INFLUENCES DETERMINING SEX. 327 



From this table it "will be seen that in June, the month when the 

 birth-rate was smallest, the ratio of boys to each 100 girls was highest, 

 and very much above the avei'age for the whole year ; while in March, 

 the month when the birth-rate was greatest, the ratio of boys was 

 smallest. 



More than 6,000,000 births took place in the seven months when the 

 ratio of boys was below the average for the year, and only 4,000,000 

 in the five months when it was above the average ; and the table shows 

 clearly that an increase in prosperity, as measured by the birth-rate, is 

 accompanied by a decrease in the ratio of boy-births, and vice versa. 



Among the lower animals, satisfactory statistics are wanting ; but 

 During states that, while domesticated animals are much more prolific 

 than their wild allies, there is also a much greater preponderance of 

 female births ; that when animals are taken from a warm to a cold 

 climate, the ratio of male births increases ; and that leather-dealers 

 say that they obtain most female skins from fertile countries where the 

 pastures are rich, and most male skins from more barren regions; and 

 he thinks we may safely conclude that the lower animals, as well as 

 man, give birth to the greatest number of females when placed in a 

 favorable environment, and to most males in an unfavorable environ- 

 ment. 



An extreme instance is furnished by those animals which, during 

 the seasons when food is abundant, lose the power to copulate and mul- 

 tiply parthenogenetically at a marvelous rate of increase, giving birth 

 to generation after generation of parthenogenetic females, so long as 

 the environment remains favorable, but giving birth, as soon as the 

 conditions of life become less favorable, to males and to females which 

 require fertilization. 



The cladocera and aphides furnish the most striking instances of 

 this kind of parthenogenesis, which has apparently been acquired, not 

 to secure fertilization, but to enable the animals to utilize to the 

 utmost the conditions which are most favorable to them, and to expand 

 and contract their numbers in conformity to changes in their environ- 

 ment. 



Among the parthenogenetic cladoceras both males and females are 

 to be found in the fall, and a few males are found in the early spring ; 

 but during the warm months of spring and summer only females are 

 found. These multiply very rapidly through the summer by partheno- 

 genesis, generation after generation, and they differ from the females 

 which are fertilized by a male in many features, all of which are of 

 such a character as to render the parthenogenetic females unusually 

 fertile. 



They produce small eggs, which are discharged from the ovary 

 while immature, and are nourished in a vascular broad pouch. They 

 have little or no yolk ; they are not protected by a hard shell, and 

 they develop immediately into parthenogenetic females, which mature 



