REPRODUCTION. 923 



of male births. It is claimed that ethnic intermixture causes a decrease in the 

 relative number of males born. This is strongly supported by a recent sta- 

 tistical study by Ripley 1 of the two races inhabiting Belgium, the Walloons, 

 of the same origin as the Kelts in France, and the Flemish, of German stock. 

 Where these races are purest, the number of boys born to 1000 girls is 1064; 

 along the region where the two races come into contact, however, the number 

 may fall as low as 1043. Maupas 2 found that sex in the rotifer, Hydatina 

 senta, could be controlled by altering the temperature of the medium surround- 

 ing the egg-laying females. In various experiments at a temperature of 26- 

 28 C., 81-100 per cent, of the eggs gave rise to males, the rest to females ; at 

 14-15 C. only 5-24 per cent, were males, the much larger majority females. 

 The above considerations are highly interesting and suggestive, but they 

 have not yet been brought under general laws sufficiently to make their bear- 

 ing upon the main problem wholly clear. It is probable that numerous 

 factors are of influence in the determination of sex. The general deduction 

 from all the facts seems justified that unfavorable nutritive conditions sur- 

 rounding the parents tend to the production of males, favorable conditions 

 to the production of females. The experimental results indicate, moreover, 

 that the conditions surrounding the parents or the developing embryo are 

 largely responsible for the resulting sex. Watase 3 regards the embryo as 

 neutral as regards sex from the time of fertilization up to a certain stage of 

 its development ; external conditions act as a stimulus to the sexless proto- 

 plasm, and the resulting response is a development in the direction of either 

 maleness or femaleness according to the nature of the stimulus. How largely 

 and in what manner this may be true of the human species is wholly unknown. 

 Dusing urges that the various factors determining sex have arisen through 

 natural selection ; they are conducive to the continuance of the species, and 

 they act in such a way that sex is in a certain sense self-regulating the 

 scarcity of one sex tends to the greater production of individuals of that sex ; 

 this is instanced by the fact mentioned above that after the destruction of 

 males by war relatively more males are born than previously. 



E. EPOCHS IN THE PHYSIOLOGICAL LIFE OF THE INDIVIDUAL. 



Fertilization begins, somatic death ends, the physiological life of the indi- 

 vidual. Between these two events the life-processes go on gradually, and, 

 with the exception of birth, are marked by few abrupt changes. It is some- 

 times convenient to divide the individual life into a number of successive 

 stages, as follows : the embryonic period, the fetal period, infancy, childhood, 

 youth, or adolescence, maturity, and old age, or senescence. Such a division, 

 however, is not physiologically exact, the stages are not sharply limited, and 

 the terms are employed in very different senses by different writers. Between 

 fertilization and birth the functions originate and are developed gradually. 



1 W. Z. Kipley : Quarterly Publications of the American Statistical Association, v., March, 1896. 



2 E. Maupas : Comptes rendus de V Academic des sciences, Paris, cxiii., 1891. 



3 S. Watase : Journal of Morphology, vi., 1892. 



