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THE AMERICAN NATURALIST [Vol. XLI 



Let us suppose that a species possesses the maximum reproduc- 

 tive power when there are x males to n females, the relative magni- 

 tudes of X and n depending upon the breeding habits of the species. 

 Let us suppose also that a males and b females die before reaching 

 sexual maturity. The number of males born will therefore be 

 a + X, and the number of females b + n. The sexual ratio will 

 be , J or k , X 1000 if we wish to express the number of 

 male births to 1000 female births. 



In a monogamous species, such as the American robin, the repro- 

 ductive power of the species would be at a maximum when there 

 were equal numbers of sexually mature males and females. If 

 the males are more likely to be killed off than the females, a would 

 be greater than b, and the sexual ratio would be greater than unity. 

 Surplus males or females would die off without reproducing. In 

 a polygamous species, such as the ox, it is not necessary that there 

 be equal numbers of sexually mature males and females to give 

 the species its maximum reproductive power, and a + x might 

 well be less than b + n. The sexual ratio would in this instance 

 be less than unity. The relative proportions of the sexes in any 

 species may, therefore, be looked upon as one of the physiological 

 adaptations of the species, determined by the conditions of its 

 existence. 



V. The Experimental Point of Attack. 

 If sex is inherited according to Galton's law it should be possible, 

 by suitable selection of the parents, to establish a strain of animals 

 or plants in which males or females occur with any desired degree 

 of frequency compatible with perpetuation of the species. The 

 practical benefits of the favorable results of such an experiment 

 to the dairy and grazing interests of the country would be difficult 

 to estimate. Castle has recently ])ublislifil some experiuuMits on 

 the effects of inbreeding, cross-brmiinu', and selection upon the 

 fertility of flies (Drosophila) in wliieh lie foniid no marke.l elinnge 



