate it be- 

 comes necessary for the biologist either to 

 affirm or deny the two principles stated 

 above. If he affirms them, then he must 

 look upon all races as tending to progress 

 in definite directions — not necessarily one, 

 but possibly several different directions, 

 according to the characters with which fer- 

 tility may be correlated — the moment na- 

 tural selection is suspended ; the organism 

 carries in itself, in virtue of the laws of in- 

 heritance and the correlation of its charac- 

 ters, a tendency to progressive change. I f 

 on the other hand, the biologist denies these 

 principles, then he must be prepared to 

 meet the weight of evidence in favor of the 

 inheritance of fertility and fecundity con- 

 tained in Parts II and III of the present 

 memoir. 



2. The theory discussed in Part I opens 

 with the proof that if fertility be a function 

 of any physical characters which are them- 

 selves inherited according to the law of an- 

 cestral heredity, then it must itself be in- 

 herited according to that law. As fertility 

 would certainly appear to be associated 

 M^ith physique, we have thus an a priori 

 argument in favor of its inheritance. 



3. In the next place the influence of 

 ' record ' making on apparent fertility is 

 considered. The mother with more ofi"- 

 spring has a greater chance than one with 

 fewer of getting into the record which ex- 

 tends over several generations, and, further, 

 if every possible entry be taken from the 



* Essay on Eeproductive Selection in 'The Chances 

 of Death and other Studies in Evolution,' Vol. 1, p. 63. 



